[PATCH v2 00/14] mm/shmem: various (urgent?) fixes

[PATCH v2 00/14] mm/shmem: various (urgent?) fixes

[David Hildenbrand (Arm)]

Hi,

see v1 for details. TL;DR various fixes.

I now rebased on top of mm-stable (thanks Zi!) and retested with
Gregory's tests (thanks!). Thanks everyone for the fast review!

When sending this upstream there will be one conflict with the following
upstream commit:

	737362c7012a ("fs: port ->set_acl() to pass const mnt_idmap")

Which should get resolved through something like

diff --git a/mm/shmem.c b/mm/shmem.c
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@
-static int shmem_set_acl(struct mnt_idmap *idmap, struct dentry *dentry,
+static int shmem_set_acl(const struct mnt_idmap *idmap, struct dentry *dentry,
 		struct posix_acl *acl, int type)

Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
---
Changes in v2:
- Rebased on mm-stable
 - Fixup shmem_set_acl() compilation issue after rebase
 - Dropped: "mm/shmem: avoid moving the truncation start backwards"
 - Fixed up: "mm/shmem: stop retrying folios that cannot be split"
- In "hugetlbfs: enforce F_SEAL_EXEC in setattr", check for S_ISREG
- Smaller patch description fixups
- Picked up tags
- Link to v1: https://patch.msgid.link/20261026-fixes_shmem-v1-0-703206d5fa0c@kernel.org

---

Chris Mason (4):
      mm/mempolicy: charge shared policy nodes
      mm/shmem: honour F_SEAL_EXEC when an ACL is set
      mm/shmem: stop retrying folios that cannot be split
      mm/shmem: preserve data in clean folios when splitting fails

David Hildenbrand (Arm) (7):
      mm/shmem: discard whole new large folios on fallocate rollback
      mm/shmem: rescan after nonuniform folio splitting
      mm/shmem: fix truncate hang in large swap entry
      mm/shmem: preserve allocation errors when swapping in large entries
      mm/shmem: fix truncate loop after boundary swapin failure
      mm/shmem: fix accounting of swapin-error markers
      mm/shmem: fix truncation of files containing only swapin-error markers

Eva Kurchatova (1):
      hugetlbfs: enforce F_SEAL_EXEC in setattr

Josef Bacik (2):
      tmpfs: keep the quota soft limits of an unused id
      tmpfs: preserve explicitly disabled quota limits

 fs/hugetlbfs/inode.c  |   8 +++
 include/linux/memfd.h |   9 ++++
 mm/mempolicy.c        |   4 +-
 mm/shmem.c            | 132 ++++++++++++++++++++++++++++++++------------------
 mm/shmem_quota.c      |  28 ++++++-----
 5 files changed, 119 insertions(+), 62 deletions(-)

---

base-commit: 183895979c441e52dbd651b388553b09eaa35cfa

change-id: 20261025-fixes_shmem-2782719b130e

--

Cheers,

David

[PATCH v2 01/14] mm/mempolicy: charge shared policy nodes

[David Hildenbrand (Arm)]

From: Chris Mason <mason@kernel.org>

Any local user can bypass memory.max with mbind() on a tmpfs file, and
allocate unbounded kernel memory.

mbind() sets a NUMA memory policy for a range of a mapping. If the
mapping is of a shmem and friends, the kernel also stores the policy on
the inode, in a tree with one entry per range of the file.

Each mbind() on a single page of a mapping can add one shared policy node
(struct sp_node) and one struct mempolicy to that tree. They outlive the
mapping and stay around until the inode is destroyed. The tree is not
bounded by the file size, so the file can be empty.

Memory that user space can allocate must be charged to a memory cgroup. A
slab cache does that for all of its objects if it is created with
SLAB_ACCOUNT. numa_policy_init() creates the slab caches for both
structs without SLAB_ACCOUNT, so they are not charged to any cgroup.

Reproduced with mbind() on single pages of an empty tmpfs file, from a
cgroup with memory.max=8M: the global OOM killer killed tasks owned by
root, and the machine panicked.

Fix this by creating both caches with SLAB_ACCOUNT. This change implies
that mbind() can now fail when running into memcg limits. Further,
the policy_cache also serves task and VMA policies, so they are
similarly accounted now.

Note that per-file limits are not really helpful, as the user could
simply use many files; having limits per user is tricky as well.

Fix generated by an LLM and submitted by Chris Mason in private.

Fixes: 5d097056c9a0 ("kmemcg: account certain kmem allocations to memcg")
Cc: stable@vger.kernel.org
Signed-off-by: Chris Mason <mason@kernel.org>
Acked-by: Pedro Falcato <pfalcato@suse.de>
Acked-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
Acked-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Tested-by: Gregory Price (Meta) <gourry@gourry.net>
[ david: smaller description tweaks ]
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
---
 mm/mempolicy.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 0884c629670dd..4da1cb8777f68 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -3372,11 +3372,11 @@ void __init numa_policy_init(void)
 
 	policy_cache = kmem_cache_create("numa_policy",
 					 sizeof(struct mempolicy),
-					 0, SLAB_PANIC, NULL);
+					 0, SLAB_PANIC | SLAB_ACCOUNT, NULL);
 
 	sn_cache = kmem_cache_create("shared_policy_node",
 				     sizeof(struct sp_node),
-				     0, SLAB_PANIC, NULL);
+				     0, SLAB_PANIC | SLAB_ACCOUNT, NULL);
 
 	for_each_node(nid) {
 		preferred_node_policy[nid] = (struct mempolicy) {

-- 
2.43.0

[PATCH v2 02/14] tmpfs: keep the quota soft limits of an unused id

[David Hildenbrand (Arm)]

From: Josef Bacik <josef@toxicpanda.com>

When the last reference to a dquot goes away, shmem_release_dquot()
stores its limits in the rb tree of the mount, unless
shmem_is_empty_dquot() says that there is nothing worth storing. That
function looks at the usage and at the two hard limits only.

So if the administrator sets nothing but soft limits for an id that owns
no file yet, the entry is thrown away as soon as the dquot that
Q_SETQUOTA used is released, and the id never gets to see a limit.

To fix it, compare the soft limits as well: an entry can be empty only if
both soft limits are zero as well, which is exactly what
shmem_acquire_dquot() sets for a new entry.

Fix generated by an LLM and sent by Josef Bacik in private.

