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Thu, 11 Apr 2024 12:01:30 -0700 (PDT) MIME-Version: 1.0 References: <20240411153232.169560-1-zi.yan@sent.com> In-Reply-To: From: Yang Shi Date: Thu, 11 Apr 2024 12:01:19 -0700 Message-ID: Subject: Re: [PATCH] mm/rmap: do not add fully unmapped large folio to deferred split list To: David Hildenbrand Cc: Zi Yan , linux-mm@kvack.org, Andrew Morton , "Matthew Wilcox (Oracle)" , Ryan Roberts , Barry Song <21cnbao@gmail.com>, linux-kernel@vger.kernel.org Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Queue-Id: 5D08F1A0025 X-Rspam-User: X-Rspamd-Server: rspam02 X-Stat-Signature: 3pbuzqsfgox4fy5cnanfwephyf4tdn3h X-HE-Tag: 1712862092-753639 X-HE-Meta: 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 4F2sLdEw XEZi2MGrSm3LBeJm0jx1MDvRkD6cRdLxLlUsv6vJpyECtnzwZatdwd+BtJDj8J/X2GP/biUAAcurnDGGtoYq4/wFRVMuE2yx4uYEX2SsQSfi+12YOvuJ2mi1vb+h+EucMFO191dhRjGdW8DsdthdVKHlD5a+/LZEFaEmhmozx678DXNKfdZDnaOszUR5r6l7ejLYVV/mNPdsLO95rYfFw+zUvScMkbfX/kYPrTdGqSDsD8I6itfLF2hGSBbrTSHfbGZmX8zFeFlUfPrq7VCzqXs8ZbstRaHlmVXQ0o5+gVSTBKBnIJhCnYwMYRJ1z9YSfd/MFOFZ5Je1aFDjhXthKRroIHq22SNN4DTIjnfvZiC3ANu2ua6uNME+5tqYyFlLE1pfjO7U4tpH7LLw= 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 Thu, Apr 11, 2024 at 8:46=E2=80=AFAM David Hildenbrand wrote: > > On 11.04.24 17:32, 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. 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 mapcount before > > adding a folio to deferred split list. > > > > Signed-off-by: Zi Yan > > --- > > mm/rmap.c | 9 ++++++--- > > 1 file changed, 6 insertions(+), 3 deletions(-) > > > > diff --git a/mm/rmap.c b/mm/rmap.c > > index 2608c40dffad..d599a772e282 100644 > > --- a/mm/rmap.c > > +++ b/mm/rmap.c > > @@ -1494,7 +1494,7 @@ static __always_inline void __folio_remove_rmap(s= truct folio *folio, > > enum rmap_level level) > > { > > atomic_t *mapped =3D &folio->_nr_pages_mapped; > > - int last, nr =3D 0, nr_pmdmapped =3D 0; > > + int last, nr =3D 0, nr_pmdmapped =3D 0, mapcount =3D 0; > > enum node_stat_item idx; > > > > __folio_rmap_sanity_checks(folio, page, nr_pages, level); > > @@ -1506,7 +1506,8 @@ static __always_inline void __folio_remove_rmap(s= truct folio *folio, > > break; > > } > > > > - atomic_sub(nr_pages, &folio->_large_mapcount); > > + mapcount =3D atomic_sub_return(nr_pages, > > + &folio->_large_mapcount) + 1= ; > > That becomes a new memory barrier on some archs. Rather just re-read it > below. Re-reading should be fine here. > > > do { > > last =3D atomic_add_negative(-1, &page->_mapcount= ); > > if (last) { > > @@ -1554,7 +1555,9 @@ static __always_inline void __folio_remove_rmap(s= truct folio *folio, > > * is still mapped. > > */ > > if (folio_test_large(folio) && folio_test_anon(folio)) > > - if (level =3D=3D RMAP_LEVEL_PTE || nr < nr_pmdmap= ped) > > + if ((level =3D=3D RMAP_LEVEL_PTE && > > + mapcount !=3D 0) || > > + (level =3D=3D RMAP_LEVEL_PMD && nr < nr_pmdma= pped)) > > deferred_split_folio(folio); > > } > > But I do wonder if we really care? Usually the folio will simply get > freed afterwards, where we simply remove it from the list. > > If it's pinned, we won't be able to free or reclaim, but it's rather a > corner case ... > > Is it really worth the added code? Not convinced. It is actually not only an optimization, but also fixed the broken thp_deferred_split_page counter in /proc/vmstat. The counter actually counted the partially unmapped huge pages (so they are on deferred split queue), but it counts the fully unmapped mTHP as well now. For example, when a 64K THP is fully unmapped, the thp_deferred_split_page is not supposed to get inc'ed, but it does now. The counter is also useful for performance analysis, for example, whether a workload did a lot of partial unmap or not. So fixing the counter seems worthy. Zi Yan should have mentioned this in the commit log. > > -- > Cheers, > > David / dhildenb >