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Wed, 24 Apr 2024 15:54:00 -0700 (PDT) MIME-Version: 1.0 References: <20240424211031.475756-1-zi.yan@sent.com> In-Reply-To: From: Yang Shi Date: Wed, 24 Apr 2024 15:53:49 -0700 Message-ID: Subject: Re: [PATCH v2] mm/rmap: do not add fully unmapped large folio to deferred split list To: Zi Yan Cc: linux-mm@kvack.org, Andrew Morton , "Matthew Wilcox (Oracle)" , 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-Stat-Signature: sn8u1qtscq3zfdxzinn1s7qum8qsuwyw X-Rspamd-Queue-Id: 61D5B40007 X-Rspamd-Server: rspam06 X-Rspam-User: X-HE-Tag: 1713999242-573890 X-HE-Meta: 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 uP+xaJei EVB0dLC4uFRCjSrYcwii6bFQo+Y65dtudEMuw6RX/nZYoG8OxuprA+nU6cSy7/8DauMuIQv159zqdm5jl6UaKofZ6ybBjZIzZwt66KRsWb5LCMf+q3Jmpha+MCd4AOvwLCNvhoYgmQx1dyULe5RdkLy64nIkoYh9FDE73q506on7bXftK1t8ncM8S7hJ/M+t5stc7HTaqUqtu+RULdY7AdKittepbpa3JQCUJZviesSmOQoGgli3JXCXW2jQuQZi7qANjvLZFI0iZTLT2dZ59TLUvDpqFIP4ZOatO6fUY2BdBou5iauufeKZDXMlTIreVI//Uv5TRnFLshD8OJ72c/53UaciD1pBp16bXQWv9M6qgMpe54rZcp/pIhddKHYzOU95O7FqQN9L+xEcP1hgSfqKlBuXQqrNZJjmOePFDHzPACm5xZbRVEhy2UVEqAP7L3bIh2v4AynXxzq4= 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 Wed, Apr 24, 2024 at 3:39=E2=80=AFPM Zi Yan wrote: > > On 24 Apr 2024, at 18:32, Yang Shi wrote: > > > On Wed, Apr 24, 2024 at 2:10=E2=80=AFPM Zi Yan wrote: > >> > >> From: Zi Yan > >> > >> In __folio_remove_rmap(), a large folio is added to deferred split lis= t > >> if any page in a folio loses its final mapping. It is possible that > >> the folio is unmapped fully, but it is unnecessary to add the folio > >> to deferred split list at all. Fix it by checking folio->_nr_pages_map= ped > >> before adding a folio to deferred split list. If the folio is already > >> on the deferred split list, it will be skipped. > >> > >> 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 no= t > >> fix everything. A fully unmapped PTE-mapped order-9 THP was also added= to > >> deferred split list and counted as THP_DEFERRED_SPLIT_PAGE, since nr i= s > >> 512 (non zero), level is RMAP_LEVEL_PTE, and inside deferred_split_fol= io() > >> the order-9 folio is folio_test_pmd_mappable(). However, this miscount > >> was present even earlier due to implementation, since PTEs are unmappe= d > >> individually and first PTE unmapping adds the THP into the deferred sp= lit > >> list. > > > > Shall you mention the miscounting for mTHP too? There is another patch > > series adding the counter support for mTHP. > > OK, will add it. > > > >> > >> With commit b06dc281aa99 ("mm/rmap: introduce > >> folio_remove_rmap_[pte|ptes|pmd]()"), kernel is able to unmap PTE-mapp= ed > >> folios in one shot without causing the miscount, hence this patch. > >> > >> Signed-off-by: Zi Yan > >> --- > >> 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 =3D=3D RMAP_LEVEL_PTE || nr < nr_pmd= mapped) > >> - deferred_split_folio(folio); > >> + if (folio_test_large(folio) && folio_test_anon(folio) = && > >> + list_empty(&folio->_deferred_list) && > > > > Do we really need this check? deferred_split_folio() does the same > > check too. Bailing out earlier sounds ok too, but there may not be too > > much gain. > > Sure, I can remove it. > > > > >> + ((level =3D=3D RMAP_LEVEL_PTE && atomic_read(mappe= d)) || > >> + (level =3D=3D RMAP_LEVEL_PMD && nr < nr_pmdmapped= ))) > > > > IIUC, this line is used to cover the case which has both partial > > PTE-mapping and PMD-mapping, then PMD mapping is unmapped fully. IIRC > > this case was not handled correctly before, the THP actually skipped > > deferred split queue. If so please add some description in the commit > > log. > > It is properly handled before, since the original code is > (level =3D=3D RMAP_LEVEL_PTE || nr < nr_pmdmapped), meaning > if either level is RMAP_LEVEL_PTE or > (level =3D=3D RMAP_LEVEL_PMD && nr < nr_pmdmapped), the folio > is added to the deferred split list. So only level =3D=3D RMAP_LEVEL_PTE > part of logic needs to be fixed. Oh, yes. I misread "||" to "&&". Thanks for correcting me and fixing the problem. > > > > > Otherwise the patch looks good to me. Reviewed-by: Yang Shi > > > > > > Thanks. > >> + deferred_split_folio(folio); > >> } > >> > >> /* > >> > >> base-commit: 2541ee5668b019c486dd3e815114130e35c1495d > >> -- > >> 2.43.0 > >> > > > -- > Best Regards, > Yan, Zi