Fixes: de4c0e7ca8b5 ("shmem: Add default quota limit mount options")
Cc: stable@vger.kernel.org
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Tested-by: Gregory Price (Meta) <gourry@gourry.net>
[ david: slightly adjusted description ]
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
---
 mm/shmem_quota.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/shmem_quota.c b/mm/shmem_quota.c
index d0b92d6da50f5..a83392f75e56b 100644
--- a/mm/shmem_quota.c
+++ b/mm/shmem_quota.c
@@ -251,6 +251,8 @@ static bool shmem_is_empty_dquot(struct dquot *dquot)
 	if (test_bit(DQ_FAKE_B, &dquot->dq_flags) ||
 		(dquot->dq_dqb.dqb_curspace == 0 &&
 		 dquot->dq_dqb.dqb_curinodes == 0 &&
+		 dquot->dq_dqb.dqb_bsoftlimit == 0 &&
+		 dquot->dq_dqb.dqb_isoftlimit == 0 &&
 		 dquot->dq_dqb.dqb_bhardlimit == bhardlimit &&
 		 dquot->dq_dqb.dqb_ihardlimit == ihardlimit))
 		return true;

-- 
2.43.0

[PATCH v2 03/14] tmpfs: preserve explicitly disabled quota limits

[David Hildenbrand (Arm)]

From: Josef Bacik <josef@toxicpanda.com>

An administrator’s decision to disable a user’s or group’s quota
limits can be silently lost when tmpfs restores the mount defaults
after releasing and reacquiring the quota record. This can cause
later file creation or writes to fail with EDQUOT, even though the
administrator has explicitly disabled quota limits for the user or
group.

do_set_dqblk() sets DQ_FAKE_B when all block and inode limits are zero.
This means quota enforcement is disabled for that user or group.
shmem_is_empty_dquot() treats the flag as a reason to discard the entry
from the mount's quota tree. The next shmem_acquire_dquot() finds no
saved entry and recreates it with the mount's default hard limits.

If the cached dquot is reused, DQ_FAKE_B remains set: Q_GETQUOTA reports
the restored defaults, but enforcement is still disabled. Once that
dquot has been reclaimed, a new dquot enforces the restored defaults.

Remove the DQ_FAKE_B test. Only discard entries with zero usage, zero
soft limits and hard limits equal to the mount defaults. Explicitly
disabled limits are then preserved whenever the mount defaults are
nonzero.

Explicitly disabled limits now retain a quota-tree entry, just like other
limits that differ from the mount defaults.

Rename shmem_is_empty_dquot() to shmem_is_default_dquot() to describe
the check for default values.

Fix generated by an LLM and sent by Josef Bacik in private.

Fixes: de4c0e7ca8b5 ("shmem: Add default quota limit mount options")
Cc: stable@vger.kernel.org
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Tested-by: Gregory Price (Meta) <gourry@gourry.net>
[ david: code cleanups; rewrite description ]
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
---
 mm/shmem_quota.c | 30 +++++++++++++++---------------
 1 file changed, 15 insertions(+), 15 deletions(-)

diff --git a/mm/shmem_quota.c b/mm/shmem_quota.c
index a83392f75e56b..eeca9efd448a2 100644
--- a/mm/shmem_quota.c
+++ b/mm/shmem_quota.c
@@ -234,7 +234,7 @@ static int shmem_acquire_dquot(struct dquot *dquot)
 	return ret;
 }
 
-static bool shmem_is_empty_dquot(struct dquot *dquot)
+static bool shmem_is_default_dquot(struct dquot *dquot)
 {
 	struct shmem_sb_info *sbinfo = dquot->dq_sb->s_fs_info;
 	qsize_t bhardlimit;
@@ -248,21 +248,21 @@ static bool shmem_is_empty_dquot(struct dquot *dquot)
 		ihardlimit = sbinfo->qlimits.grpquota_ihardlimit;
 	}
 
-	if (test_bit(DQ_FAKE_B, &dquot->dq_flags) ||
-		(dquot->dq_dqb.dqb_curspace == 0 &&
-		 dquot->dq_dqb.dqb_curinodes == 0 &&
-		 dquot->dq_dqb.dqb_bsoftlimit == 0 &&
-		 dquot->dq_dqb.dqb_isoftlimit == 0 &&
-		 dquot->dq_dqb.dqb_bhardlimit == bhardlimit &&
-		 dquot->dq_dqb.dqb_ihardlimit == ihardlimit))
-		return true;
-
-	return false;
+	/*
+	 * Defaults are zero usage and soft limits, and the mount's hard limits.
+	 * DQ_FAKE_B only means that *all* limits are zero, which need not match
+	 * the mount defaults.
+	 */
+	return dquot->dq_dqb.dqb_curspace == 0 &&
+	       dquot->dq_dqb.dqb_curinodes == 0 &&
+	       dquot->dq_dqb.dqb_bsoftlimit == 0 &&
+	       dquot->dq_dqb.dqb_isoftlimit == 0 &&
+	       dquot->dq_dqb.dqb_bhardlimit == bhardlimit &&
+	       dquot->dq_dqb.dqb_ihardlimit == ihardlimit;
 }
 /*
- * Store limits from dquot in the tree unless it's fake. If it is fake
- * remove the id from the tree since there is no useful information in
- * there.
+ * Remove the ID from the tree if it has no usage and its limits match
+ * the mount defaults. Otherwise, save its limits for the next acquire.
  */
 static int shmem_release_dquot(struct dquot *dquot)
 {
@@ -297,7 +297,7 @@ static int shmem_release_dquot(struct dquot *dquot)
 	return -ENOENT;
 
 found:
-	if (shmem_is_empty_dquot(dquot)) {
+	if (shmem_is_default_dquot(dquot)) {
 		/* Remove entry from the tree */
 		rb_erase(&entry->node, info->dqi_priv);
 		kfree(entry);

-- 
2.43.0

[PATCH v2 04/14] mm/shmem: honour F_SEAL_EXEC when an ACL is set

[David Hildenbrand (Arm)]

From: Chris Mason <mason@kernel.org>

The owner of a memfd sealed with F_SEAL_EXEC can still change its
execute bits by setting a POSIX access ACL. With CONFIG_TMPFS_POSIX_ACL,
this lets an unprivileged owner make a non-executable memfd executable,
bypassing vm.memfd_noexec=2.

simple_set_acl() updates i_mode directly, bypassing the seal check in
shmem_setattr(). Add shmem_set_acl() to check the resulting mode. Share
the F_SEAL_EXEC check with shmem_setattr() using memfd_check_seals_mode()
in include/linux/memfd.h.

Fix generated by an LLM and sent by Chris Mason in private.

Fixes: 6fd7353829ca ("mm/memfd: add F_SEAL_EXEC")
Cc: stable@vger.kernel.org
Signed-off-by: Chris Mason <mason@kernel.org>
Acked-by: Pedro Falcato <pfalcato@suse.de>
Acked-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
Acked-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Tested-by: Gregory Price (Meta) <gourry@gourry.net>
[ david: use common helper; simplify description ]
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
---
 include/linux/memfd.h |  9 +++++++++
 mm/shmem.c            | 35 ++++++++++++++++++++++++++++++-----
 2 files changed, 39 insertions(+), 5 deletions(-)

diff --git a/include/linux/memfd.h b/include/linux/memfd.h
index c159e40e3f34a..272479317cf10 100644
--- a/include/linux/memfd.h
+++ b/include/linux/memfd.h
@@ -2,10 +2,19 @@
 #ifndef __LINUX_MEMFD_H
 #define __LINUX_MEMFD_H
 
+#include <linux/fcntl.h>
 #include <linux/file.h>
 
 #define MEMFD_ANON_NAME "[memfd]"
 
