Message-ID: <d198e62d-4c3a-44ae-a93f-7964e4f05fc8@redhat.com>
Date: Thu, 20 Feb 2025 10:36:05 +0100
From: David Hildenbrand <david@...hat.com>
To: Barry Song <21cnbao@...il.com>
Cc: Suren Baghdasaryan <surenb@...gle.com>,
 Lokesh Gidra <lokeshgidra@...gle.com>, linux-mm@...ck.org,
 akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
 zhengtangquan@...o.com, Barry Song <v-songbaohua@...o.com>,
 Andrea Arcangeli <aarcange@...hat.com>, Al Viro <viro@...iv.linux.org.uk>,
 Axel Rasmussen <axelrasmussen@...gle.com>, Brian Geffon
 <bgeffon@...gle.com>, Christian Brauner <brauner@...nel.org>,
 Hugh Dickins <hughd@...gle.com>, Jann Horn <jannh@...gle.com>,
 Kalesh Singh <kaleshsingh@...gle.com>,
 "Liam R . Howlett" <Liam.Howlett@...cle.com>,
 Matthew Wilcox <willy@...radead.org>, Michal Hocko <mhocko@...e.com>,
 Mike Rapoport <rppt@...nel.org>, Nicolas Geoffray <ngeoffray@...gle.com>,
 Peter Xu <peterx@...hat.com>, Ryan Roberts <ryan.roberts@....com>,
 Shuah Khan <shuah@...nel.org>, ZhangPeng <zhangpeng362@...wei.com>,
 Yu Zhao <yuzhao@...gle.com>
Subject: Re: [PATCH RFC] mm: Fix kernel BUG when userfaultfd_move encounters
 swapcache

