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Message-ID: <c364015e-ab37-411d-b2e9-4e7b10effdf5@bytedance.com>
Date: Wed, 16 Oct 2024 18:58:33 +0800
From: Qi Zheng <zhengqi.arch@...edance.com>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
syzbot+7d917f67c05066cec295@...kaller.appspotmail.com,
Andrew Morton <akpm@...ux-foundation.org>, Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH v1] mm/pagewalk: fix usage of pmd_leaf()/pud_leaf()
without present check
On 2024/10/15 21:13, David Hildenbrand wrote:
> On 15.10.24 13:12, David Hildenbrand wrote:
>> pmd_leaf()/pud_leaf() only implies a pmd_present()/pud_present() check on
>> some architectures. We really should check for
>> pmd_present()/pud_present() first.
>>
>> This should explain the report we got on ppc64 (which has
>> CONFIG_PGTABLE_HAS_HUGE_LEAVES set in the config) that triggered:
>> VM_WARN_ON_ONCE(pmd_leaf(pmdp_get_lockless(pmdp)));
>>
>> Likely we had a PMD migration entry for which pmd_leaf() did not
>> trigger. We raced with restoring the PMD migration entry, and suddenly
>> saw a pmd_leaf(). In this case, pte_offset_map_lock() saved us from more
>> trouble, because it rechecks the PMD value, but we would not have
>> processed
>> the migration entry -- which is not too bad because the only user of
>> FW_MIGRATION is KSM for unsharing, and KSM only applies to small folios.
>>
>> Further, we shouldn't re-read the PMD/PUD value for our warning, the
>> primary purpose of the VM_WARN_ON_ONCE() is to find spurious use of
>> pmd_leaf()/pud_leaf() without CONFIG_PGTABLE_HAS_HUGE_LEAVES.
>>
>> As a side note, we are currently not implementing FW_MIGRATION support
>> for PUD migration entries, which likely should exist due to hugetlb. Add
>> a TODO so this won't fall through the cracks if more FW_MIGRATION users
>> get added.
>>
>> Fixes: aa39ca6940f1 ("mm/pagewalk: introduce folio_walk_start() +
>> folio_walk_end()")
>> Reported-by: syzbot+7d917f67c05066cec295@...kaller.appspotmail.com
>> Closes:
>> https://lkml.kernel.org/r/670d3248.050a0220.3e960.0064.GAE@google.com
>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>> Cc: Jann Horn <jannh@...gle.com>
>> Signed-off-by: David Hildenbrand <david@...hat.com>
>> ---
>
> Was able to write a quick reproducer and verify that the issue no longer
> triggers with this fix.
>
> https://gitlab.com/davidhildenbrand/scratchspace/-/blob/main/reproducers/move-pages-pmd-leaf.c
>
> Without this fix after a couple of seconds in a VM with 2 NUMA nodes:
>
> [ 54.333753] ------------[ cut here ]------------
> [ 54.334901] WARNING: CPU: 20 PID: 1704 at mm/pagewalk.c:815
> folio_walk_start+0x48f/0x6e0
> [ 54.336455] Modules linked in: ...
> [ 54.345009] CPU: 20 UID: 0 PID: 1704 Comm: move-pages-pmd- Not
> tainted 6.12.0-rc2+ #81
> [ 54.346529] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS
> 1.16.3-2.fc40 04/01/2014
> [ 54.348191] RIP: 0010:folio_walk_start+0x48f/0x6e0
> [ 54.349134] Code: b5 ad 48 8d 35 00 00 00 00 e8 6d 59 d7 ff e8 08 74
> da ff e9 9c fe ff ff 4c 8b 7c 24 08 4c 89 ff e8 26 2b be 00 e9 8a fe ff
> ff <0f> 0b e9 ec fe ff ff f7 c2 ff 0f 00 00 0f 85 81 fe ff ff 48 8b 02
> [ 54.352660] RSP: 0018:ffffb7e4c430bc78 EFLAGS: 00010282
> [ 54.353679] RAX: 80000002a3e008e7 RBX: ffff9946039aa580 RCX:
> ffff994380000000
> [ 54.355056] RDX: ffff994606aec000 RSI: 00007f004b000000 RDI:
> 0000000000000000
> [ 54.356440] RBP: 00007f004b000000 R08: 0000000000000591 R09:
> 0000000000000001
> [ 54.357820] R10: 0000000000000200 R11: 0000000000000001 R12:
> ffffb7e4c430bd10
> [ 54.359198] R13: ffff994606aec2c0 R14: 0000000000000002 R15:
> ffff994604a89b00
> [ 54.360564] FS: 00007f004ae006c0(0000) GS:ffff9947f7400000(0000)
> knlGS:0000000000000000
> [ 54.362111] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [ 54.363242] CR2: 00007f004adffe58 CR3: 0000000281e12005 CR4:
> 0000000000770ef0
> [ 54.364615] PKRU: 55555554
> [ 54.365153] Call Trace:
> [ 54.365646] <TASK>
> [ 54.366073] ? __warn.cold+0xb7/0x14d
> [ 54.366796] ? folio_walk_start+0x48f/0x6e0
> [ 54.367628] ? report_bug+0xff/0x140
> [ 54.368324] ? handle_bug+0x58/0x90
> [ 54.369019] ? exc_invalid_op+0x17/0x70
> [ 54.369771] ? asm_exc_invalid_op+0x1a/0x20
> [ 54.370606] ? folio_walk_start+0x48f/0x6e0
> [ 54.371415] ? folio_walk_start+0x9e/0x6e0
> [ 54.372227] do_pages_move+0x1c5/0x680
> [ 54.372972] kernel_move_pages+0x1a1/0x2b0
> [ 54.373804] __x64_sys_move_pages+0x25/0x30
It would be better to add this call stack to the commit message, which
can help people find this fix patch when they encounter same problem. ;)
Otherwise, LGTM.
Acked-by: Qi Zheng <zhengqi.arch@...edance.com>
Thanks!
>
>
>
>
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