+static inline int memfd_check_seals_mode(unsigned int seals, umode_t old_mode,
+		umode_t new_mode)
+{
+	if ((seals & F_SEAL_EXEC) && ((old_mode ^ new_mode) & 0111))
+		return -EPERM;
+	return 0;
+}
+
 #ifdef CONFIG_MEMFD_CREATE
 extern long memfd_fcntl(struct file *file, unsigned int cmd, unsigned int arg);
 struct folio *memfd_alloc_folio(struct file *memfd, pgoff_t idx);
diff --git a/mm/shmem.c b/mm/shmem.c
index fadf23f9e21ed..ad644a268c0d2 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -77,6 +77,7 @@ static struct vfsmount *shm_mnt __ro_after_init;
 #include <linux/magic.h>
 #include <linux/syscalls.h>
 #include <linux/fcntl.h>
+#include <linux/memfd.h>
 #include <uapi/linux/memfd.h>
 #include <linux/rmap.h>
 #include <linux/uuid.h>
@@ -1546,10 +1547,11 @@ static int shmem_setattr(struct mnt_idmap *idmap,
 	if (error)
 		return error;
 
-	if ((info->seals & F_SEAL_EXEC) && (attr->ia_valid & ATTR_MODE)) {
-		if ((inode->i_mode ^ attr->ia_mode) & 0111) {
-			return -EPERM;
-		}
+	if (attr->ia_valid & ATTR_MODE) {
+		error = memfd_check_seals_mode(info->seals, inode->i_mode,
+					       attr->ia_mode);
+		if (error)
+			return error;
 	}
 
 	if (S_ISREG(inode->i_mode) && (attr->ia_valid & ATTR_SIZE)) {
@@ -5397,12 +5399,35 @@ static const struct file_operations shmem_file_operations = {
 #endif
 };
 
+#ifdef CONFIG_TMPFS_POSIX_ACL
+static int shmem_set_acl(struct mnt_idmap *idmap, struct dentry *dentry,
+		struct posix_acl *acl, int type)
+{
+	struct inode *inode = d_inode(dentry);
+	umode_t mode = inode->i_mode;
+	int error;
+
+	if (type == ACL_TYPE_ACCESS) {
+		error = posix_acl_equiv_mode(acl, &mode);
+		if (error < 0)
+			return error;
+		error = memfd_check_seals_mode(SHMEM_I(inode)->seals,
+					       inode->i_mode, mode);
+		if (error)
+			return error;
+	}
+	return simple_set_acl(idmap, dentry, acl, type);
+}
+#else
+#define shmem_set_acl NULL
+#endif
+
 static const struct inode_operations shmem_inode_operations = {
 	.getattr	= shmem_getattr,
 	.setattr	= shmem_setattr,
 #ifdef CONFIG_TMPFS_XATTR
 	.listxattr	= shmem_listxattr,
-	.set_acl	= simple_set_acl,
+	.set_acl	= shmem_set_acl,
 	.fileattr_get	= shmem_fileattr_get,
 	.fileattr_set	= shmem_fileattr_set,
 #endif

-- 
2.43.0

Re: [PATCH v2 04/14] mm/shmem: honour F_SEAL_EXEC when an ACL is set

[Pratyush Yadav]

On Tue, Oct 27 2026, David Hildenbrand (Arm) wrote:

> From: Chris Mason <mason@kernel.org>
>
> The owner of a memfd sealed with F_SEAL_EXEC can still change its
> execute bits by setting a POSIX access ACL. With CONFIG_TMPFS_POSIX_ACL,
> this lets an unprivileged owner make a non-executable memfd executable,
> bypassing vm.memfd_noexec=2.
>
> simple_set_acl() updates i_mode directly, bypassing the seal check in
> shmem_setattr(). Add shmem_set_acl() to check the resulting mode. Share
> the F_SEAL_EXEC check with shmem_setattr() using memfd_check_seals_mode()
> in include/linux/memfd.h.
>
> Fix generated by an LLM and sent by Chris Mason in private.
>
> Fixes: 6fd7353829ca ("mm/memfd: add F_SEAL_EXEC")
> Cc: stable@vger.kernel.org
> Signed-off-by: Chris Mason <mason@kernel.org>
> Acked-by: Pedro Falcato <pfalcato@suse.de>
> Acked-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
> Acked-by: Zi Yan <ziy@nvidia.com>
> Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> Tested-by: Gregory Price (Meta) <gourry@gourry.net>
> [ david: use common helper; simplify description ]
> Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>

Reviewed-by: Pratyush Yadav (Google) <pratyush@kernel.org>

[...]

-- 
Regards,
Pratyush Yadav

[PATCH v2 05/14] hugetlbfs: enforce F_SEAL_EXEC in setattr

[David Hildenbrand (Arm)]

From: Eva Kurchatova <eva.kurchatova@virtuozzo.com>

F_SEAL_EXEC is enforced by shmem_setattr(), but hugetlbfs_setattr() does
not check it. As a result, fchmod() can change the execute bits of a
sealed hugetlbfs memfd.

Use memfd_check_seals_mode() in hugetlbfs_setattr() to reject changes
to execute bits when F_SEAL_EXEC is set.

This also fixes the memfd_test hugetlbfs SEAL-EXEC subtest, where
fchmod() incorrectly succeeded after applying F_SEAL_EXEC.

Fixes: 6fd7353829ca ("mm/memfd: add F_SEAL_EXEC")
Cc: stable@vger.kernel.org
Signed-off-by: Eva Kurchatova <eva.kurchatova@virtuozzo.com>
Link: https://patch.msgid.link/20260629180247.2343229-1-eva.kurchatova@virtuozzo.com
Acked-by: Pedro Falcato <pfalcato@suse.de>
Acked-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
Acked-by: Zi Yan <ziy@nvidia.com>
Tested-by: Gregory Price (Meta) <gourry@gourry.net>
[ david: use common helper; fixup Fixes:; check only regular files ]
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
---
 fs/hugetlbfs/inode.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
index ab1e4dec3f77b..1fa05b4d26531 100644
--- a/fs/hugetlbfs/inode.c
+++ b/fs/hugetlbfs/inode.c
@@ -28,6 +28,7 @@
 #include <linux/folio_batch.h>
 #include <linux/fs_parser.h>
 #include <linux/mman.h>
+#include <linux/memfd.h>
 #include <linux/slab.h>
 #include <linux/dnotify.h>
 #include <linux/statfs.h>
@@ -840,6 +841,13 @@ static int hugetlbfs_setattr(struct mnt_idmap *idmap,
 	if (error)
 		return error;
 
+	if (S_ISREG(inode->i_mode) && (ia_valid & ATTR_MODE)) {
+		error = memfd_check_seals_mode(info->seals, inode->i_mode,
+					       attr->ia_mode);
+		if (error)
+			return error;
+	}
+
 	trace_hugetlbfs_setattr(inode, dentry, attr);
 
 	if (ia_valid & ATTR_SIZE) {

-- 
2.43.0

Re: [PATCH v2 05/14] hugetlbfs: enforce F_SEAL_EXEC in setattr

[Muchun Song]

