Message-ID: <CAGsJ_4yzOXE1KS7J927QSjPRUEyCdgs4VKH7fi_7kQ72a5XtUA@mail.gmail.com>
Date: Wed, 26 Feb 2025 18:37:38 +1300
From: Barry Song <21cnbao@...il.com>
To: David Hildenbrand <david@...hat.com>
Cc: Liam.Howlett@...cle.com, aarcange@...hat.com, akpm@...ux-foundation.org, 
	axelrasmussen@...gle.com, bgeffon@...gle.com, brauner@...nel.org, 
	hughd@...gle.com, jannh@...gle.com, kaleshsingh@...gle.com, 
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, lokeshgidra@...gle.com, 
	mhocko@...e.com, ngeoffray@...gle.com, peterx@...hat.com, rppt@...nel.org, 
	ryan.roberts@....com, shuah@...nel.org, surenb@...gle.com, 
	v-songbaohua@...o.com, viro@...iv.linux.org.uk, willy@...radead.org, 
	zhangpeng362@...wei.com, zhengtangquan@...o.com, yuzhao@...gle.com, 
	stable@...r.kernel.org
Subject: Re: [PATCH RFC] mm: Fix kernel BUG when userfaultfd_move encounters swapcache

On Thu, Feb 20, 2025 at 11:24 PM David Hildenbrand <david@...hat.com> wrote:
>
> On 20.02.25 10:21, Barry Song wrote:
> > On Thu, Feb 20, 2025 at 9:40 PM David Hildenbrand <david@...hat.com> wrote:
> >>
> >> On 19.02.25 19:58, Suren Baghdasaryan wrote:
> >>> On Wed, Feb 19, 2025 at 10:30 AM David Hildenbrand <david@...hat.com> wrote:
> >>>>
> >>>> On 19.02.25 19:26, Suren Baghdasaryan 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?
> >>>>
> >>>> If the entry is pte_swp_exclusive(), and the folio is order-0, it cannot
> >>>> be pinned and we may be able to move it I think.
> >>>>
> >>>> So all that's required is to check pte_swp_exclusive() and the folio size.
> >>>>
> >>>> ... in theory :) Not sure about the swap details.
> >>>
> >>> Looking some more into it, I think we would have to perform all the
> >>> folio and anon_vma locking and pinning that we do for present pages in
> >>> move_pages_pte(). If that's correct then maybe treating swapcache
> >>> pages like a present page inside move_pages_pte() would be simpler?
> >>
> >> I'd be more in favor of not doing that. Maybe there are parts we can
> >> move out into helper functions instead, so we can reuse them?
> >
> > I actually have a v2 ready. Maybe we can discuss if some of the code can be
> > extracted as a helper based on the below before I send it formally?
> >
> > I’d say there are many parts that can be shared with present PTE, but there
> > are two major differences:
> >
> > 1. Page exclusivity – swapcache doesn’t require it (try_to_unmap_one has remove
> > Exclusive flag;)
> > 2. src_anon_vma and its lock – swapcache doesn’t require it（folio is not mapped）
> >
>
> That's a lot of complicated code you have there (not your fault, it's
> complicated stuff ... ) :)
>
> Some of it might be compressed/simplified by the use of "else if".
>
> I'll try to take a closer look later (will have to apply it to see the
> context better). Just one independent comment because I stumbled over
> this recently:
>
> [...]
>
> > @@ -1062,10 +1063,13 @@ static int move_present_pte(struct mm_struct *mm,
> >       folio_move_anon_rmap(src_folio, dst_vma);
> >       src_folio->index = linear_page_index(dst_vma, dst_addr);
> >
> > -     orig_dst_pte = mk_pte(&src_folio->page, dst_vma->vm_page_prot);
> > -     /* Follow mremap() behavior and treat the entry dirty after the move */
> > -     orig_dst_pte = pte_mkwrite(pte_mkdirty(orig_dst_pte), dst_vma);
> > -
> > +     if (pte_present(orig_src_pte)) {
> > +             orig_dst_pte = mk_pte(&src_folio->page, dst_vma->vm_page_prot);
> > +             /* Follow mremap() behavior and treat the entry dirty after the move */
> > +             orig_dst_pte = pte_mkwrite(pte_mkdirty(orig_dst_pte), dst_vma);
>
> I'll note that the comment and mkdirty is misleading/wrong. It's
> softdirty that we care about only. But that is something independent of
> this change.
>
> For swp PTEs, we maybe also would want to set softdirty.
>
> See move_soft_dirty_pte() on what is actually done on the mremap path.

I actually don't quite understand the changelog in  commit 0f8975ec4db2
(" mm: soft-dirty bits for user memory changes tracking").

"    Another thing to note, is that when mremap moves PTEs they are marked
    with soft-dirty as well, since from the user perspective mremap modifies
    the virtual memory at mremap's new address."

Why is the hardware-dirty bit not relevant? From the user's perspective,
the memory at the destination virtual address of mremap/userfaultfd_move
has changed.

For systems where CONFIG_HAVE_ARCH_SOFT_DIRTY is false, how can the dirty status
be determined?

Or is the answer that we only care about soft-dirty changes?

For the hardware-dirty bit, do we only care about actual modifications to the
physical page content rather than changes at the virtual address level?

>
> --
> Cheers,
>
> David / dhildenb
>

Thanks
Barry

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