Re: [PATCH v4] mm/rmap: do not add fully unmapped large folio to deferred split list

[Barry Song <21cnbao@gmail.com>]

On Fri, Apr 26, 2024 at 9:55 AM Zi Yan <ziy@nvidia.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@sent.com> wrote:
> >>
> >> From: Zi Yan <ziy@nvidia.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@nvidia.com>
> >> Reviewed-by: Yang Shi <shy828301@gmail.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;



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