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Sun, 21 Apr 2024 19:45:00 -0700 (PDT) Received: from [192.168.255.10] ([43.132.141.20]) by smtp.gmail.com with ESMTPSA id fq9-20020a056a0060c900b006e6b3c4e70bsm7026267pfb.171.2024.04.21.19.44.58 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sun, 21 Apr 2024 19:45:00 -0700 (PDT) Message-ID: <1dd938aa-3066-4e3d-a7eb-e1dd83ebbf25@gmail.com> Date: Mon, 22 Apr 2024 10:44:56 +0800 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v1] mm/ksm: remove page_mapcount() usage in stable_tree_search() To: David Hildenbrand , linux-kernel@vger.kernel.org Cc: linux-mm@kvack.org, Andrew Morton , Alex Shi References: <20240416172533.663418-1-david@redhat.com> Content-Language: en-US From: Alex Shi In-Reply-To: <20240416172533.663418-1-david@redhat.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: B426840004 X-Stat-Signature: i3s1ww7pdi7yzochhsgyrgwfoko3tmkt X-Rspam-User: X-HE-Tag: 1713753901-161124 X-HE-Meta: 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 D9e76/49 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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 4/17/24 1:25 AM, David Hildenbrand wrote: > We want to limit the use of page_mapcount() to the places where it is > absolutely necessary. > > If our folio has a stable node, it is a (small) KSM folio -- see > folio_stable_node(). Let's use folio_mapcount() in stable_tree_search() > instead, which results in no functional change. > > The mapcount > 1 check is a bit confusing, because that's usually a check > for page sharing. Looks like the reason is that we are guaranteed to not > exceed ksm_max_page_sharing for the tree KSM folio when merging with > that. Let's update the documentation to make that clearer. > > Cc: Andrew Morton > Cc: Alex Shi (tencent) > Signed-off-by: David Hildenbrand > --- > mm/ksm.c | 13 ++++++++----- > 1 file changed, 8 insertions(+), 5 deletions(-) > > diff --git a/mm/ksm.c b/mm/ksm.c > index 486c9974f8e20..159604ad47799 100644 > --- a/mm/ksm.c > +++ b/mm/ksm.c > @@ -1909,12 +1909,15 @@ static struct page *stable_tree_search(struct page *page) > if (page_node) { > VM_BUG_ON(page_node->head != &migrate_nodes); > /* > - * Test if the migrated page should be merged > - * into a stable node dup. If the mapcount is > - * 1 we can migrate it with another KSM page > - * without adding it to the chain. > + * If the mapcount of our migrated KSM folio is > + * at most 1, we can merge it with another > + * KSM folio where we know that we have space > + * for one more mapping without exceeding the > + * ksm_max_page_sharing limit: see > + * chain_prune(). This way, we can avoid adding > + * this stable node to the chain. > */ > - if (page_mapcount(page) > 1) > + if (folio_mapcount(folio) > 1) > goto chain_append; > } > LGTM Reviewed-by: Alex Shi