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Date: Thu, 20 Jun 2024 21:59:10 +1200
From: Barry Song <21cnbao@...il.com>
To: David Hildenbrand <david@...hat.com>
Cc: akpm@...ux-foundation.org, linux-mm@...ck.org,
baolin.wang@...ux.alibaba.com, chrisl@...nel.org,
linux-kernel@...r.kernel.org, mhocko@...e.com, ryan.roberts@....com,
shy828301@...il.com, surenb@...gle.com, v-songbaohua@...o.com,
willy@...radead.org, ying.huang@...el.com, yosryahmed@...gle.com,
yuzhao@...gle.com, Shuai Yuan <yuanshuai@...o.com>
Subject: Re: [PATCH v2 2/3] mm: use folio_add_new_anon_rmap() if folio_test_anon(folio)==false
On Thu, Jun 20, 2024 at 8:49 PM David Hildenbrand <david@...hat.com> wrote:
>
> On 20.06.24 10:33, Barry Song wrote:
> > On Thu, Jun 20, 2024 at 7:46 PM David Hildenbrand <david@...hat.com> wrote:
> >>
> >> On 18.06.24 01:11, Barry Song wrote:
> >>> From: Barry Song <v-songbaohua@...o.com>
> >>>
> >>> For the !folio_test_anon(folio) case, we can now invoke folio_add_new_anon_rmap()
> >>> with the rmap flags set to either EXCLUSIVE or non-EXCLUSIVE. This action will
> >>> suppress the VM_WARN_ON_FOLIO check within __folio_add_anon_rmap() while initiating
> >>> the process of bringing up mTHP swapin.
> >>>
> >>> static __always_inline void __folio_add_anon_rmap(struct folio *folio,
> >>> struct page *page, int nr_pages, struct vm_area_struct *vma,
> >>> unsigned long address, rmap_t flags, enum rmap_level level)
> >>> {
> >>> ...
> >>> if (unlikely(!folio_test_anon(folio))) {
> >>> VM_WARN_ON_FOLIO(folio_test_large(folio) &&
> >>> level != RMAP_LEVEL_PMD, folio);
> >>> }
> >>> ...
> >>> }
> >>>
> >>> It also improves the code’s readability. Currently, all new anonymous
> >>> folios calling folio_add_anon_rmap_ptes() are order-0. This ensures
> >>> that new folios cannot be partially exclusive; they are either entirely
> >>> exclusive or entirely shared.
> >>>
> >>> Suggested-by: David Hildenbrand <david@...hat.com>
> >>> Signed-off-by: Barry Song <v-songbaohua@...o.com>
> >>> Tested-by: Shuai Yuan <yuanshuai@...o.com>
> >>> ---
> >>> mm/memory.c | 8 ++++++++
> >>> mm/swapfile.c | 13 +++++++++++--
> >>> 2 files changed, 19 insertions(+), 2 deletions(-)
> >>>
> >>> diff --git a/mm/memory.c b/mm/memory.c
> >>> index 1f24ecdafe05..620654c13b2f 100644
> >>> --- a/mm/memory.c
> >>> +++ b/mm/memory.c
> >>> @@ -4339,6 +4339,14 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >>> if (unlikely(folio != swapcache && swapcache)) {
> >>> folio_add_new_anon_rmap(folio, vma, address, RMAP_EXCLUSIVE);
> >>> folio_add_lru_vma(folio, vma);
> >>> + } else if (!folio_test_anon(folio)) {
> >>> + /*
> >>> + * We currently only expect small !anon folios, for which we now
> >>> + * that they are either fully exclusive or fully shared. If we
> >>> + * ever get large folios here, we have to be careful.
> >>> + */
> >>> + VM_WARN_ON_ONCE(folio_test_large(folio));
> >>> + folio_add_new_anon_rmap(folio, vma, address, rmap_flags);
> >>> } else {
> >>> folio_add_anon_rmap_ptes(folio, page, nr_pages, vma, address,
> >>> rmap_flags);
> >>> diff --git a/mm/swapfile.c b/mm/swapfile.c
> >>> index ae1d2700f6a3..69efa1a57087 100644
> >>> --- a/mm/swapfile.c
> >>> +++ b/mm/swapfile.c
> >>> @@ -1908,8 +1908,17 @@ static int unuse_pte(struct vm_area_struct *vma, pmd_t *pmd,
> >>> VM_BUG_ON_FOLIO(folio_test_writeback(folio), folio);
> >>> if (pte_swp_exclusive(old_pte))
> >>> rmap_flags |= RMAP_EXCLUSIVE;
> >>> -
> >>> - folio_add_anon_rmap_pte(folio, page, vma, addr, rmap_flags);
> >>> + /*
> >>> + * We currently only expect small !anon folios, for which we now that
> >>> + * they are either fully exclusive or fully shared. If we ever get
> >>> + * large folios here, we have to be careful.
> >>> + */
> >>> + if (!folio_test_anon(folio)) {
> >>> + VM_WARN_ON_ONCE(folio_test_large(folio));
> >>
> >> (comment applies to both cases)
> >>
> >> Thinking about Hugh's comment, we should likely add here:
> >>
> >> VM_WARN_ON_FOLIO(!folio_test_locked(folio), folio);
> >>
> >> [the check we are removing from __folio_add_anon_rmap()]
> >>
> >> and document for folio_add_new_anon_rmap() in patch #1, that when
> >> dealing with folios that might be mapped concurrently by others, the
> >> folio lock must be held.
> >
> > I assume you mean something like the following for patch#1?
> >
> > diff --git a/mm/rmap.c b/mm/rmap.c
> > index df1a43295c85..20986b25f1b2 100644
> > --- a/mm/rmap.c
> > +++ b/mm/rmap.c
> > @@ -1394,7 +1394,8 @@ void folio_add_anon_rmap_pmd(struct folio
> > *folio, struct page *page,
> > *
> > * Like folio_add_anon_rmap_*() but must only be called on *new* folios.
> > * This means the inc-and-test can be bypassed.
> > - * The folio does not have to be locked.
> > + * The folio doesn't necessarily need to be locked while it's
> > exclusive unless two threads
> > + * map it concurrently. However, the folio must be locked if it's shared.
> > *
> > * If the folio is pmd-mappable, it is accounted as a THP.
> > */
> > @@ -1406,6 +1407,7 @@ void folio_add_new_anon_rmap(struct folio
> > *folio, struct vm_area_struct *vma,
> > int nr_pmdmapped = 0;
> >
> > VM_WARN_ON_FOLIO(folio_test_hugetlb(folio), folio);
> > + VM_WARN_ON_FOLIO(!exclusive && !folio_test_locked(folio), folio);
>
> For now this would likely do. I was concerned about a concurrent
> scenario in the exclusive case, but that shouldn't really happen I guess.
>
Since this is primarily a documentation update, I'll wait for two or
three days to see if
there are any more Linux-next reports before sending v3 combining these fixes
together(I've already fixed another doc warn reported by lkp) and seek Andrew's
assistance to drop v2 and apply v3.
> --
> Cheers,
>
> David / dhildenb
>
Thanks
Barry
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