Re: [PATCH 1/3] mm: set folio swapbacked iff folios are dirty in try_to_unmap_one

[Lance Yang]

On Mon, Jan 6, 2025 at 5:34 PM Baolin Wang
<baolin.wang@xxxxxxxxxxxxxxxxx> wrote:
>
>
>
> On 2025/1/6 17:03, Barry Song wrote:
> > On Mon, Jan 6, 2025 at 7:40 PM Baolin Wang
> > <baolin.wang@xxxxxxxxxxxxxxxxx> wrote:
> >>
> >>
> >>
> >> On 2025/1/6 11:17, Barry Song wrote:
> >>> From: Barry Song <v-songbaohua@xxxxxxxx>
> >>>
> >>> The refcount may be temporarily or long-term increased, but this does
> >>> not change the fundamental nature of the folio already being lazy-
> >>> freed. Therefore, we only reset 'swapbacked' when we are certain the
> >>> folio is dirty and not droppable.
> >>>
> >>> Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
> >>> Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx>
> >>
> >> The changes look good to me. While we are at it, could you also change
> >> the __discard_anon_folio_pmd_locked() to follow the same strategy for
> >> lazy-freed PMD-sized folio?
> >
> > it seems you mean __discard_anon_folio_pmd_locked() is lacking
> > folio_set_swapbacked(folio) for dirty pmd-mapped folios?

Good catch!

Hmm... I don't recall why we don't call folio_set_swapbacked for dirty
THPs in __discard_anon_folio_pmd_locked() - possibly to align with
previous behavior ;)

If a dirty PMD-mapped THP cannot be discarded, we just split it and
restart the page walk to process the PTE-mapped THP. After that, we
will only mark each folio within the THP as swap-backed individually.

It seems like we could cut the work by calling folio_set_swapbacked()
for dirty THPs directly in __discard_anon_folio_pmd_locked(), skipping
the restart of the page walk after splitting the THP, IMHO ;)

Thanks,
Lance


> > and it seems !(vma->vm_flags & VM_DROPPABLE) is also not
> > handled properly?


>
> Right.