> On Oct 27, 2026, at 21:03, David Hildenbrand (Arm) <david@kernel.org> wrote:
> 
> From: Eva Kurchatova <eva.kurchatova@virtuozzo.com>
> 
> F_SEAL_EXEC is enforced by shmem_setattr(), but hugetlbfs_setattr() does
> not check it. As a result, fchmod() can change the execute bits of a
> sealed hugetlbfs memfd.
> 
> Use memfd_check_seals_mode() in hugetlbfs_setattr() to reject changes
> to execute bits when F_SEAL_EXEC is set.
> 
> This also fixes the memfd_test hugetlbfs SEAL-EXEC subtest, where
> fchmod() incorrectly succeeded after applying F_SEAL_EXEC.
> 
> Fixes: 6fd7353829ca ("mm/memfd: add F_SEAL_EXEC")
> Cc: stable@vger.kernel.org
> Signed-off-by: Eva Kurchatova <eva.kurchatova@virtuozzo.com>
> Link: https://patch.msgid.link/20260629180247.2343229-1-eva.kurchatova@virtuozzo.com
> Acked-by: Pedro Falcato <pfalcato@suse.de>
> Acked-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
> Acked-by: Zi Yan <ziy@nvidia.com>
> Tested-by: Gregory Price (Meta) <gourry@gourry.net>
> [ david: use common helper; fixup Fixes:; check only regular files ]
> Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>

Reviewed-by: Muchun Song <muchun.song@linux.dev>

Thanks.

[PATCH v2 06/14] mm/shmem: stop retrying folios that cannot be split

[David Hildenbrand (Arm)]

From: Chris Mason <mason@kernel.org>

A fault or collapse can insert a large folio crossing EOF after
shmem_undo_range() has handled the boundary folios. If splitting that
folio fails, truncate_inode_partial_folio() retains it when it is dirty.
The final scan then repeatedly finds it because it restarts from an
unchanged start.

Truncation keeps the inode lock and ignores fatal signals while looping.
A concurrent unlink() can then block other directory operations while
holding the directory lock and waiting for the file's lock.

Advance start past the retained folio, whose truncated part has already
been zeroed. Do not move start backwards if the failed split has shrunk
the folio.

Fix generated by an LLM and sent by Chris Mason in private.

Fixes: 55ac8bbe358b ("mm/shmem: fix race in shmem_undo_range w/THP")
Cc: stable@vger.kernel.org
Signed-off-by: Chris Mason <mason@kernel.org>
Tested-by: Gregory Price (Meta) <gourry@gourry.net>
[ david: split fixes, simplify descriptions, adjust comments ]
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
---
 mm/shmem.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/mm/shmem.c b/mm/shmem.c
index ad644a268c0d2..5d21cdf5fb0a5 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1481,6 +1481,15 @@ static void shmem_undo_range(struct inode *inode, loff_t lstart, uoff_t lend,
 						index = start;
 						break;
 					}
+				} else if (folio->mapping) {
+					/*
+					 * Splitting failed, but the part in the
+					 * range is already zeroed. The folio
+					 * was not dropped, so skip it when
+					 * restarting the scan to avoid retrying
+					 * it forever.
+					 */
+					start = max(start, folio_next_index(folio));
 				}
 			}
 			folio_unlock(folio);

-- 
2.43.0

[PATCH v2 07/14] mm/shmem: preserve data in clean folios when splitting fails

[David Hildenbrand (Arm)]

From: Chris Mason <mason@kernel.org>

A shared writable mapping can have dirty PTEs while its shmem folio is
still clean. A read fault can install a writable PTE, and a later store
sets the PTE's dirty bit without marking the folio dirty.

After shmem_undo_range() has handled the boundary folios, a concurrent
read fault into a hole below the new EOF can allocate a large folio.
For example, on a tmpfs mount with huge=always, the fault's offset must
be within the file, but the allocated folio can extend beyond i_size.
The final scan then finds this folio and calls
truncate_inode_partial_folio() without marking it dirty.

Splitting normally collects the PTE dirty bits when unmapping the
folio. An extra reference, for example from vmsplice(), can make it
fail before that step. The fallback unmapping on split failure excludes
shmem, so the helper still sees a clean folio and drops it, losing data
outside the range.

CPU0: ftruncate(fd, 3M)                CPU1: MAP_SHARED, tmpfs huge=always
─────────────────────────────────────  ──────────────────────────────────
i_size = 3M
shmem_undo_range(3M, -1)
  first pass: hole, nothing to do
  boundary folio at 3M: hole, none
                                       read fault at 2M+x (below EOF)
                                         PMD folio [2M,4M), crosses EOF
                                         writable PTE, folio clean
                                       store to 2M+x
                                         PTE dirty, folio still clean
                                       vmsplice(2M+x)
                                         extra folio reference
  whole_folios: finds [2M,4M) folio
  truncate_inode_partial_folio()
    zero [3M,4M)
    folio_split()
      unmap_folio(): extra ref
        → -EAGAIN, PTE dirty bit
          never reaches the folio
    folio clean
      → truncate_inode_folio(whole)
        [2M,3M) dropped
                                       read 2M+x → refault → zeroes

Mark large folios dirty before partial truncation in the final scan,
as the boundary handling already does. Exclude failed-fallocate
rollback, which must discard the folios it added.

Fix generated by an LLM and sent by Chris Mason in private.

Fixes: b9a8a4195c7d ("truncate,shmem: Handle truncates that split large folios")
Cc: stable@vger.kernel.org
Signed-off-by: Chris Mason <mason@kernel.org>
Acked-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Tested-by: Gregory Price (Meta) <gourry@gourry.net>
[ david: simplify descriptions, adjust comments ]
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
---
 mm/shmem.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/mm/shmem.c b/mm/shmem.c
index 5d21cdf5fb0a5..b615301a311e0 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1464,6 +1464,15 @@ static void shmem_undo_range(struct inode *inode, loff_t lstart, uoff_t lend,
 				VM_BUG_ON_FOLIO(folio_test_writeback(folio),
 						folio);
 
+				/*
+				 * A shared mapping can have dirty PTEs while
+				 * the folio is still clean. Mark the folio
+				 * dirty so a failed split cannot discard data
+				 * outside the range.
+				 */
+				if (!unfalloc && folio_test_large(folio))
+					folio_mark_dirty(folio);
+
 				if (!folio_test_large(folio)) {
 					truncate_inode_folio(mapping, folio);
 				} else if (truncate_inode_partial_folio(folio,

-- 
2.43.0

[PATCH v2 08/14] mm/shmem: discard whole new large folios on fallocate rollback

[David Hildenbrand (Arm)]

When undoing a failed fallocate(), shmem_undo_range() must discard each
newly allocated folio. Its second scan instead passes large
folios to truncate_inode_partial_folio(), which does not necessarily
discard the entire large folio, but instead tries splitting it to
discard only the requested range.

For example, with huge=always, a failed allocation starting at offset
4 KiB can leave page 0 allocated in an otherwise empty file.

Discard the whole folio when unfalloc is set. Note that we enter this
path only for folios that are not uptodate, and can consequently be
dropped without losing any data.

