Message-ID: <CAGsJ_4zPTvnD3oai3HpurURvUeBRX+BKBr0cy0=mCyCOFyNBZA@mail.gmail.com>
Date: Fri, 21 Feb 2025 10:45:23 +1300
From: Barry Song <21cnbao@...il.com>
To: David Hildenbrand <david@...hat.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 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:

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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