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Message-ID: <CAK1f24kaGo3PJSd83=-t_uAFTTJiSsZvJTmsX9co4ueFDiPneA@mail.gmail.com>
Date: Tue, 18 Jun 2024 09:56:51 +0800
From: Lance Yang <ioworker0@...il.com>
To: David Hildenbrand <david@...hat.com>
Cc: akpm@...ux-foundation.org, willy@...radead.org, sj@...nel.org,
baolin.wang@...ux.alibaba.com, maskray@...gle.com, ziy@...dia.com,
ryan.roberts@....com, 21cnbao@...il.com, mhocko@...e.com,
fengwei.yin@...el.com, zokeefe@...gle.com, shy828301@...il.com,
xiehuan09@...il.com, libang.li@...group.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 v8 3/3] mm/vmscan: avoid split lazyfree THP during shrink_folio_list()
On Tue, Jun 18, 2024 at 2:04 AM David Hildenbrand <david@...hat.com> wrote:
>
> Sorry for taking so long to review ... getting there. Mostly nits.
No worries at all :)
Thanks for taking time to review!
>
> > @@ -497,6 +499,13 @@ static inline void split_huge_pmd_locked(struct vm_area_struct *vma,
> > unsigned long address, pmd_t *pmd,
> > bool freeze, struct folio *folio) {}
> >
> > +static inline bool unmap_huge_pmd_locked(struct vm_area_struct *vma,
> > + unsigned long addr, pmd_t *pmdp,
> > + struct folio *folio)
> > +{
> > + return false;
> > +}
> > +
> > #define split_huge_pud(__vma, __pmd, __address) \
> > do { } while (0)
> >
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index e766d3f3a302..425374ae06ed 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -2688,6 +2688,82 @@ static void unmap_folio(struct folio *folio)
> > try_to_unmap_flush();
> > }
> >
> > +static bool __discard_anon_folio_pmd_locked(struct vm_area_struct *vma,
> > + unsigned long addr, pmd_t *pmdp,
> > + struct folio *folio)
> > +{
> > + VM_WARN_ON_FOLIO(folio_test_swapbacked(folio), folio);
> > + VM_WARN_ON_FOLIO(!folio_test_anon(folio), folio);
>
> I would drop these (that's exactly what the single caller checks). In
Agreed. I will drop these.
> any case don't place them above the variable declaration ;)
Yep, I see.
>
> > +
> > + struct mm_struct *mm = vma->vm_mm;
> > + int ref_count, map_count;
> > + pmd_t orig_pmd = *pmdp;
> > + struct page *page;
> > +
> > + if (unlikely(!pmd_present(orig_pmd) || !pmd_trans_huge(orig_pmd)))
> > + return false;
> > +
> > + page = pmd_page(orig_pmd);
> > + if (unlikely(page_folio(page) != folio))
> > + return false;
>
> I'm curious, how could that happen? And how could it happen that we have
> !pmd_trans_huge() ? Didn't rmap walking code make sure that this PMD
> maps the folio already, and we are holding the PTL?
Makes sense to me. I was adding these just in case, but it's probably too much.
Let's drop them ;)
>
> > +
> > + if (folio_test_dirty(folio) || pmd_dirty(orig_pmd))
> > + return false;
> > +
> > + orig_pmd = pmdp_huge_clear_flush(vma, addr, pmdp);
> > +
> > + /*
> > + * Syncing against concurrent GUP-fast:
> > + * - clear PMD; barrier; read refcount
> > + * - inc refcount; barrier; read PMD
> > + */
> > + smp_mb();
> > +
> > + ref_count = folio_ref_count(folio);
> > + map_count = folio_mapcount(folio);
> > +
> > + /*
> > + * Order reads for folio refcount and dirty flag
> > + * (see comments in __remove_mapping()).
> > + */
> > + smp_rmb();
> > +
> > + /*
> > + * If the folio or its PMD is redirtied at this point, or if there
> > + * are unexpected references, we will give up to discard this folio
> > + * and remap it.
> > + *
> > + * The only folio refs must be one from isolation plus the rmap(s).
> > + */
> > + if (folio_test_dirty(folio) || pmd_dirty(orig_pmd) ||
> > + ref_count != map_count + 1) {
> > + set_pmd_at(mm, addr, pmdp, orig_pmd);
> > + return false;
> > + }
> > +
> > + folio_remove_rmap_pmd(folio, page, vma);
> > + zap_deposited_table(mm, pmdp);
> > + add_mm_counter(mm, MM_ANONPAGES, -HPAGE_PMD_NR);
> > + if (vma->vm_flags & VM_LOCKED)
> > + mlock_drain_local();
> > + folio_put(folio);
> > +
> > + return true;
> > +}
> > +
> > +bool unmap_huge_pmd_locked(struct vm_area_struct *vma, unsigned long addr,
> > + pmd_t *pmdp, struct folio *folio)
> > +{
> > + VM_WARN_ON_FOLIO(!folio_test_pmd_mappable(folio), folio);
> > + VM_WARN_ON_FOLIO(!folio_test_locked(folio), folio);
> > + VM_WARN_ON_ONCE(!IS_ALIGNED(addr, HPAGE_PMD_SIZE));
> > +
> > + if (folio_test_anon(folio) && !folio_test_swapbacked(folio))
> > + return __discard_anon_folio_pmd_locked(vma, addr, pmdp, folio);
> > +
> > + return false;
> > +}
> > +
> > static void remap_page(struct folio *folio, unsigned long nr)
> > {
> > int i = 0;
> > diff --git a/mm/rmap.c b/mm/rmap.c
> > index dacf24bc82f0..7d97806f74cd 100644
> > --- a/mm/rmap.c
> > +++ b/mm/rmap.c
> > @@ -1678,16 +1678,23 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
> > goto walk_abort;
> > }
> >
> > - if (!pvmw.pte && (flags & TTU_SPLIT_HUGE_PMD)) {
> > - /*
> > - * We temporarily have to drop the PTL and start once
> > - * again from that now-PTE-mapped page table.
> > - */
> > - split_huge_pmd_locked(vma, pvmw.address, pvmw.pmd,
> > - false, folio);
> > - flags &= ~TTU_SPLIT_HUGE_PMD;
> > - page_vma_mapped_walk_restart(&pvmw);
> > - continue;
> > + if (!pvmw.pte) {
> > + if (unmap_huge_pmd_locked(vma, pvmw.address, pvmw.pmd,
> > + folio))
> > + goto walk_done;
> > +
> > + if (flags & TTU_SPLIT_HUGE_PMD) {
> > + /*
> > + * We temporarily have to drop the PTL and start
> > + * once again from that now-PTE-mapped page
> > + * table.
>
> Nit: it's not a PTE-mapped page table.
>
> Maybe
>
> "... restart so we can process the PTE-mapped THP."
Nice. Will adjust as you suggested.
>
>
>
> > }
> >
> > /* Unexpected PMD-mapped THP? */
>
> Nothing else jumped at me :)
Thanks again for your time!
Lance
>
> --
> Cheers,
>
> David / dhildenb
>
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