On 20.02.25 10:31, Barry Song wrote:
> On Thu, Feb 20, 2025 at 9:51 PM David Hildenbrand <david@...hat.com> wrote:
>>
>> On 19.02.25 21:37, Barry Song wrote:
>>> On Thu, Feb 20, 2025 at 7:27 AM Suren Baghdasaryan <surenb@...gle.com> wrote:
>>>>
>>>> On Wed, Feb 19, 2025 at 3:25 AM Barry Song <21cnbao@...il.com> wrote:
>>>>>
>>>>> From: Barry Song <v-songbaohua@...o.com>
>>>>>
>>>>> userfaultfd_move() checks whether the PTE entry is present or a
>>>>> swap entry.
>>>>>
>>>>> - If the PTE entry is present, move_present_pte() handles folio
>>>>>     migration by setting:
>>>>>
>>>>>     src_folio->index = linear_page_index(dst_vma, dst_addr);
>>>>>
>>>>> - If the PTE entry is a swap entry, move_swap_pte() simply copies
>>>>>     the PTE to the new dst_addr.
>>>>>
>>>>> This approach is incorrect because even if the PTE is a swap
>>>>> entry, it can still reference a folio that remains in the swap
>>>>> cache.
>>>>>
>>>>> If do_swap_page() is triggered, it may locate the folio in the
>>>>> swap cache. However, during add_rmap operations, a kernel panic
>>>>> can occur due to:
>>>>>    page_pgoff(folio, page) != linear_page_index(vma, address)
>>>>
>>>> Thanks for the report and reproducer!
>>>>
>>>>>
>>>>> $./a.out > /dev/null
>>>>> [   13.336953] page: refcount:6 mapcount:1 mapping:00000000f43db19c index:0xffffaf150 pfn:0x4667c
>>>>> [   13.337520] head: order:2 mapcount:1 entire_mapcount:0 nr_pages_mapped:1 pincount:0
>>>>> [   13.337716] memcg:ffff00000405f000
>>>>> [   13.337849] anon flags: 0x3fffc0000020459(locked|uptodate|dirty|owner_priv_1|head|swapbacked|node=0|zone=0|lastcpupid=0xffff)
>>>>> [   13.338630] raw: 03fffc0000020459 ffff80008507b538 ffff80008507b538 ffff000006260361
>>>>> [   13.338831] raw: 0000000ffffaf150 0000000000004000 0000000600000000 ffff00000405f000
>>>>> [   13.339031] head: 03fffc0000020459 ffff80008507b538 ffff80008507b538 ffff000006260361
>>>>> [   13.339204] head: 0000000ffffaf150 0000000000004000 0000000600000000 ffff00000405f000
>>>>> [   13.339375] head: 03fffc0000000202 fffffdffc0199f01 ffffffff00000000 0000000000000001
>>>>> [   13.339546] head: 0000000000000004 0000000000000000 00000000ffffffff 0000000000000000
>>>>> [   13.339736] page dumped because: VM_BUG_ON_PAGE(page_pgoff(folio, page) != linear_page_index(vma, address))
>>>>> [   13.340190] ------------[ cut here ]------------
>>>>> [   13.340316] kernel BUG at mm/rmap.c:1380!
>>>>> [   13.340683] Internal error: Oops - BUG: 00000000f2000800 [#1] PREEMPT SMP
>>>>> [   13.340969] Modules linked in:
>>>>> [   13.341257] CPU: 1 UID: 0 PID: 107 Comm: a.out Not tainted 6.14.0-rc3-gcf42737e247a-dirty #299
>>>>> [   13.341470] Hardware name: linux,dummy-virt (DT)
>>>>> [   13.341671] pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
>>>>> [   13.341815] pc : __page_check_anon_rmap+0xa0/0xb0
>>>>> [   13.341920] lr : __page_check_anon_rmap+0xa0/0xb0
>>>>> [   13.342018] sp : ffff80008752bb20
>>>>> [   13.342093] x29: ffff80008752bb20 x28: fffffdffc0199f00 x27: 0000000000000001
>>>>> [   13.342404] x26: 0000000000000000 x25: 0000000000000001 x24: 0000000000000001
>>>>> [   13.342575] x23: 0000ffffaf0d0000 x22: 0000ffffaf0d0000 x21: fffffdffc0199f00
>>>>> [   13.342731] x20: fffffdffc0199f00 x19: ffff000006210700 x18: 00000000ffffffff
>>>>> [   13.342881] x17: 6c203d2120296567 x16: 6170202c6f696c6f x15: 662866666f67705f
>>>>> [   13.343033] x14: 6567617028454741 x13: 2929737365726464 x12: ffff800083728ab0
>>>>> [   13.343183] x11: ffff800082996bf8 x10: 0000000000000fd7 x9 : ffff80008011bc40
>>>>> [   13.343351] x8 : 0000000000017fe8 x7 : 00000000fffff000 x6 : ffff8000829eebf8
>>>>> [   13.343498] x5 : c0000000fffff000 x4 : 0000000000000000 x3 : 0000000000000000
>>>>> [   13.343645] x2 : 0000000000000000 x1 : ffff0000062db980 x0 : 000000000000005f
>>>>> [   13.343876] Call trace:
>>>>> [   13.344045]  __page_check_anon_rmap+0xa0/0xb0 (P)
>>>>> [   13.344234]  folio_add_anon_rmap_ptes+0x22c/0x320
>>>>> [   13.344333]  do_swap_page+0x1060/0x1400
>>>>> [   13.344417]  __handle_mm_fault+0x61c/0xbc8
>>>>> [   13.344504]  handle_mm_fault+0xd8/0x2e8
>>>>> [   13.344586]  do_page_fault+0x20c/0x770
>>>>> [   13.344673]  do_translation_fault+0xb4/0xf0
>>>>> [   13.344759]  do_mem_abort+0x48/0xa0
>>>>> [   13.344842]  el0_da+0x58/0x130
>>>>> [   13.344914]  el0t_64_sync_handler+0xc4/0x138
>>>>> [   13.345002]  el0t_64_sync+0x1ac/0x1b0
>>>>> [   13.345208] Code: aa1503e0 f000f801 910f6021 97ff5779 (d4210000)
