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Message-ID: <CAGsJ_4xREM-P1mFqeM-s3-cJ9czb6PXwizb-3hOhwaF6+QM5QA@mail.gmail.com>
Date: Thu, 7 Mar 2024 21:10:10 +1300
From: Barry Song <21cnbao@...il.com>
To: Lance Yang <ioworker0@...il.com>, david@...hat.com,
Vishal Moola <vishal.moola@...il.com>
Cc: akpm@...ux-foundation.org, zokeefe@...gle.com, ryan.roberts@....com,
shy828301@...il.com, mhocko@...e.com, fengwei.yin@...el.com,
xiehuan09@...il.com, wangkefeng.wang@...wei.com, songmuchun@...edance.com,
peterx@...hat.com, minchan@...nel.org, linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/1] mm/madvise: enhance lazyfreeing with mTHP in madvise_free
On Thu, Mar 7, 2024 at 9:00 PM Lance Yang <ioworker0@...il.com> wrote:
>
> Hey Barry,
>
> Thanks for taking time to review!
>
> On Thu, Mar 7, 2024 at 3:00 PM Barry Song <21cnbao@...il.com> wrote:
> >
> > On Thu, Mar 7, 2024 at 7:15 PM Lance Yang <ioworker0@...il.com> wrote:
> > >
> [...]
> > > +static inline bool can_mark_large_folio_lazyfree(unsigned long addr,
> > > + struct folio *folio, pte_t *start_pte)
> > > +{
> > > + int nr_pages = folio_nr_pages(folio);
> > > + fpb_t flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY;
> > > +
> > > + for (int i = 0; i < nr_pages; i++)
> > > + if (page_mapcount(folio_page(folio, i)) != 1)
> > > + return false;
> >
> > we have moved to folio_estimated_sharers though it is not precise, so
> > we don't do
> > this check with lots of loops and depending on the subpage's mapcount.
>
> If we don't check the subpage’s mapcount, and there is a cow folio associated
> with this folio and the cow folio has smaller size than this folio,
> should we still
> mark this folio as lazyfree?
I agree, this is true. However, we've somehow accepted the fact that
folio_likely_mapped_shared
can result in false negatives or false positives to balance the
overhead. So I really don't know :-)
Maybe David and Vishal can give some comments here.
>
> > BTW, do we need to rebase our work against David's changes[1]?
> > [1] https://lore.kernel.org/linux-mm/20240227201548.857831-1-david@redhat.com/
>
> Yes, we should rebase our work against David’s changes.
>
> >
> > > +
> > > + return nr_pages == folio_pte_batch(folio, addr, start_pte,
> > > + ptep_get(start_pte), nr_pages, flags, NULL);
> > > +}
> > > +
> > > static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
> > > unsigned long end, struct mm_walk *walk)
> > >
> > > @@ -676,11 +690,45 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
> > > */
> > > if (folio_test_large(folio)) {
> > > int err;
> > > + unsigned long next_addr, align;
> > >
> > > - if (folio_estimated_sharers(folio) != 1)
> > > - break;
> > > - if (!folio_trylock(folio))
> > > - break;
> > > + if (folio_estimated_sharers(folio) != 1 ||
> > > + !folio_trylock(folio))
> > > + goto skip_large_folio;
> >
> >
> > I don't think we can skip all the PTEs for nr_pages, as some of them might be
> > pointing to other folios.
> >
> > for example, for a large folio with 16PTEs, you do MADV_DONTNEED(15-16),
> > and write the memory of PTE15 and PTE16, you get page faults, thus PTE15
> > and PTE16 will point to two different small folios. We can only skip when we
> > are sure nr_pages == folio_pte_batch() is sure.
>
> Agreed. Thanks for pointing that out.
>
> >
> > > +
> > > + align = folio_nr_pages(folio) * PAGE_SIZE;
> > > + next_addr = ALIGN_DOWN(addr + align, align);
> > > +
> > > + /*
> > > + * If we mark only the subpages as lazyfree, or
> > > + * cannot mark the entire large folio as lazyfree,
> > > + * then just split it.
> > > + */
> > > + if (next_addr > end || next_addr - addr != align ||
> > > + !can_mark_large_folio_lazyfree(addr, folio, pte))
> > > + goto split_large_folio;
> > > +
> > > + /*
> > > + * Avoid unnecessary folio splitting if the large
> > > + * folio is entirely within the given range.
> > > + */
> > > + folio_clear_dirty(folio);
> > > + folio_unlock(folio);
> > > + for (; addr != next_addr; pte++, addr += PAGE_SIZE) {
> > > + ptent = ptep_get(pte);
> > > + if (pte_young(ptent) || pte_dirty(ptent)) {
> > > + ptent = ptep_get_and_clear_full(
> > > + mm, addr, pte, tlb->fullmm);
> > > + ptent = pte_mkold(ptent);
> > > + ptent = pte_mkclean(ptent);
> > > + set_pte_at(mm, addr, pte, ptent);
> > > + tlb_remove_tlb_entry(tlb, pte, addr);
> > > + }
> >
> > Can we do this in batches? for a CONT-PTE mapped large folio, you are unfolding
> > and folding again. It seems quite expensive.
>
> Thanks for your suggestion. I'll do this in batches in v3.
>
> Thanks again for your time!
>
> Best,
> Lance
>
> >
> > > + }
> > > + folio_mark_lazyfree(folio);
> > > + goto next_folio;
> > > +
> > > +split_large_folio:
> > > folio_get(folio);
> > > arch_leave_lazy_mmu_mode();
> > > pte_unmap_unlock(start_pte, ptl);
> > > @@ -688,13 +736,28 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
> > > err = split_folio(folio);
> > > folio_unlock(folio);
> > > folio_put(folio);
> > > - if (err)
> > > - break;
> > > - start_pte = pte =
> > > - pte_offset_map_lock(mm, pmd, addr, &ptl);
> > > - if (!start_pte)
> > > - break;
> > > - arch_enter_lazy_mmu_mode();
> > > +
> > > + /*
> > > + * If the large folio is locked or cannot be split,
> > > + * we just skip it.
> > > + */
> > > + if (err) {
> > > +skip_large_folio:
> > > + if (next_addr >= end)
> > > + break;
> > > + pte += (next_addr - addr) / PAGE_SIZE;
> > > + addr = next_addr;
> > > + }
> > > +
> > > + if (!start_pte) {
> > > + start_pte = pte = pte_offset_map_lock(
> > > + mm, pmd, addr, &ptl);
> > > + if (!start_pte)
> > > + break;
> > > + arch_enter_lazy_mmu_mode();
> > > + }
> > > +
> > > +next_folio:
> > > pte--;
> > > addr -= PAGE_SIZE;
> > > continue;
> > > --
> > > 2.33.1
> > >
Thanks
Barry
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