Re: [PATCH v5] mm/rmap: do not add fully unmapped large folio to deferred split list

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

On Sat, Apr 27, 2024 at 3:20 AM Zi Yan <ziy@nvidia.com> wrote:
>
> On 26 Apr 2024, at 15:08, David Hildenbrand wrote:
>
> > On 26.04.24 21:02, Zi Yan 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().
> >>
> >> Suggested-by: David Hildenbrand <david@redhat.com>
> >> Signed-off-by: Zi Yan <ziy@nvidia.com>
> >> ---
> >>   mm/rmap.c | 12 +++++++++---
> >>   1 file changed, 9 insertions(+), 3 deletions(-)
> >>
> >> diff --git a/mm/rmap.c b/mm/rmap.c
> >> index 2608c40dffad..a9bd64ebdd9a 100644
> >> --- a/mm/rmap.c
> >> +++ b/mm/rmap.c
> >> @@ -1495,6 +1495,7 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
> >>   {
> >>      atomic_t *mapped = &folio->_nr_pages_mapped;
> >>      int last, nr = 0, nr_pmdmapped = 0;
> >> +    bool partially_mapped = false;
> >>      enum node_stat_item idx;
> >>      __folio_rmap_sanity_checks(folio, page, nr_pages, level);
> >> @@ -1515,6 +1516,8 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
> >>                                      nr++;
> >>                      }
> >>              } while (page++, --nr_pages > 0);
> >> +
> >> +            partially_mapped = !!nr && !!atomic_read(mapped);
> >
> > Nit: The && should remove the need for both !!.
>
> My impression was that !! is needed to convert from int to bool and I do
> find "!!int && !!int" use in the kernel. If this is unnecessary, Andrew
> can apply the fixup below. I can send a new version if it is really needed.
>
> diff --git a/mm/rmap.c b/mm/rmap.c
> index a9bd64ebdd9a..c1fd5828409b 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1517,7 +1517,7 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
>                         }
>                 } while (page++, --nr_pages > 0);
>
> -               partially_mapped = !!nr && !!atomic_read(mapped);
> +               partially_mapped = nr && atomic_read(mapped

Reviewed-by：Barry Song <baohua@kernel.org>

>                 break;
>         case RMAP_LEVEL_PMD:
>                 atomic_dec(&folio->_large_mapcount);
>
> >
> > Reviewed-by: David Hildenbrand <david@redhat.com>
>
> Thanks.
>
> --
> Best Regards,
> Yan, Zi