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Fri, 26 Apr 2024 21:09:47 -0700 (PDT) MIME-Version: 1.0 References: <20240426190253.541419-1-zi.yan@sent.com> In-Reply-To: <20240426190253.541419-1-zi.yan@sent.com> From: Lance Yang Date: Sat, 27 Apr 2024 12:09:36 +0800 Message-ID: Subject: Re: [PATCH v5] mm/rmap: do not add fully unmapped large folio to deferred split list To: Zi Yan Cc: Andrew Morton , linux-mm@kvack.org, "Matthew Wilcox (Oracle)" , Yang Shi , Ryan Roberts , Barry Song <21cnbao@gmail.com>, David Hildenbrand , linux-kernel@vger.kernel.org Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspam-User: X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: C3A904000A X-Stat-Signature: git5ogo1n1dd1pezec8y6d1thz3j4w7d X-HE-Tag: 1714190989-877455 X-HE-Meta: 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 Bpz2LFW4 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Sat, Apr 27, 2024 at 3:02=E2=80=AFAM Zi Yan wrote: > > From: Zi Yan > > 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 > Signed-off-by: Zi Yan Reviewed-by: Lance Yang Thanks, Lance > --- > 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(str= uct folio *folio, > { > atomic_t *mapped =3D &folio->_nr_pages_mapped; > int last, nr =3D 0, nr_pmdmapped =3D 0; > + bool partially_mapped =3D 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(str= uct folio *folio, > nr++; > } > } while (page++, --nr_pages > 0); > + > + partially_mapped =3D !!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(str= uct folio *folio, > nr =3D 0; > } > } > + > + partially_mapped =3D nr < nr_pmdmapped; > break; > } > > @@ -1553,9 +1558,10 @@ static __always_inline void __folio_remove_rmap(st= ruct 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 =3D=3D RMAP_LEVEL_PTE || nr < nr_pmdmap= ped) > - 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 >