From: Yang Shi <shy828301@gmail.com>
To: Zi Yan <ziy@nvidia.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
linux-mm@kvack.org,
"Matthew Wilcox (Oracle)" <willy@infradead.org>,
Ryan Roberts <ryan.roberts@arm.com>,
Barry Song <21cnbao@gmail.com>,
David Hildenbrand <david@redhat.com>,
Lance Yang <ioworker0@gmail.com>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH v5] mm/rmap: do not add fully unmapped large folio to deferred split list
Date: Fri, 26 Apr 2024 13:42:53 -0700 [thread overview]
Message-ID: <CAHbLzkr9t_Kfrjddn2Xu22vSQUO55Z5wHVi8_oz=qL1oEQ9wvg@mail.gmail.com> (raw)
In-Reply-To: <20240426190253.541419-1-zi.yan@sent.com>
On Fri, Apr 26, 2024 at 12:02 PM 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().
>
> Suggested-by: David Hildenbrand <david@redhat.com>
> Signed-off-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Yang Shi <shy828301@gmail.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);
> break;
> case RMAP_LEVEL_PMD:
> atomic_dec(&folio->_large_mapcount);
> @@ -1532,6 +1535,8 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
> nr = 0;
> }
> }
> +
> + partially_mapped = nr < nr_pmdmapped;
> break;
> }
>
> @@ -1553,9 +1558,10 @@ 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_anon(folio) &&
> + list_empty(&folio->_deferred_list) &&
> + partially_mapped)
> + deferred_split_folio(folio);
> }
>
> /*
>
> base-commit: 3dba658670af22074cc6f26dc92efe0013ac3359
> --
> 2.43.0
>
next prev parent reply other threads:[~2024-04-26 20:43 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-04-26 19:02 [PATCH v5] mm/rmap: do not add fully unmapped large folio to deferred split list Zi Yan
2024-04-26 19:08 ` David Hildenbrand
2024-04-26 19:20 ` Zi Yan
2024-04-26 20:15 ` David Hildenbrand
2024-04-26 20:22 ` Zi Yan
2024-04-27 4:06 ` Lance Yang
2024-04-27 6:51 ` David Hildenbrand
2024-04-27 9:32 ` Barry Song
2024-04-26 20:42 ` Yang Shi [this message]
2024-04-27 4:09 ` Lance Yang
2024-05-01 13:24 ` Alexander Gordeev
2024-05-01 13:38 ` Zi Yan
2024-05-01 15:54 ` David Hildenbrand
2024-05-02 13:18 ` Alexander Gordeev
2024-05-02 13:20 ` Zi Yan
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