>>>>> [   13.345504] ---[ end trace 0000000000000000 ]---
>>>>> [   13.345715] note: a.out[107] exited with irqs disabled
>>>>> [   13.345954] note: a.out[107] exited with preempt_count 2
>>>>>
>>>>> Fully fixing it would be quite complex, requiring similar handling
>>>>> of folios as done in move_present_pte.
>>>>
>>>> How complex would that be? Is it a matter of adding
>>>> folio_maybe_dma_pinned() checks, doing folio_move_anon_rmap() and
>>>> folio->index = linear_page_index like in move_present_pte() or
>>>> something more?
>>>
>>> My main concern is still with large folios that require a split_folio()
>>> during move_pages(), as the entire folio shares the same index and
>>> anon_vma. However, userfaultfd_move() moves pages individually,
>>> making a split necessary.
>>>
>>> However, in split_huge_page_to_list_to_order(), there is a:
>>>
>>>           if (folio_test_writeback(folio))
>>>                   return -EBUSY;
>>>
>>> This is likely true for swapcache, right? However, even for move_present_pte(),
>>> it simply returns -EBUSY:
>>>
>>> move_pages_pte()
>>> {
>>>                   /* at this point we have src_folio locked */
>>>                   if (folio_test_large(src_folio)) {
>>>                           /* split_folio() can block */
>>>                           pte_unmap(&orig_src_pte);
>>>                           pte_unmap(&orig_dst_pte);
>>>                           src_pte = dst_pte = NULL;
>>>                           err = split_folio(src_folio);
>>>                           if (err)
>>>                                   goto out;
>>>
>>>                           /* have to reacquire the folio after it got split */
>>>                           folio_unlock(src_folio);
>>>                           folio_put(src_folio);
>>>                           src_folio = NULL;
>>>                           goto retry;
>>>                   }
>>> }
>>>
>>> Do we need a folio_wait_writeback() before calling split_folio()?
>>>
>>> By the way, I have also reported that userfaultfd_move() has a fundamental
>>> conflict with TAO (Cc'ed Yu Zhao), which has been part of the Android common
>>> kernel. In this scenario, folios in the virtual zone won’t be split in
>>> split_folio(). Instead, the large folio migrates into nr_pages small folios.
>>   > > Thus, the best-case scenario would be:
>>>
>>> mTHP -> migrate to small folios in split_folio() -> move small folios to
>>> dst_addr
>>>
>>> While this works, it negates the performance benefits of
>>> userfaultfd_move(), as it introduces two PTE operations (migration in
>>> split_folio() and move in userfaultfd_move() while retry), nr_pages memory
>>> allocations, and still requires one memcpy(). This could end up
>>> performing even worse than userfaultfd_copy(), I guess.
>>   > > The worst-case scenario would be failing to allocate small folios in
>>> split_folio(), then userfaultfd_move() might return -ENOMEM?
>>
>> Although that's an Android problem and not an upstream problem, I'll
>> note that there are other reasons why the split / move might fail, and
>> user space either must retry or fallback to a COPY.
>>
>> Regarding mTHP, we could move the whole folio if the user space-provided
>> range allows for batching over multiple PTEs (nr_ptes), they are in a
>> single VMA, and folio_mapcount() == nr_ptes.
>>
>> There are corner cases to handle, such as moving mTHPs such that they
>> suddenly cross two page tables I assume, that are harder to handle when
>> not moving individual PTEs where that cannot happen.
> 
> This is a useful suggestion. I’ve heard that Lokesh is also interested in
> modifying ART to perform moves at the mTHP granularity, which would require
> kernel modifications as well. It’s likely the direction we’ll take after
> fixing the current urgent bugs. The current split_folio() really isn’t ideal.
> 
> The corner cases you mentioned are definitely worth considering. However,
> once we can perform batch UFFDIO_MOVE, I believe that in most cases,
> the conflict between userfaultfd_move() and TAO will be resolved ?

Well, as soon as you would have varying mTHP sizes, you'd still run into 
the split with TAO. Maybe that doesn't apply with Android today, but I 
can just guess that performing sub-mTHP moving would still be required 
for GC at some point.

-- 
Cheers,

David / dhildenb

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