Message-ID: <CAGsJ_4yNSKdft-G=4X2Dr5HJ-axh-8SohGW7nkUfTYrsNFr9_Q@mail.gmail.com>
Date: Fri, 26 Apr 2024 11:28:23 +0800
From: Barry Song <21cnbao@...il.com>
To: Zi Yan <ziy@...dia.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org, 
	"Matthew Wilcox (Oracle)" <willy@...radead.org>, Yang Shi <shy828301@...il.com>, 
	Ryan Roberts <ryan.roberts@....com>, David Hildenbrand <david@...hat.com>, 
	Lance Yang <ioworker0@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4] mm/rmap: do not add fully unmapped large folio to
 deferred split list

On Fri, Apr 26, 2024 at 10:50 AM Zi Yan <ziy@...dia.com> wrote:
>
> On 25 Apr 2024, at 22:23, Barry Song wrote:
>
> > On Fri, Apr 26, 2024 at 9:55 AM Zi Yan <ziy@...dia.com> wrote:
> >>
> >> On 25 Apr 2024, at 21:45, Barry Song wrote:
> >>
> >>> On Fri, Apr 26, 2024 at 5:11 AM Zi Yan <zi.yan@...t.com> wrote:
> >>>>
> >>>> From: Zi Yan <ziy@...dia.com>
> >>>>
> >>>> In __folio_remove_rmap(), a large folio is added to deferred split list
> >>>> if any page in a folio loses its final mapping. But it is possible that
> >>>> the folio is fully unmapped and adding it to deferred split list is
> >>>> unnecessary.
> >>>>
> >>>> For PMD-mapped THPs, that was not really an issue, because removing the
> >>>> last PMD mapping in the absence of PTE mappings would not have added the
> >>>> folio to the deferred split queue.
> >>>>
> >>>> However, for PTE-mapped THPs, which are now more prominent due to mTHP,
> >>>> they are always added to the deferred split queue. One side effect
> >>>> is that the THP_DEFERRED_SPLIT_PAGE stat for a PTE-mapped folio can be
> >>>> unintentionally increased, making it look like there are many partially
> >>>> mapped folios -- although the whole folio is fully unmapped stepwise.
> >>>>
> >>>> Core-mm now tries batch-unmapping consecutive PTEs of PTE-mapped THPs
> >>>> where possible starting from commit b06dc281aa99 ("mm/rmap: introduce
> >>>> folio_remove_rmap_[pte|ptes|pmd]()"). When it happens, a whole PTE-mapped
> >>>> folio is unmapped in one go and can avoid being added to deferred split
> >>>> list, reducing the THP_DEFERRED_SPLIT_PAGE noise. But there will still be
> >>>> noise when we cannot batch-unmap a complete PTE-mapped folio in one go
> >>>> -- or where this type of batching is not implemented yet, e.g., migration.
> >>>>
> >>>> To avoid the unnecessary addition, folio->_nr_pages_mapped is checked
> >>>> to tell if the whole folio is unmapped. If the folio is already on
> >>>> deferred split list, it will be skipped, too.
> >>>>
> >>>> Note: commit 98046944a159 ("mm: huge_memory: add the missing
> >>>> folio_test_pmd_mappable() for THP split statistics") tried to exclude
> >>>> mTHP deferred split stats from THP_DEFERRED_SPLIT_PAGE, but it does not
> >>>> fix the above issue. A fully unmapped PTE-mapped order-9 THP was still
> >>>> added to deferred split list and counted as THP_DEFERRED_SPLIT_PAGE,
> >>>> since nr is 512 (non zero), level is RMAP_LEVEL_PTE, and inside
> >>>> deferred_split_folio() the order-9 folio is folio_test_pmd_mappable().
> >>>>
> >>>> Signed-off-by: Zi Yan <ziy@...dia.com>
> >>>> Reviewed-by: Yang Shi <shy828301@...il.com>
> >>>> ---
> >>>>  mm/rmap.c | 8 +++++---
> >>>>  1 file changed, 5 insertions(+), 3 deletions(-)
> >>>>
> >>>> diff --git a/mm/rmap.c b/mm/rmap.c
> >>>> index a7913a454028..220ad8a83589 100644
> >>>> --- a/mm/rmap.c
> >>>> +++ b/mm/rmap.c
> >>>> @@ -1553,9 +1553,11 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
> >>>>                  * page of the folio is unmapped and at least one page
> >>>>                  * is still mapped.
> >>>>                  */
> >>>> -               if (folio_test_large(folio) && folio_test_anon(folio))
> >>>> -                       if (level == RMAP_LEVEL_PTE || nr < nr_pmdmapped)
> >>>> -                               deferred_split_folio(folio);
> >>>> +               if (folio_test_large(folio) && folio_test_anon(folio) &&
> >>>> +                   list_empty(&folio->_deferred_list) &&
> >>>> +                   ((level == RMAP_LEVEL_PTE && atomic_read(mapped)) ||
> >>>> +                    (level == RMAP_LEVEL_PMD && nr < nr_pmdmapped)))
> >>>> +                       deferred_split_folio(folio);
> >>>
> >>> Hi Zi Yan,
> >>> in case a mTHP is mapped by two processed (forked but not CoW yet), if we
> >>> unmap the whole folio by pte level in one process only, are we still adding this
> >>> folio into deferred list?
> >>
> >> No. Because the mTHP is still fully mapped by the other process. In terms of code,
> >> nr will be 0 in that case and this if condition is skipped. nr is only increased
> >> from 0 when one of the subpages in the mTHP has no mapping, namely page->_mapcount
> >> becomes negative and last is true in the case RMAP_LEVEL_PTE.
> >
> > Ok. i see, so "last" won't be true?
> >
> > case RMAP_LEVEL_PTE:
> > do {
> > last = atomic_add_negative(-1, &page->_mapcount);
> >    if (last && folio_test_large(folio)) {
> >        last = atomic_dec_return_relaxed(mapped);
> >        last = (last < ENTIRELY_MAPPED);
> > }
> >
> > if (last)
> >      nr++;
> > } while (page++, --nr_pages > 0);
> > break;
>
> Right, because for every subpage its corresponding
> last = atomic_add_negative(-1, &page->_mapcount); is not true after the unmapping.2

if a mTHP is mapped only by one process, and we unmap it entirely, we will
get nr > 0, then we are executing adding it into deferred_list? so it seems
atomic_read(mapped) is preventing this case from adding deferred_list?

I wonder if  it is possible to fixup nr to 0 from the first place?
for example
/* we are doing an entire unmapping */
if (page==&folio->page && nr_pages ==  folio_nr_pages(folio))
..

>
>
> --
> Best Regards,
> Yan, Zi

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