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Tue, 03 Mar 2026 04:17:28 -0800 (PST) Date: Tue, 3 Mar 2026 12:17:27 +0000 From: Wei Yang To: Dev Jain Cc: akpm@linux-foundation.org, david@kernel.org, lorenzo.stoakes@oracle.com, riel@surriel.com, Liam.Howlett@oracle.com, vbabka@kernel.org, harry.yoo@oracle.com, jannh@google.com, baohua@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, ryan.roberts@arm.com, anshuman.khandual@arm.com, stable Subject: Re: [PATCH v3] mm/rmap: fix incorrect pte restoration for lazyfree folios Message-ID: <20260303121727.ss3d3gbzituygb6p@master> Reply-To: Wei Yang References: <20260303061528.2429162-1-dev.jain@arm.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260303061528.2429162-1-dev.jain@arm.com> User-Agent: NeoMutt/20170113 (1.7.2) On Tue, Mar 03, 2026 at 11:45:28AM +0530, Dev Jain wrote: >We batch unmap anonymous lazyfree folios by folio_unmap_pte_batch. >If the batch has a mix of writable and non-writable bits, we may end up >setting the entire batch writable. Fix this by respecting writable bit >during batching. >Although on a successful unmap of a lazyfree folio, the soft-dirty bit is >lost, preserve it on pte restoration by respecting the bit during batching, >to make the fix consistent w.r.t both writable bit and soft-dirty bit. > >I was able to write the below reproducer and crash the kernel. >Explanation of reproducer (set 64K mTHP to always): > >Fault in a 64K large folio. Split the VMA at mid-point with MADV_DONTFORK. >fork() - parent points to the folio with 8 writable ptes and 8 non-writable >ptes. Merge the VMAs with MADV_DOFORK so that folio_unmap_pte_batch() can >determine all the 16 ptes as a batch. Do MADV_FREE on the range to mark >the folio as lazyfree. Write to the memory to dirty the pte, eventually >rmap will dirty the folio. Then trigger reclaim, we will hit the pte >restoration path, and the kernel will crash with the following trace: > >[ 21.134473] kernel BUG at mm/page_table_check.c:118! >[ 21.134497] Internal error: Oops - BUG: 00000000f2000800 [#1] SMP >[ 21.135917] Modules linked in: >[ 21.136085] CPU: 1 UID: 0 PID: 1735 Comm: dup-lazyfree Not tainted 7.0.0-rc1-00116-g018018a17770 #1028 PREEMPT >[ 21.136858] Hardware name: linux,dummy-virt (DT) >[ 21.137019] pstate: 21400005 (nzCv daif +PAN -UAO -TCO +DIT -SSBS BTYPE=--) >[ 21.137308] pc : page_table_check_set+0x28c/0x2a8 >[ 21.137607] lr : page_table_check_set+0x134/0x2a8 >[ 21.137885] sp : ffff80008a3b3340 >[ 21.138124] x29: ffff80008a3b3340 x28: fffffdffc3d14400 x27: ffffd1a55e03d000 >[ 21.138623] x26: 0040000000000040 x25: ffffd1a55f7dd000 x24: 0000000000000001 >[ 21.139045] x23: 0000000000000001 x22: 0000000000000001 x21: ffffd1a55f217f30 >[ 21.139629] x20: 0000000000134521 x19: 0000000000134519 x18: 005c43e000040000 >[ 21.140027] x17: 0001400000000000 x16: 0001700000000000 x15: 000000000000ffff >[ 21.140578] x14: 000000000000000c x13: 005c006000000000 x12: 0000000000000020 >[ 21.140828] x11: 0000000000000000 x10: 005c000000000000 x9 : ffffd1a55c079ee0 >[ 21.141077] x8 : 0000000000000001 x7 : 005c03e000040000 x6 : 000000004000ffff >[ 21.141490] x5 : ffff00017fffce00 x4 : 0000000000000001 x3 : 0000000000000002 >[ 21.141741] x2 : 0000000000134510 x1 : 0000000000000000 x0 : ffff0000c08228c0 >[ 21.141991] Call trace: >[ 21.142093] page_table_check_set+0x28c/0x2a8 (P) >[ 21.142265] __page_table_check_ptes_set+0x144/0x1e8 >[ 21.142441] __set_ptes_anysz.constprop.0+0x160/0x1a8 >[ 21.142766] contpte_set_ptes+0xe8/0x140 >[ 21.142907] try_to_unmap_one+0x10c4/0x10d0 >[ 21.143177] rmap_walk_anon+0x100/0x250 >[ 21.143315] try_to_unmap+0xa0/0xc8 >[ 21.143441] shrink_folio_list+0x59c/0x18a8 >[ 21.143759] shrink_lruvec+0x664/0xbf0 >[ 21.144043] shrink_node+0x218/0x878 >[ 21.144285] __node_reclaim.constprop.0+0x98/0x338 >[ 21.144763] user_proactive_reclaim+0x2a4/0x340 >[ 21.145056] reclaim_store+0x3c/0x60 >[ 21.145216] dev_attr_store+0x20/0x40 >[ 21.145585] sysfs_kf_write+0x84/0xa8 >[ 21.145835] kernfs_fop_write_iter+0x130/0x1c8 >[ 21.145994] vfs_write+0x2b8/0x368 >[ 21.146119] ksys_write+0x70/0x110 >[ 21.146240] __arm64_sys_write+0x24/0x38 >[ 21.146380] invoke_syscall+0x50/0x120 >[ 21.146513] el0_svc_common.constprop.0+0x48/0xf8 >[ 21.146679] do_el0_svc+0x28/0x40 >[ 21.146798] el0_svc+0x34/0x110 >[ 21.146926] el0t_64_sync_handler+0xa0/0xe8 >[ 21.147074] el0t_64_sync+0x198/0x1a0 >[ 21.147225] Code: f9400441 b4fff241 17ffff94 d4210000 (d4210000) >[ 21.147440] ---[ end trace 0000000000000000 ]--- > > >#define _GNU_SOURCE >#include >#include >#include >#include >#include >#include >#include >#include > >void write_to_reclaim() { > const char *path = "/sys/devices/system/node/node0/reclaim"; > const char *value = "409600000000"; > int fd = open(path, O_WRONLY); > if (fd == -1) { > perror("open"); > exit(EXIT_FAILURE); > } > > if (write(fd, value, sizeof("409600000000") - 1) == -1) { > perror("write"); > close(fd); > exit(EXIT_FAILURE); > } > > printf("Successfully wrote %s to %s\n", value, path); > close(fd); >} > >int main() >{ > char *ptr = mmap((void *)(1UL << 30), 1UL << 16, PROT_READ | PROT_WRITE, > MAP_PRIVATE | MAP_ANONYMOUS, -1, 0); > if ((unsigned long)ptr != (1UL << 30)) { > perror("mmap"); > return 1; > } > > /* a 64K folio gets faulted in */ > memset(ptr, 0, 1UL << 16); > > /* 32K half will not be shared into child */ > if (madvise(ptr, 1UL << 15, MADV_DONTFORK)) { > perror("madvise madv dontfork"); > return 1; > } > > pid_t pid = fork(); > > if (pid < 0) { > perror("fork"); > return 1; > } else if (pid == 0) { > sleep(15); > } else { > /* merge VMAs. now first half of the 16 ptes are writable, the other half not. */ > if (madvise(ptr, 1UL << 15, MADV_DOFORK)) { > perror("madvise madv fork"); > return 1; > } > if (madvise(ptr, (1UL << 16), MADV_FREE)) { > perror("madvise madv free"); > return 1; > } > > /* dirty the large folio */ > (*ptr) += 10; > > write_to_reclaim(); > // sleep(10); > waitpid(pid, NULL, 0); > > } >} > >Fixes: 354dffd29575 ("mm: support batched unmap for lazyfree large folios during reclamation") >Cc: stable >Signed-off-by: Dev Jain >--- >Patch applies on mm-unstable (9af4957ef127). > >v2->v3: > - Don't special case for anon folios > >v1->v2: > - Just respect the writable bit instead of hacking in a pte_wrprotect() in > failure path > - Also handle soft-dirty bit > > mm/rmap.c | 9 ++++++++- > 1 file changed, 8 insertions(+), 1 deletion(-) > >diff --git a/mm/rmap.c b/mm/rmap.c >index bff8f222004e4..5a3e408e3f179 100644 >--- a/mm/rmap.c >+++ b/mm/rmap.c >@@ -1955,7 +1955,14 @@ static inline unsigned int folio_unmap_pte_batch(struct folio *folio, > if (userfaultfd_wp(vma)) > return 1; > >- return folio_pte_batch(folio, pvmw->pte, pte, max_nr); >+ /* >+ * If unmap fails, we need to restore the ptes. To avoid accidentally >+ * upgrading write permissions for ptes that were not originally >+ * writable, and to avoid losing the soft-dirty bit, use the >+ * appropriate FPB flags. >+ */ >+ return folio_pte_batch_flags(folio, vma, pvmw->pte, &pte, max_nr, >+ FPB_RESPECT_WRITE | FPB_RESPECT_SOFT_DIRTY); > } > Hi, Dev When reading the code, I got one confusion. Current call flow is like below: try_to_unmap_one(); nr_pages = folio_unmap_pte_batch(folio, &pvmw, flags, pteval); .. pteval = get_and_clear_ptes(mm, address, pvmw.pte, nr_pages); .. set_ptes(mm, address, pvmw.pte, pteval, nr_pages); We get pteval by folio_unmap_pte_batch() but it is set again by get_and_clear_ptes(), which maybe a different value. Then we use this pteval to restore ptes. So even we fix folio_unmap_pte_batch(), how this impact on the final restored value? Hope I don't miss something. > /* >-- >2.34.1 > -- Wei Yang Help you, Help me