Message-ID: <CA+EESO7VFAGCgh0xu3W84Te0zGbL4QfWXYDcVA7Toy5qMrmcpA@mail.gmail.com>
Date: Thu, 20 Feb 2025 14:33:55 -0800
From: Lokesh Gidra <lokeshgidra@...gle.com>
To: Barry Song <21cnbao@...il.com>
Cc: David Hildenbrand <david@...hat.com>, 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 Thu, Feb 20, 2025 at 2:27 PM Barry Song <21cnbao@...il.com> 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.
>
> So I'd say for mTHP, returning -EBUSY as early as possible is the
> better choice to avoid wasting much time and eventually returning
> -EBUSY anyway unless we want to modify split_folio() things.
>
Great! In this case, can we please fix the kernel panic bug as soon as
possible. Until that is fixed, the ioctl is practically unusable.
> > >
> > > struct anon_vma *folio_get_anon_vma(const struct folio *folio)
> > > {
> > >         ...
> > >         if (!folio_mapped(folio))
> > >                 goto out;
> > >          ...
> > > }
> > >
> > > We likely need to modify split_folio() to support splitting unmapped anon
> > > folios within the swap cache or introduce a new function like
> > > split_unmapped_anon_folio()? Otherwise, userspace will have to fall back
> > > to UFFDIO_COPY or retry.
> > >
> > > As it stands, I see no way for sub-mTHP to survive moving with the current
> > > code and within the existing raced window. For mTHP, there is essentially
> > > no difference between returning -EBUSY immediately upon detecting that it
> > > is within the swap cache, as proposed in v1.
> > >
> > > [1] https://lore.kernel.org/linux-mm/20250220092101.71966-1-21cnbao@gmail.com/
> > >
> > > >
> > > > --
> > > > Cheers,
> > > >
> > > > David / dhildenb
> > > >
> > >
> Thanks
> Barry

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