Re: [syzbot] [mm?] WARNING in deferred_split_folio

["David Hildenbrand (Arm)" <david@kernel.org>]

On 4/1/26 10:59, Lance Yang wrote:
> 
> On Wed, Apr 01, 2026 at 04:10:25PM +0800, Lance Yang wrote:
>>
>> +Cc Usama
>>
>> On Tue, Mar 31, 2026 at 11:08:27PM -0700, syzbot wrote:
>>> Hello,
>>>
>>> syzbot found the following issue on:
>>>
>>> HEAD commit:    cf7c3c02fdd0 Add linux-next specific files for 20260330
>>> git tree:       linux-next
>>> console output: https://syzkaller.appspot.com/x/log.txt?x=154ee46a580000
>>> kernel config:  https://syzkaller.appspot.com/x/.config?x=3944d875fa9bfb67
>>> dashboard link: https://syzkaller.appspot.com/bug?extid=a7067a757858ac8eb085
>>> compiler:       Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
>>> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=12c846ba580000
>>>
>>> Downloadable assets:
>>> disk image: https://storage.googleapis.com/syzbot-assets/053d3b49a360/disk-cf7c3c02.raw.xz
>>> vmlinux: https://storage.googleapis.com/syzbot-assets/faabb37d41d0/vmlinux-cf7c3c02.xz
>>> kernel image: https://storage.googleapis.com/syzbot-assets/8d47fe92aaa8/bzImage-cf7c3c02.xz
>>>
>>> IMPORTANT: if you fix the issue, please add the following tag to the commit:
>>> Reported-by: syzbot+a7067a757858ac8eb085@syzkaller.appspotmail.com
>>>
>>> free_pages_and_swap_cache+0x2b9/0x490 mm/swap_state.c:401
>>> __tlb_batch_free_encoded_pages mm/mmu_gather.c:138 [inline]
>>> tlb_batch_pages_flush mm/mmu_gather.c:151 [inline]
>>> tlb_flush_mmu_free mm/mmu_gather.c:417 [inline]
>>> tlb_flush_mmu+0x6d3/0xa30 mm/mmu_gather.c:424
>>> tlb_finish_mmu+0xf9/0x230 mm/mmu_gather.c:549
>>> exit_mmap+0x498/0x9e0 mm/mmap.c:1313
>>> __mmput+0x118/0x430 kernel/fork.c:1177
>>> exit_mm+0x18e/0x250 kernel/exit.c:581
>>> do_exit+0x6a2/0x22c0 kernel/exit.c:962
>>> do_group_exit+0x21b/0x2d0 kernel/exit.c:1116
>>> __do_sys_exit_group kernel/exit.c:1127 [inline]
>>> __se_sys_exit_group kernel/exit.c:1125 [inline]
>>> __x64_sys_exit_group+0x3f/0x40 kernel/exit.c:1125
>>> x64_sys_call+0x221a/0x2240 arch/x86/include/generated/asm/syscalls_64.h:232
>>> do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>>> do_syscall_64+0x15f/0xf80 arch/x86/entry/syscall_64.c:94
>>> entry_SYSCALL_64_after_hwframe+0x77/0x7f
>>> ------------[ cut here ]------------
>>> 1
>>> WARNING: mm/huge_memory.c:4371 at deferred_split_folio+0x974/0xaa0 mm/huge_memory.c:4371, CPU#1: syz.3.1110/10500
>>> Modules linked in:
>>> CPU: 1 UID: 0 PID: 10500 Comm: syz.3.1110 Not tainted syzkaller #0 PREEMPT(full) 
>>> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2026
>>> RIP: 0010:deferred_split_folio+0x974/0xaa0 mm/huge_memory.c:4371
>>> Code: 31 48 8d 65 d8 5b 41 5c 41 5d 41 5e 41 5f 5d e9 c2 67 8d 09 cc e8 8c 73 93 ff 48 89 df 48 c7 c6 20 5b fc 8b e8 dd 2b f5 fe 90 <0f> 0b 90 e9 d4 fe ff ff e8 9f 7a 8a 09 e8 6a 73 93 ff 48 89 df 48
>>> RSP: 0018:ffffc900047ef540 EFLAGS: 00010046
>>> RAX: 1c05fb65cfaab100 RBX: ffffea0001840000 RCX: 0000000080000001
>>> RDX: 0000000000000002 RSI: ffffffff8e4da1c7 RDI: ffff88807d6f9e80
>>> RBP: ffffc900047ef610 R08: ffff8880b87247d3 R09: 1ffff110170e48fa
>>> R10: dffffc0000000000 R11: ffffed10170e48fb R12: ffffea0001840040
>>> R13: 0000000000000000 R14: 0000000000010000 R15: 1ffff920008fdeb0
>>> FS:  00007f32e32a76c0(0000) GS:ffff8881250e8000(0000) knlGS:0000000000000000
>>> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>>> CR2: 00007f5825757930 CR3: 0000000034ad8000 CR4: 00000000003526f0
>>> Call Trace:
>>> <TASK>
>>> migrate_folio_move mm/migrate.c:1411 [inline]
>>
>> Looks like a race introduced by commit[1] ("mm: migrate: requeue
>> destination folio on deferred split queue").
>>
>> Between folio migration (mbind) and rmap removal (exit_mmap), I guess :)
>>
>> migrate_folio_move() snapshots src_partially_mapped from src before
>> migration:
>>
>> 	if (folio_order(src) > 1 &&
>> 	    !data_race(list_empty(&src->_deferred_list))) {
>> 		src_deferred_split = true;
>> 		src_partially_mapped = folio_test_partially_mapped(src);
>> 	}
>>
>> Then move_to_new_folio() eventually unqueues src in
>> __folio_migrate_mapping():
>>
>> 	folio_unqueue_deferred_split(src);
>>
>> After that, migration restores mappings to dst:
>>
>> 	if (old_page_state & PAGE_WAS_MAPPED)
>> 		remove_migration_ptes(src, dst, 0);
>>
>> At that point, dst is already visible again. A concurrent unmap path
>>from another sharer can then remove some of those mappings and reach
>> deferred_split_folio(dst, true), which sets PG_partially_mapped on
>> dst.
>>
>> Migration then resumes and does:
>>
>> 	if (src_deferred_split)
>> 		deferred_split_folio(dst, src_partially_mapped);
>>
>> If the earlier snapshot from src was false, this becomes
>> deferred_split_folio(dst, false), but dst may already have been marked
>> partially mapped by the concurrent rmap-removal path, so the WARN in
>> deferred_split_folio() fires:
>>
>> 	if (partially_mapped) {
>> 		...
>> 	} else {
>> 		/* partially mapped folios cannot become non-partially mapped */
>> 		VM_WARN_ON_FOLIO(folio_test_partially_mapped(folio), folio);
>> 	}
>>
>> [1] https://lore.kernel.org/all/20260312104723.1351321-1-usama.arif@linux.dev/
>>
> 
> Perhaps the WARN is simply too strict there :)
> 
> Migration already holds the folio lock on dst, while the competing
> rmap-removal path runs under the page-table lock. So once
> remove_migration_ptes(src, dst, 0) makes dst visible again, this race
> looks hard to avoid.
> 
> So maybe the simplest fix is just to drop the WARN in the
> !partially_mapped path:
> 
> ---8<---
> Subject: [PATCH 1/1] mm/thp: avoid false warning in deferred_split_folio()
> 
> From: Lance Yang <lance.yang@linux.dev>
> 
> migrate_folio_move() snapshots src_partially_mapped from src before
> migration and later requeues dst after remove_migration_ptes(src, dst, 0).
> 
> Once dst is visible again, a competing rmap-removal path can legally set
> PG_partially_mapped before the migration path reaches
> deferred_split_folio(dst, src_partially_mapped).
> 
> Migration already holds the folio lock on dst, while the competing
> rmap-removal path runs under the page-table lock. So once
> remove_migration_ptes(src, dst, 0) makes dst visible again, this race
> looks hard to avoid.
> 
> So just drop the WARN in the !partially_mapped path and preserve an
> already-set PG_partially_mapped bit.
> 
> Link: https://lore.kernel.org/linux-mm/69ccb65b.050a0220.183828.003a.GAE@google.com/
> Fixes: 8a8ca142a488 ("mm: migrate: requeue destination folio on deferred split queue")
> Reported-by: syzbot+a7067a757858ac8eb085@syzkaller.appspotmail.com
> Signed-off-by: Lance Yang <lance.yang@linux.dev>
> ---
>  mm/huge_memory.c | 3 ---
>  1 file changed, 3 deletions(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 745eb3d0d4a7..8ea8e293dc7c 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -4433,9 +4433,6 @@ void deferred_split_folio(struct folio *folio, bool partially_mapped)
>  			mod_mthp_stat(folio_order(folio), MTHP_STAT_NR_ANON_PARTIALLY_MAPPED, 1);
> 
>  		}
> -	} else {
> -		/* partially mapped folios cannot become non-partially mapped */
> -		VM_WARN_ON_FOLIO(folio_test_partially_mapped(folio), folio);
>  	}

