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

[Peter Xu <peterx@redhat.com>]

On Thu, Feb 20, 2025 at 09:37:50AM +1300, 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()?

Maybe no need in the first version to fix the immediate bug?

It's also not always the case to hit writeback here. IIUC, writeback only
happens for a short window when the folio was just added into swapcache.
MOVE can happen much later after that anytime before a swapin.  My
understanding is that's also what Matthew wanted to point out.  It may be
better justified of that in a separate change with some performance
measurements.

Thanks,

-- 
Peter Xu