Fixes: 55ac8bbe358b ("mm/shmem: fix race in shmem_undo_range w/THP")
Cc: stable@vger.kernel.org
Reviewed-by: Zi Yan <ziy@nvidia.com>
Acked-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Tested-by: Gregory Price (Meta) <gourry@gourry.net>
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
---
 mm/shmem.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index b615301a311e0..e6532c9b0cbdf 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1473,7 +1473,7 @@ static void shmem_undo_range(struct inode *inode, loff_t lstart, uoff_t lend,
 				if (!unfalloc && folio_test_large(folio))
 					folio_mark_dirty(folio);
 
-				if (!folio_test_large(folio)) {
+				if (unfalloc || !folio_test_large(folio)) {
 					truncate_inode_folio(mapping, folio);
 				} else if (truncate_inode_partial_folio(folio,
 						lstart, lend, NULL, NULL)) {

-- 
2.43.0

[PATCH v2 09/14] mm/shmem: rescan after nonuniform folio splitting

[David Hildenbrand (Arm)]

The final scan in shmem_undo_range() restarts after a successful split
only if the original folio is no longer large. A nonuniform split can
leave that folio large, so the scan skips the newly created folios.
Hole punching then leaves pages within the hole allocated even though
they could be discarded.

For example, a fault below the hole can allocate a 2 MiB folio after
boundary handling. Punching a 1 MiB hole at offset 64 KiB splits the
folio successfully but leaves the original folio at order 4. The scan
continues past all 512 pages, leaving 2 MiB allocated instead of 1 MiB.
The hole reads as zero, but the pages and their block charges remain.

Fixes: 7460b470a131 ("mm/truncate: use folio_split() in truncate operation")
Cc: stable@vger.kernel.org
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Acked-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
Tested-by: Gregory Price (Meta) <gourry@gourry.net>
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
---
 mm/shmem.c | 16 +++++-----------
 1 file changed, 5 insertions(+), 11 deletions(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index e6532c9b0cbdf..d7cc45d7077a5 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1478,18 +1478,12 @@ static void shmem_undo_range(struct inode *inode, loff_t lstart, uoff_t lend,
 				} else if (truncate_inode_partial_folio(folio,
 						lstart, lend, NULL, NULL)) {
 					/*
-					 * If we split a page, reset the loop so
-					 * that we pick up the new sub pages.
-					 * Otherwise the THP was entirely
-					 * dropped or the target range was
-					 * zeroed, so just continue the loop as
-					 * is.
+					 * The folio was split. Retry the scan
+					 * to find the split folios.
 					 */
-					if (!folio_test_large(folio)) {
-						folio_unlock(folio);
-						index = start;
-						break;
-					}
+					folio_unlock(folio);
+					index = start;
+					break;
 				} else if (folio->mapping) {
 					/*
 					 * Splitting failed, but the part in the

-- 
2.43.0

[PATCH v2 10/14] mm/shmem: fix truncate hang in large swap entry

[David Hildenbrand (Arm)]

Truncating a tmpfs file or memfd to a page-aligned size inside a large
swap entry can loop indefinitely in shmem_undo_range(). The task holds
the inode lock and cannot be killed.

A large swap entry represents a whole swapped-out folio. For example,
truncate a file whose pages 0-7 share one swap entry to four pages:

    [ 0  1  2  3 | 4  5  6  7 ]
         keep    |   discard
                 ^
            new EOF, start = 4

i_size has already been updated when boundary handling asks
shmem_get_partial_folio() for page 4. Since it is swapped out, the helper
calls shmem_get_folio() with SGP_READ. That rejects page 4 with -EINVAL
because its starting offset equals i_size. Nothing is read back.

The final scan cannot free the entry without losing pages 0-3. It skips
it, restarts from start = 4, and finds it again indefinitely. An unaligned
truncate avoids this particular failure because the requested page starts
below i_size. Folios already in the page-cache cannot trigger it as the
direct lookup will return them beyond EOF.

Fix it by letting shmem_get_partial_folio() call shmem_swapin_folio()
directly for swap entries. This bypasses the read path's i_size check,
allowing the existing boundary handling to zero and split the folio. If
swap-in returns a smaller folio, it also splits the swap entry so the
final scan can free the entries beyond EOF. Retry the lookup if the entry
changes during swap-in.

Handling swap-in allocation failures remains broken and is unchanged by
this commit. Even when shmem_swapin_folio() correctly returns -ENOMEM
instead of -EEXIST, shmem_get_partial_folio() returns NULL and
shmem_undo_range() skips boundary handling. If the large swap entry
remains intact, truncation can still loop in the final scan, while hole
punching can return without zeroing the boundary.

This may read an entire large folio back from swap while holding
i_rwsem, only to discard part of it. Avoiding that I/O would require
splitting both the swap entry and any large folio still in the swap
cache. That, however, is much bigger work.

Fixes: 8a1968bd997f ("mm/shmem, swap: fix race of truncate and swap entry split")
Cc: stable@vger.kernel.org
Reported-by: Chris Mason <mason@kernel.org>
Acked-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Tested-by: Gregory Price (Meta) <gourry@gourry.net>
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
---
 mm/shmem.c | 15 +++++++--------
 1 file changed, 7 insertions(+), 8 deletions(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index d7cc45d7077a5..a538b37fe739b 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1298,12 +1298,10 @@ void shmem_unlock_mapping(struct address_space *mapping)
 
 static struct folio *shmem_get_partial_folio(struct inode *inode, pgoff_t index)
 {
+	const gfp_t gfp = mapping_gfp_mask(inode->i_mapping);
 	struct folio *folio;
 
-	/*
-	 * At first avoid shmem_get_folio(,,,SGP_READ): that fails
-	 * beyond i_size, and reports fallocated folios as holes.
-	 */
+retry:
 	folio = filemap_get_entry(inode->i_mapping, index);
 	if (!folio)
 		return folio;
@@ -1314,13 +1312,14 @@ static struct folio *shmem_get_partial_folio(struct inode *inode, pgoff_t index)
 		/* The folio has been swapped out */
 		folio_unlock(folio);
 		folio_put(folio);
+		goto retry;
 	}
 	/*
-	 * But read a folio back from swap if any of it is within i_size
-	 * (although in some cases this is just a waste of time).
+	 * Truncation can look up a large swap entry at or beyond i_size.
+	 * Swap it in directly to bypass the size check in shmem_get_folio().
 	 */
-	folio = NULL;
-	shmem_get_folio(inode, index, 0, &folio, SGP_READ);
+	if (shmem_swapin_folio(inode, index, &folio, SGP_READ, gfp, NULL, NULL) == -EEXIST)
+		goto retry;
 	return folio;
 }
 

-- 
2.43.0

[PATCH v2 11/14] mm/shmem: preserve allocation errors when swapping in large entries

[David Hildenbrand (Arm)]

shmem_swapin_folio() adjusts swap to the requested slot within a large
swap entry before allocating a folio. If allocation or charging fails,
the error path compares this adjusted value with the original entry
still stored in the XArray in shmem_confirm_swap().

Unless the index is at the start of the entry, this turns -ENOMEM into
-EEXIST even though nothing changed.

The caller retries -EEXIST, so persistent allocation failures can leave
the task spinning instead of returning the error.

Restore swap to index_entry on allocation failure so the existing error
path checks the original entry. This preserves the allocation error if
the entry is unchanged and still retries if it changed concurrently.

