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

On Tue, Jan 7, 2025 at 9:52 AM Barry Song [off-list ref] wrote:

On Tue, Jan 7, 2025 at 3:40 AM Lance Yang [off-list ref] wrote:

quoted

On Mon, Jan 6, 2025 at 5:34 PM Baolin Wang
[off-list ref] wrote:

quoted

On 2025/1/6 17:03, Barry Song wrote:

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On Mon, Jan 6, 2025 at 7:40 PM Baolin Wang
[off-list ref] wrote:

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On 2025/1/6 11:17, Barry Song wrote:

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From: Barry Song <redacted>

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 <redacted>
Signed-off-by: Barry Song <redacted>

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);
        }

quoted

Thanks,
Lance

quoted

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

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Right.

Thanks
Barry