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[70.30.145.63]) by smtp.gmail.com with ESMTPSA id d194-20020a3768cb000000b006fb9bbb071fsm4136265qkc.29.2023.01.25.12.13.03 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 25 Jan 2023 12:13:04 -0800 (PST) Date: Wed, 25 Jan 2023 15:13:02 -0500 From: Peter Xu To: Mike Kravetz Cc: linux-mm@kvack.org, Naoya Horiguchi , David Rientjes , Michal Hocko , Matthew Wilcox , David Hildenbrand , James Houghton , Muchun Song Subject: Re: A mapcount riddle Message-ID: References: MIME-Version: 1.0 In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline X-Rspam-User: X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 23FEE120019 X-Stat-Signature: s9wcfiqnijhukcu4gz83c6ps5178nngh X-HE-Tag: 1674677587-34659 X-HE-Meta: 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 MikKFW1h 6CYmP0d5jeVJ89pRoHeU5b8vovM/FJ+Czep8hrU2o8nzkULkbXps9DKDh/6s+I8jJkLcnuD2bvkFUIDJYJVqrqR4M1UeufciYx9KfkRS1nNaHQqGLWsWekDzmMceupRDi8aCV+XvSnAdYfXKMe7VkZfug+qciItKP09qxrpDBerf+kpmA3qf3yzSPsRc6MVGdaEl2n/r38OcDM8mtrRVjpvrQv6zLJ2QRCE4Ok9OPUE0VaQkv4QaDO2pw6DjCqEbAf1+89xum2DgG25+UC2OfuREVW21UEG9mxf9lNxnNinLZ3ugjHD5Y4Fkrg6sKIhA7fr/9fKiUEGxoYR6LzTFB1fgB0lGLqkKSI77KoYWKdheEXPZxi55bEbvci89FapHCrJKP 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: On Wed, Jan 25, 2023 at 10:16:58AM -0800, Mike Kravetz wrote: > On 01/25/23 11:46, Peter Xu wrote: > > On Tue, Jan 24, 2023 at 03:35:38PM -0800, Mike Kravetz wrote: > > > On 01/24/23 18:00, Peter Xu wrote: > > > > On Tue, Jan 24, 2023 at 12:56:24PM -0800, Mike Kravetz wrote: > > > > Besides, I'm also curious on the planned fix too regarding the two issues > > > > mentioned. > > > > > > My planned 'fix' is to simply check for shared a PMD > > > (page_count(virt_to_page(pte))) to determine if page with mapcount == 1 > > > is shared. > > > > I think having the current pte* won't easily work, we'll need to walk all > > the pgtable that mapped this page. > > > > To be explicit, one page can be mapped at pgtable1 which is shared by proc1 > > & proc2, and it can also be mapped at pgtable2 which is shared by proc3 & > > proc4. Then (assuming pte1* points to pgtable1): > > > > page_count(virt_to_page(pte1)) + page_mapcount(page) > > > > Won't be the right mapcount we're looking for. > > That assumes we want an accurate mapcount. In the two code segments I pointed > out, we only need to know if more than one process maps the page. We can > get that with 'page_count(virt_to_page(pte)) + page_mapcount(page)'. I see. Yes it sounds working. We can check mapcount==1 first and only go for the pgtable if mapcount>1, then we also know that's the only pmd it got mapped at. > > For now (and stable releases), I propose just fixing the improper behavior. > If we really need an accurate mapcount in the shared PMD case (and I am not > 100% sure we do), then we can add code to maintain or compute that. > > Part of my reason for sending this email was to determine if there may > be some places where we really do need an accurate mapcount in the shared > PMD case. I have not found any, but could have easily missed something. Nothing I noticed either. Thanks, -- Peter Xu