Fixes: 91ab656ece13 ("mm/shmem, swap: tidy up swap entry splitting")
Cc: stable@vger.kernel.org
Reported-by: Josef Bacik <josef@toxicpanda.com>
Acked-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Tested-by: Gregory Price (Meta) <gourry@gourry.net>
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
---
 mm/shmem.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/shmem.c b/mm/shmem.c
index a538b37fe739b..9a11de010ed7b 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2528,6 +2528,8 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
 			else
 				error = -ENOMEM;
 			folio = NULL;
+			/* Make sure to validate the original value. */
+			swap = index_entry;
 			goto failed;
 		}
 		if (fault_type) {

-- 
2.43.0

[PATCH v2 12/14] mm/shmem: fix truncate loop after boundary swapin failure

[David Hildenbrand (Arm)]

If swapping in a large entry at the new EOF fails, shmem_undo_range()
leaves the entry in place. Its final scan skips the entry, then restarts
from the same start index and finds it again. Truncation can loop forever
while holding i_rwsem.

Advance start past the skipped entry so that a restarted scan can finish,
as we already do for folios that cannot be split.

Note that this does not fix the missing zeroing when boundary swapin fails,
it merely prevents the endless loop.

Fixes: 8a1968bd997f ("mm/shmem, swap: fix race of truncate and swap entry split")
Cc: stable@vger.kernel.org
Acked-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
Tested-by: Gregory Price (Meta) <gourry@gourry.net>
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
---
 mm/shmem.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index 9a11de010ed7b..4ece296a1ccf1 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1434,14 +1434,16 @@ static void shmem_undo_range(struct inode *inode, loff_t lstart, uoff_t lend,
 					order = shmem_confirm_swap(mapping, indices[i],
 								   radix_to_swp_entry(folio));
 					/*
-					 * If found a large swap entry cross the end or start
-					 * border, skip it as the truncate_inode_partial_folio
-					 * above should have at least zerod its content once.
+					 * Skip boundary entries when restarting
+					 * the scan to avoid endless loops after
+					 * swapin failure.
 					 */
 					if (order > 0) {
 						base = round_down(base, 1 << order);
-						if (base < start || base + (1 << order) > end)
+						if (base < start || base + (1 << order) > end) {
+							start = max(start, base + (1 << order));
 							continue;
+						}
 					}
 					/* Swap was replaced by page or extended, retry */
 					index = base;

-- 
2.43.0

[PATCH v2 13/14] mm/shmem: fix accounting of swapin-error markers

[David Hildenbrand (Arm)]

A failed swap read replaces the swap entry with a swapin-error marker in
the page-cache and subtracts its pages from "alloced" and "swapped";
however, it leaves their block charge in place.

Removing the marker during truncate then subtracts from "swapped" again.
This can underflow the counters or make swapped reach zero while other
pages remain swapped out, causing swapoff to loop without finding them.

When installing the marker, subtract only from "swapped" and let
shmem_recalc_inode() reduce "alloced" and release the block charge.

Make shmem_free_swap() report successful removal separately from the
number of swap pages freed, so removing a swapin-error marker does not
subtract from "swapped" again.

It's a bit confusing, but replacing a swapped out page by a swapin-error
marker in shmem_set_folio_swapin_error() behaves as follows:

Initially:
	alloced=1, swapped=1, nrpages=0
Calling shmem_recalc_inode(inode, 0, -1)
	-> swapped = 0
	-> freed =  1 - 0 - 0 = 1
	-> alloced -= freed
	-> alloced=0, swapped=0, block charge released

Fixes: 6cec2b95dadf ("mm/shmem: fix infinite loop when swap in shmem error at swapoff time")
Cc: stable@vger.kernel.org
Tested-by: Gregory Price (Meta) <gourry@gourry.net>
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
---
 mm/shmem.c | 31 ++++++++++++++-----------------
 1 file changed, 14 insertions(+), 17 deletions(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index 4ece296a1ccf1..2800dbdf031c1 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1177,12 +1177,11 @@ static void shmem_delete_from_page_cache(struct folio *folio, void *radswap)
 }
 
 /*
- * Remove swap entry from page cache, free the swap and its page cache. Returns
- * the number of pages being freed. 0 means entry not found in XArray (0 pages
- * being freed).
+ * Remove a swap entry or swapin-error marker wholly within [index, end].
+ * Return whether it was removed, counting freed swap pages in nr_swaps_freed.
  */
-static long shmem_free_swap(struct address_space *mapping,
-			    pgoff_t index, pgoff_t end, void *radswap)
+static bool shmem_free_swap(struct address_space *mapping,
+		pgoff_t index, pgoff_t end, void *radswap, long *nr_swaps_freed)
 {
 	XA_STATE(xas, &mapping->i_pages, index);
 	const softleaf_t swp = radix_to_swp_entry(radswap);
@@ -1203,8 +1202,10 @@ static long shmem_free_swap(struct address_space *mapping,
 	xas_unlock_irq(&xas);
 
 	/* A swapin-error marker holds no swap slot, so just drop it. */
-	if (nr_pages && softleaf_is_swap(swp))
+	if (nr_pages && softleaf_is_swap(swp)) {
 		swap_put_entries_direct(swp, nr_pages);
+		*nr_swaps_freed += nr_pages;
+	}
 
 	return nr_pages;
 }
@@ -1358,8 +1359,8 @@ static void shmem_undo_range(struct inode *inode, loff_t lstart, uoff_t lend,
 			if (xa_is_value(folio)) {
 				if (unfalloc)
 					continue;
-				nr_swaps_freed += shmem_free_swap(mapping, indices[i],
-								  end - 1, folio);
+				shmem_free_swap(mapping, indices[i], end - 1,
+						folio, &nr_swaps_freed);
 				continue;
 			}
 
@@ -1422,13 +1423,11 @@ static void shmem_undo_range(struct inode *inode, loff_t lstart, uoff_t lend,
 
 			if (xa_is_value(folio)) {
 				int order;
-				long swaps_freed;
 
 				if (unfalloc)
 					continue;
-				swaps_freed = shmem_free_swap(mapping, indices[i],
-							      end - 1, folio);
-				if (!swaps_freed) {
+				if (!shmem_free_swap(mapping, indices[i], end - 1,
+						     folio, &nr_swaps_freed)) {
 					pgoff_t base = indices[i];
 
 					order = shmem_confirm_swap(mapping, indices[i],
@@ -1449,7 +1448,6 @@ static void shmem_undo_range(struct inode *inode, loff_t lstart, uoff_t lend,
 					index = base;
 					break;
 				}
-				nr_swaps_freed += swaps_freed;
 				continue;
 			}
 
@@ -2385,11 +2383,10 @@ static void shmem_set_folio_swapin_error(struct inode *inode, pgoff_t index,
 	folio_put_swap(folio, NULL);
 	swap_cache_del_folio(folio);
 	/*
-	 * Don't treat swapin error folio as alloced. Otherwise inode->i_blocks
-	 * won't be 0 when inode is released and thus trigger WARN_ON(i_blocks)
-	 * in shmem_evict_inode().
+	 * Let shmem_recalc_inode() reduce alloced and release the block charge
+	 * for the freed swap pages.
 	 */
-	shmem_recalc_inode(inode, -nr_pages, -nr_pages);
+	shmem_recalc_inode(inode, 0, -nr_pages);
 }
 
 static int shmem_split_large_entry(struct inode *inode, pgoff_t index,

-- 
2.43.0

[PATCH v2 14/14] mm/shmem: fix truncation of files containing only swapin-error markers

[David Hildenbrand (Arm)]

It might not be very common, but we can end up with shmem files that are
composed only of swapin-error/poison markers.

