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[79.242.56.157]) by smtp.gmail.com with ESMTPSA id l13-20020ac2430d000000b00518c0034e1esm808919lfh.50.2024.04.16.03.53.58 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 16 Apr 2024 03:54:00 -0700 (PDT) Message-ID: Date: Tue, 16 Apr 2024 12:53:57 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v1 05/18] mm: improve folio_likely_mapped_shared() using the mapcount of large folios To: Lance Yang Cc: akpm@linux-foundation.org, cgroups@vger.kernel.org, chris@zankel.net, corbet@lwn.net, dalias@libc.org, fengwei.yin@intel.com, glaubitz@physik.fu-berlin.de, hughd@google.com, jcmvbkbc@gmail.com, linmiaohe@huawei.com, linux-doc@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-sh@vger.kernel.org, linux-trace-kernel@vger.kernel.org, muchun.song@linux.dev, naoya.horiguchi@nec.com, peterx@redhat.com, richardycc@google.com, ryan.roberts@arm.com, shy828301@gmail.com, willy@infradead.org, ysato@users.sourceforge.jp, ziy@nvidia.com References: <20240409192301.907377-6-david@redhat.com> <20240416104008.41979-1-ioworker0@gmail.com> <3ef81fe3-2584-4db8-ab66-eaa44c035707@redhat.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: B8F3220012 X-Rspam-User: X-Stat-Signature: 9fkyny9qemik6y36t4god5r6br4eq94f X-Rspamd-Server: rspam01 X-HE-Tag: 1713264849-154411 X-HE-Meta: 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 3lw8IPeA 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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 16.04.24 12:52, Lance Yang wrote: > On Tue, Apr 16, 2024 at 6:47 PM David Hildenbrand wrote: >> >> On 16.04.24 12:40, Lance Yang wrote: >>> Hey David, >>> >>> Maybe I spotted a bug below. >> >> Thanks for the review! >> >>> >>> [...] >>> static inline bool folio_likely_mapped_shared(struct folio *folio) >>> { >>> - return page_mapcount(folio_page(folio, 0)) > 1; >>> + int mapcount = folio_mapcount(folio); >>> + >>> + /* Only partially-mappable folios require more care. */ >>> + if (!folio_test_large(folio) || unlikely(folio_test_hugetlb(folio))) >>> + return mapcount > 1; >>> + >>> + /* A single mapping implies "mapped exclusively". */ >>> + if (mapcount <= 1) >>> + return false; >>> + >>> + /* If any page is mapped more than once we treat it "mapped shared". */ >>> + if (folio_entire_mapcount(folio) || mapcount > folio_nr_pages(folio)) >>> + return true; >>> >>> bug: if a PMD-mapped THP is exclusively mapped, the folio_entire_mapcount() >>> function will return 1 (atomic_read(&folio->_entire_mapcount) + 1). >> >> If it's exclusively mapped, then folio_mapcount(folio)==1. In which case >> the previous statement: >> >> if (mapcount <= 1) >> return false; >> >> Catches it. > > You're right! > >> >> IOW, once we reach this point we now that folio_mapcount(folio) > 1, and >> there must be something else besides the entire mapping ("more than once"). >> >> >> Or did I not address your concern? > > Sorry, my mistake :( No worries, thanks for the review and thinking this through! -- Cheers, David / dhildenb