linux-kernel - Re: [PATCH 1/3] mm: set folio swapbacked iff folios are dirty in try_to_unmap

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Message-ID: <CAGsJ_4zxdPREb-jmjrjNWySiVwPLbXfj5pG541kYyXXPD4_vBg@mail.gmail.com>
Date: Tue, 7 Jan 2025 09:52:29 +1300
From: Barry Song <21cnbao@...il.com>
To: Lance Yang <ioworker0@...il.com>
Cc: Baolin Wang <baolin.wang@...ux.alibaba.com>, akpm@...ux-foundation.org, 
	linux-mm@...ck.org, linux-arm-kernel@...ts.infradead.org, x86@...nel.org, 
	linux-kernel@...r.kernel.org, david@...hat.com, ryan.roberts@....com, 
	zhengtangquan@...o.com, ying.huang@...el.com, kasong@...cent.com, 
	chrisl@...nel.org, Barry Song <v-songbaohua@...o.com>
Subject: Re: [PATCH 1/3] mm: set folio swapbacked iff folios are dirty in try_to_unmap_one

On Tue, Jan 7, 2025 at 3:40 AM Lance Yang <ioworker0@...il.com> wrote:
>
> On Mon, Jan 6, 2025 at 5:34 PM Baolin Wang
> <baolin.wang@...ux.alibaba.com> wrote:
> >
> >
> >
> > On 2025/1/6 17:03, Barry Song wrote:
> > > On Mon, Jan 6, 2025 at 7:40 PM Baolin Wang
> > > <baolin.wang@...ux.alibaba.com> wrote:
> > >>
> > >>
> > >>
> > >> On 2025/1/6 11:17, Barry Song wrote:
> > >>> From: Barry Song <v-songbaohua@...o.com>
> > >>>
> > >>> 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@...hat.com>
> > >>> Signed-off-by: Barry Song <v-songbaohua@...o.com>
> > >>
> > >> 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
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