Such markers are not counted in info->alloced. Consequently,
shmem_setattr() can skip shmem_truncate_range(), leaving the markers
behind.

Truncating the file to zero and growing it again will result in reads
failing with -EIO instead of returning zeroes.

Check whether the mapping contains entries, in addition to
info->alloced, before skipping the truncation. Keep the info->alloced
check so that shmem_recalc_inode(), called at the end of
shmem_undo_range(), can fix up accounting after reclaim has emptied the
mapping.

Fixes: 6cec2b95dadf ("mm/shmem: fix infinite loop when swap in shmem error at swapoff time")
Cc: stable@vger.kernel.org
Tested-by: Gregory Price (Meta) <gourry@gourry.net>
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
---
 mm/shmem.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index 2800dbdf031c1..3c42b4f253b5e 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1591,7 +1591,8 @@ static int shmem_setattr(struct mnt_idmap *idmap,
 			if (oldsize > holebegin)
 				unmap_mapping_range(inode->i_mapping,
 							holebegin, 0, 1);
-			if (info->alloced)
+			/* Swapin-error markers are not counted in alloced. */
+			if (info->alloced || !mapping_empty(inode->i_mapping))
 				shmem_truncate_range(inode,
 							newsize, (loff_t)-1);
 			/* unmap again to remove racily COWed private pages */

-- 
2.43.0

Re: [PATCH v2 00/14] mm/shmem: various (urgent?) fixes

[Baolin Wang]

On 10/27/26 9:03 PM, David Hildenbrand (Arm) wrote:
> Hi,
> 
> see v1 for details. TL;DR various fixes.
> 
> I now rebased on top of mm-stable (thanks Zi!) and retested with
> Gregory's tests (thanks!). Thanks everyone for the fast review!
> 
> When sending this upstream there will be one conflict with the following
> upstream commit:
> 
> 	737362c7012a ("fs: port ->set_acl() to pass const mnt_idmap")
> 
> Which should get resolved through something like
> 
> diff --git a/mm/shmem.c b/mm/shmem.c
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@
> -static int shmem_set_acl(struct mnt_idmap *idmap, struct dentry *dentry,
> +static int shmem_set_acl(const struct mnt_idmap *idmap, struct dentry *dentry,
>   		struct posix_acl *acl, int type)
> 
> Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
> ---
> Changes in v2:
> - Rebased on mm-stable
>   - Fixup shmem_set_acl() compilation issue after rebase
>   - Dropped: "mm/shmem: avoid moving the truncation start backwards"
>   - Fixed up: "mm/shmem: stop retrying folios that cannot be split"
> - In "hugetlbfs: enforce F_SEAL_EXEC in setattr", check for S_ISREG
> - Smaller patch description fixups
> - Picked up tags
> - Link to v1: https://patch.msgid.link/20261026-fixes_shmem-v1-0-703206d5fa0c@kernel.org

why drop this fix "mm/shmem: hold off eviction in shmem_writeout()" from v2?

https://lore.kernel.org/all/20261026-fixes_shmem-v1-6-703206d5fa0c@kernel.org/#t

Re: [PATCH v2 00/14] mm/shmem: various (urgent?) fixes

[David Hildenbrand (Arm)]

On 10/27/26 14:20, Baolin Wang wrote:
> 
> 
> On 10/27/26 9:03 PM, David Hildenbrand (Arm) wrote:
>> Hi,
>>
>> see v1 for details. TL;DR various fixes.
>>
>> I now rebased on top of mm-stable (thanks Zi!) and retested with
>> Gregory's tests (thanks!). Thanks everyone for the fast review!
>>
>> When sending this upstream there will be one conflict with the following
>> upstream commit:
>>
>>     737362c7012a ("fs: port ->set_acl() to pass const mnt_idmap")
>>
>> Which should get resolved through something like
>>
>> diff --git a/mm/shmem.c b/mm/shmem.c
>> --- a/mm/shmem.c
>> +++ b/mm/shmem.c
>> @@
>> -static int shmem_set_acl(struct mnt_idmap *idmap, struct dentry *dentry,
>> +static int shmem_set_acl(const struct mnt_idmap *idmap, struct dentry *dentry,
>>           struct posix_acl *acl, int type)
>>
>> Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
>> ---
>> Changes in v2:
>> - Rebased on mm-stable
>>   - Fixup shmem_set_acl() compilation issue after rebase
>>   - Dropped: "mm/shmem: avoid moving the truncation start backwards"
>>   - Fixed up: "mm/shmem: stop retrying folios that cannot be split"
>> - In "hugetlbfs: enforce F_SEAL_EXEC in setattr", check for S_ISREG
>> - Smaller patch description fixups
>> - Picked up tags
>> - Link to v1: https://patch.msgid.link/20261026-fixes_shmem-
>> v1-0-703206d5fa0c@kernel.org
> 
> why drop this fix "mm/shmem: hold off eviction in shmem_writeout()" from v2?
> 
> https://lore.kernel.org/all/20261026-fixes_shmem-v1-6-703206d5fa0c@kernel.org/#t

Because I'm stupid and dropped two commit instead of one :(

-- 
Cheers,

David

Re: [PATCH v2 00/14] mm/shmem: various (urgent?) fixes

[David Hildenbrand (Arm)]

On 10/27/26 14:27, David Hildenbrand (Arm) wrote:
> On 10/27/26 14:20, Baolin Wang wrote:
>>
>>
>> On 10/27/26 9:03 PM, David Hildenbrand (Arm) wrote:
>>> Hi,
>>>
>>> see v1 for details. TL;DR various fixes.
>>>
>>> I now rebased on top of mm-stable (thanks Zi!) and retested with
>>> Gregory's tests (thanks!). Thanks everyone for the fast review!
>>>
>>> When sending this upstream there will be one conflict with the following
>>> upstream commit:
>>>
>>>     737362c7012a ("fs: port ->set_acl() to pass const mnt_idmap")
>>>
>>> Which should get resolved through something like
>>>
>>> diff --git a/mm/shmem.c b/mm/shmem.c
>>> --- a/mm/shmem.c
>>> +++ b/mm/shmem.c
>>> @@
>>> -static int shmem_set_acl(struct mnt_idmap *idmap, struct dentry *dentry,
>>> +static int shmem_set_acl(const struct mnt_idmap *idmap, struct dentry *dentry,
>>>           struct posix_acl *acl, int type)
>>>
>>> Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
>>> ---
>>> Changes in v2:
>>> - Rebased on mm-stable
>>>   - Fixup shmem_set_acl() compilation issue after rebase
>>>   - Dropped: "mm/shmem: avoid moving the truncation start backwards"
>>>   - Fixed up: "mm/shmem: stop retrying folios that cannot be split"
>>> - In "hugetlbfs: enforce F_SEAL_EXEC in setattr", check for S_ISREG
>>> - Smaller patch description fixups
>>> - Picked up tags
>>> - Link to v1: https://patch.msgid.link/20261026-fixes_shmem-
>>> v1-0-703206d5fa0c@kernel.org
>>
>> why drop this fix "mm/shmem: hold off eviction in shmem_writeout()" from v2?
>>
>> https://lore.kernel.org/all/20261026-fixes_shmem-v1-6-703206d5fa0c@kernel.org/#t
> 
> Because I'm stupid and dropped two commit instead of one :(
> 

