[PATCH] fs: rotate inodes whose page cache could not be dropped

[Nikolaos Karaolidis <nick@karaolidis.com>]

When inode_lru_isolate() drops the page cache of an unused inode, it
has to unlock the list, so it returns LRU_RETRY and the walk restarts
at the head of the list, where the inode still is. If
invalidate_mapping_pages() could not empty the mapping, the restarted
walk strips the same inode again, and so on until nr_to_walk runs out.
Every walk of that list then ends on this inode, and nothing behind it
is reclaimed for as long as its cache stays.

Before commit 2a0629834cd8 ("vfs: Don't evict inode under the inode lru
traversing context") the walker pinned the inode with a reference, and
dropping it in iput_final() set I_REFERENCED, so the restarted walk
rotated the inode to the tail. Set I_REFERENCED again when the mapping
is still populated after the invalidation, in the same i_lock section
that clears I_LRU_ISOLATING. inode_unpin_lru_isolating() now expects
i_lock to be held, like inode_pin_lru_isolating().

On CONFIG_HIGHMEM kernels this can happen to any unused inode: a folio
is left behind if it is locked, under writeback, spliced into a pipe,
or still in another CPU's LRU-add batch after being read. Without
HIGHMEM it happens to DAX inodes whose only entry is at index 0, which
mapping_shrinkable() lets onto the LRU and invalidate_mapping_pages()
never removes.

Fixes: 2a0629834cd8 ("vfs: Don't evict inode under the inode lru traversing context")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Nikolaos Karaolidis <nick@karaolidis.com>
---

Found while working on reclaim of inodes pinned by page cache in
ZONE_MOVABLE.

Tested in QEMU/KVM on master af32da41b032 (v7.3-rc6-232), with lockdep
and KASAN enabled. A small init program makes one unused inode whose
page can't be invalidated, lets it reach the head of the inode LRU,
opens 2000 other files, does one "echo 2 > /proc/sys/vm/drop_caches"
and counts the inodes left via umount; kprobes count the
invalidate_mapping_pages() calls:

                              without patch           with patch
  i386 HIGHMEM, file read     2000 of 2000 left,      none left,
  on another CPU              10300 strips, 1 inode   4 strips
  i386 HIGHMEM, page          same                    same
  spliced into a pipe
  x86_64, ext4 -o dax,        same                    same
  file mmap()ed
  controls (no leftover,      none left               none left
  shadow entry at index 0)

2a0629834cd8^ behaves like the patched kernel, 2a0629834cd8 like
master. On 5.15.y x86_64, where every unused inode with page cache is
on the LRU, the read-on-another-CPU case blocks the walk as well.

With "echo 64M > /sys/fs/cgroup/memory.reclaim" instead of
drop_caches, the i386 pipe case does 262450 strips and frees nothing
without the patch, 40 strips and frees everything with it. In the DAX
case that write takes ~400ms with the patch instead of ~4ms:
invalidate_mapping_pages() counts the DAX entry it leaves behind as
reclaimed, so unpatched, the 64M are "reclaimed" by re-stripping one
inode in a tight loop while nothing is freed.

The reproducer is a single C file; I can post it.

An LLM was used to check the analysis against the code, write the
reproducer and draft the changelog; I have reviewed all of it.

 fs/inode.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/fs/inode.c b/fs/inode.c
index a9d37be390a1..1614a97ce953 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -593,12 +593,11 @@ static void inode_pin_lru_isolating(struct inode *inode)
 
 static void inode_unpin_lru_isolating(struct inode *inode)
 {
-	spin_lock(&inode->i_lock);
+	lockdep_assert_held(&inode->i_lock);
 	WARN_ON(!(inode_state_read(inode) & I_LRU_ISOLATING));
 	inode_state_clear(inode, I_LRU_ISOLATING);
 	/* Called with inode->i_lock which ensures memory ordering. */
 	inode_wake_up_bit(inode, __I_LRU_ISOLATING);
-	spin_unlock(&inode->i_lock);
 }
 
 static void inode_wait_for_lru_isolating(struct inode *inode)
@@ -981,7 +980,13 @@ static enum lru_status inode_lru_isolate(struct list_head *item,
 		else
 			__count_vm_events(PGINODESTEAL, reap);
 		mm_account_reclaimed_pages(reap);
+
+		spin_lock(&inode->i_lock);
+		/* The walk restarts here: don't retry leftovers right away */
+		if (!mapping_empty(&inode->i_data))
+			inode_state_set(inode, I_REFERENCED);
 		inode_unpin_lru_isolating(inode);
+		spin_unlock(&inode->i_lock);
 		return LRU_RETRY;
 	}
 

base-commit: af32da41b0327b9c6a37856ba82b6760d6c8d10e
-- 
2.55.0