Message-ID: <af58fae7-5a81-4fa9-acea-a35b2c415d28@redhat.com>
Date: Thu, 20 Feb 2025 23:31:23 +0100
From: David Hildenbrand <david@...hat.com>
To: Barry Song <21cnbao@...il.com>, Lokesh Gidra <lokeshgidra@...gle.com>
Cc: Suren Baghdasaryan <surenb@...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 23:26, Barry Song wrote:
> On Fri, Feb 21, 2025 at 11:20 AM Lokesh Gidra <lokeshgidra@...gle.com> wrote:
>>
>> On Thu, Feb 20, 2025 at 1:45 PM Barry Song <21cnbao@...il.com> wrote:
>>>
>>> On Thu, Feb 20, 2025 at 10:36 PM David Hildenbrand <david@...hat.com> wrote:
>>>>
>>>> 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.
>>>
>>> With patch v2[1], as discussed in my previous email, I have observed that
>>> small folios consistently succeed without crashing. Similarly, mTHP no
>>> longer crashes; however, it still returns -EBUSY during the raced time
>>> window, even after adding folio_wait_writeback. While I previously
>>> mentioned that folio_writeback prevents mTHP from splitting, this is not
>>> the only factor. The split_folio() function still returns -EBUSY because
>>> folio_get_anon_vma(folio) returns NULL when the folio is not mapped.
>>>
>>> int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
>>>                                       unsigned int new_order)
>>> {
>>>                  anon_vma = folio_get_anon_vma(folio);
>>>                  if (!anon_vma) {
>>>                          ret = -EBUSY;
>>>                          goto out;
>>>                  }
>>>
>>>                  end = -1;
>>>                  mapping = NULL;
>>>                  anon_vma_lock_write(anon_vma);
>>> }
>>>
>>> Even if mTHP is not from TAO's virtual zone, userfaultfd_move() will still
>>> fail when performing sub-mTHP moving in the swap cache case due to:
>>
>> Just to clarify my doubt. What do you mean by sub-mTHP? Also when you
>> say 'small folio' above,  do you mean single-page folios?
> 
> This means any moving size smaller than the size of mTHP, or moving
> a partial mTHP.
> 
>>
>> Am I understanding correctly that your patch correctly handles moving
>> single swap-cache page case?
> 
> Yes, the crash is fixed for both small and large folios, and for small
> folios, moving is consistently successful(even for the swapcache case).
> The only issue is that sub-mTHP moving constantly fails for the swapcache
> case because split_folio() fails, even after waiting for writeback as
> split_folio()
> can only split mapped folios - which is false for swapcache since
> try_to_unmap_one() has been done.

I mean, we (as the caller of split_folio()) have the VMA + anon_vma in 
our hands. Do we only have to bypass that mapping check, or is there 
something else that would block us?


-- 
Cheers,

David / dhildenb

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