Re: [PATCH RFC] mm: Fix kernel BUG when userfaultfd_move encounters swapcache

[David Hildenbrand <david@redhat.com>]

On 19.02.25 21:37, Barry Song wrote:
> On Thu, Feb 20, 2025 at 7:27 AM Suren Baghdasaryan <surenb@google.com> wrote:
>>
>> On Wed, Feb 19, 2025 at 3:25 AM Barry Song <21cnbao@gmail.com> wrote:
>>>
>>> From: Barry Song <v-songbaohua@oppo.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.

-- 
Cheers,

David / dhildenb