Can't we simply move the setting before restoring migration ptes?

diff --git a/mm/migrate.c b/mm/migrate.c
index 05cb408846f2..5f222cb0ca90 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1385,6 +1385,15 @@ static int migrate_folio_move(free_folio_t put_new_folio, unsigned long private,
        if (rc)
                goto out;
 
+       /*
+        * Requeue the destination folio on the deferred split queue if
+        * the source was on the queue.  The source is unqueued in
+        * __folio_migrate_mapping(), so we recorded the state from
+        * before move_to_new_folio().
+        */
+       if (src_deferred_split)
+               deferred_split_folio(dst, src_partially_mapped);
+
        /*
         * When successful, push dst to LRU immediately: so that if it
         * turns out to be an mlocked page, remove_migration_ptes() will
@@ -1400,16 +1409,6 @@ static int migrate_folio_move(free_folio_t put_new_folio, unsigned long private,
 
        if (old_page_state & PAGE_WAS_MAPPED)
                remove_migration_ptes(src, dst, 0);
-
-       /*
-        * Requeue the destination folio on the deferred split queue if
-        * the source was on the queue.  The source is unqueued in
-        * __folio_migrate_mapping(), so we recorded the state from
-        * before move_to_new_folio().
-        */
-       if (src_deferred_split)
-               deferred_split_folio(dst, src_partially_mapped);
-
 out_unlock_both:
        folio_unlock(dst);
        folio_set_owner_migrate_reason(dst, reason);


-- 
Cheers,

David