On Thu, Feb 20, 2025 at 11:24 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> On 20.02.25 10:21, Barry Song wrote:
> > On Thu, Feb 20, 2025 at 9:40 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
> >>
> >> On 19.02.25 19:58, Suren Baghdasaryan wrote:
> >>> On Wed, Feb 19, 2025 at 10:30 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
> >>>>
> >>>> On 19.02.25 19:26, Suren Baghdasaryan wrote:
> >>>>> On Wed, Feb 19, 2025 at 3:25 AM Barry Song <21cnbao@xxxxxxxxx> wrote:
> >>>>>>
> >>>>>> From: Barry Song <v-songbaohua@xxxxxxxx>
> >>>>>>
> >>>>>> 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
