On Tue, Jan 7, 2025 at 9:52 AM Barry Song <21cnbao@xxxxxxxxx> wrote:
>
> On Tue, Jan 7, 2025 at 3:40 AM Lance Yang <ioworker0@xxxxxxxxx> wrote:
> >
> > 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 ;)
>
> Yes, the existing code for PMD-mapped THPs seems quite inefficient. It splits
> the PMD-mapped THP into smaller folios, marks each split PTE as dirty, and
Apologies for the typo, I meant splitting a PMD-mapped THP into a PTE-mapped
THP.
> then iterates over each PTE. I’m not sure why it’s designed this way—could
> there be a specific reason behind this approach?
>
> However, it does appear to handle folio_set_swapbacked() correctly, as only
> a dirty PMD will result in dirty PTEs being generated in
> __split_huge_pmd_locked():
>
> } else {
> pte_t entry;
>
> entry = mk_pte(page, READ_ONCE(vma->vm_page_prot));
> if (write)
> entry = pte_mkwrite(entry, vma);
>
> if (!young)
> entry = pte_mkold(entry);
>
> /* NOTE: this may set soft-dirty too on some archs */
> if (dirty)
> entry = pte_mkdirty(entry);
>
> if (soft_dirty)
> entry = pte_mksoft_dirty(entry);
>
> if (uffd_wp)
> entry = pte_mkuffd_wp(entry);
>
> for (i = 0; i < HPAGE_PMD_NR; i++)
> VM_WARN_ON(!pte_none(ptep_get(pte + i)));
>
> set_ptes(mm, haddr, pte, entry, HPAGE_PMD_NR);
> }
>
>
>
> >
> > Thanks,
> > Lance
> >
> >
> > > > and it seems !(vma->vm_flags & VM_DROPPABLE) is also not
> > > > handled properly?
> >
> >
> > >
> > > Right.
>
> Thanks
> Barry