I'll wait a bit and then send v3 fairly aggressively, including again:


From d342e8d7d01bb656d7e224e38acd7309d7678e53 Mon Sep 17 00:00:00 2001
From: Josef Bacik <josef@toxicpanda.com>
Date: Fri, 9 Oct 2026 00:36:37 +0000
Subject: [PATCH] mm/shmem: hold off eviction in shmem_writeout()

Once shmem_writeout() has moved the folio from the page cache to the
swap cache, the folio lock no longer keeps the inode alive: truncation
and eviction find a swap entry, free it and do not wait for anybody.
That did not matter as long as swap_writeout() was the last thing the
function did.  But since it puts the folio back when swap_writeout()
returns AOP_WRITEPAGE_ACTIVATE, it uses the mapping and the inode
afterwards:

  reclaim                               last close of an unlinked file
  shmem_writeout()
    shmem_delete_from_page_cache()
    swap_writeout()
      zswap_store() fails               shmem_evict_inode()
      writeback is disabled               shmem_truncate_range()
      return AOP_WRITEPAGE_ACTIVATE         shmem_free_swap()
                                        destroy_inode()
                                        ... RCU grace period, freed
    shmem_add_to_page_cache(mapping)
      xas_lock_irq()            <- use after free

swap_writeout() behaves that way if zswap is enabled, cannot take the
folio and the memcg or one of its ancestors has memory.zswap.writeback
set to 0.

shmem_unuse() already added infrastructure to deal with that.
Reuse info->stop_eviction to protect the inode during writeout. Increment
it before removing the folio from the page cache, and decrement it after
swap_writeout() and any attempted reinsertion and inode accounting.
In shmem_evict_inode(), wait for the counter to reach zero even if
swapoff has already removed the inode from the swap list. Add some helpers
to make the code easier to digest.

Fix generated by an LLM and sent by Josef Bacik in private.

Fixes: 6344a6d9ce13 ("mm/shmem: writeout free swap if swap_writeout() reactivates")
Cc: stable@vger.kernel.org
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Acked-by: Pedro Falcato <pfalcato@suse.de>
Acked-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
Reviewed-by: Nhat Pham <nphamcs@gmail.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Tested-by: Baolin Wang <baolin.wang@linux.alibaba.com>
[ david: code cleanups; improve description ]
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
---
 mm/shmem.c | 36 ++++++++++++++++++++++++++++++------
 1 file changed, 30 insertions(+), 6 deletions(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index ad644a268c0d2..9db820f04d378 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1616,6 +1616,17 @@ static int shmem_setattr(struct mnt_idmap *idmap,
 	return error;
 }
 
+static void shmem_disallow_eviction(struct shmem_inode_info *info)
+{
+	atomic_inc(&info->stop_eviction);
+}
+
+static void shmem_allow_eviction(struct shmem_inode_info *info)
+{
+	if (atomic_dec_and_test(&info->stop_eviction))
+		wake_up_var(&info->stop_eviction);
+}
+
 static void shmem_evict_inode(struct inode *inode)
 {
 	struct shmem_inode_info *info = SHMEM_I(inode);
@@ -1628,16 +1639,21 @@ static void shmem_evict_inode(struct inode *inode)
 		mapping_set_exiting(inode->i_mapping);
 		shmem_truncate_range(inode, 0, (loff_t)-1);
 		shmem_unused_huge_del(inode);
-		while (!list_empty(&info->swaplist)) {
-			/* Wait while shmem_unuse() is scanning this inode... */
+		/*
+		 * Writeout may still use the inode after swapoff has removed it
+		 * from the swaplist, so always wait at least once.
+		 */
+		do {
 			wait_var_event(&info->stop_eviction,
 				       !atomic_read(&info->stop_eviction));
+			if (list_empty(&info->swaplist))
+				break;
 			spin_lock(&shmem_swaplist_lock);
 			/* ...but beware of the race if we peeked too early */
 			if (!atomic_read(&info->stop_eviction))
 				list_del_init(&info->swaplist);
 			spin_unlock(&shmem_swaplist_lock);
-		}
+		} while (!list_empty(&info->swaplist));
 	}
 
 	simple_xattrs_free(&sbinfo->xa_cache, &info->xattrs, sbinfo->max_inodes ? &freed : NULL);
@@ -1776,15 +1792,14 @@ int shmem_unuse(unsigned int type)
 		 * remove placeholder inode from swaplist, nor let it be freed
 		 * (igrab() would protect from unlink, but not from unmount).
 		 */
-		atomic_inc(&info->stop_eviction);
+		shmem_disallow_eviction(info);
 		spin_unlock(&shmem_swaplist_lock);
 
 		error = shmem_unuse_inode(&info->vfs_inode, type);
 		cond_resched();
 
 		spin_lock(&shmem_swaplist_lock);
-		if (atomic_dec_and_test(&info->stop_eviction))
-			wake_up_var(&info->stop_eviction);
+		shmem_allow_eviction(info);
 		if (error)
 			break;
 		if (list_empty(&info->swaplist))
@@ -1915,6 +1930,13 @@ int shmem_writeout(struct swap_io_ctx *ctx, struct folio *folio,
 			spin_unlock(&shmem_swaplist_lock);
 		}
 
+		/*
+		 * Once the folio has left the page cache, its lock no longer
+		 * keeps the inode from being evicted and freed: hold eviction
+		 * off, in case the folio has to be put back below.
+		 */
+		shmem_disallow_eviction(info);
+
 		folio_dup_swap(folio, NULL);
 		shmem_delete_from_page_cache(folio, swp_to_radix_entry(folio->swap));
 
@@ -1922,6 +1944,7 @@ int shmem_writeout(struct swap_io_ctx *ctx, struct folio *folio,
 		error = swap_writeout(ctx, folio);
 		if (error != AOP_WRITEPAGE_ACTIVATE) {
 			/* folio has been unlocked */
+			shmem_allow_eviction(info);
 			return error;
 		}
 
@@ -1938,6 +1961,7 @@ int shmem_writeout(struct swap_io_ctx *ctx, struct folio *folio,
 			shmem_recalc_inode(inode, 0, -nr_pages);
 			folio_put_swap(folio, NULL);
 		}
+		shmem_allow_eviction(info);
 
 		/*
 		 * The swap_cache_del_folio() below could be left for
-- 
2.43.0


-- 
Cheers,

David