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[79.242.56.157]) by smtp.gmail.com with ESMTPSA id t11-20020adfe10b000000b00347edb20b6esm4119390wrz.106.2024.04.16.03.47.08 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 16 Apr 2024 03:47:09 -0700 (PDT) Message-ID: <3ef81fe3-2584-4db8-ab66-eaa44c035707@redhat.com> Date: Tue, 16 Apr 2024 12:47:07 +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> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 62D57C0005 X-Stat-Signature: cmskj53kog5n44rjdfmp5z1bwds8pgzh X-Rspam-User: X-HE-Tag: 1713264433-890361 X-HE-Meta: U2FsdGVkX1/E2z6+3TO58MfZ7w+i6MOT0NWGhiG3MHyXR1pFv2A2Qy1tvEk9HeRZ+VhN9iDvKu2utyIENBpsTgszc6EgHcKLjvgpCCqEdnYMXT3aXK61ijX8SzUcCW8rQCKDQrv6gMTsNmehpRdMUyri0Wjuen2JdLiHMas0u3qgvBVI90y52XwF9MscbX1ET8NjWwJZWyHl65DU9Pj9ZgxO80asECWzCNhyq91ZOSJG4znoxRgUYYXA1LjlvBokUrXFp+2h3JpDi4aDJZ0/yQWchGSTV7Fe5vnwT5pz3dRgDLqejTnhVZ8I0LvjUv8aisGEFM20srN2q/22y51rzG8st4tkCKiFKm/9YUds/MZdH8xCpQUusS/zIj+oZTHkNyfSGRubLzFH0+5NQOd5z/oAq3YSUF8cilGZKKjSWQKPL+F4V7fDggSbhpBoJKwyu/ZEpy4ipMswDtXJEXBZ/8VDiriebTs97S6veUoi0fIcOVjR6EifypuGVs7ggeWmEJfys3Sl/xZO6GYM+AkcOQegGnxEYP0yCt3YBSkgKZWpwY3NPKi7Bn0HciMwtZ5+ZM/gq2R0vfkm8eq7xI9P6XRBvIedftaEB9DZhhK1ZMUYdqIkbJhybeVpS3LAP8lUBkW7JMXRk7KwUhUfOrHwpb8KUdJ6nDw3wpB1c3/9+hnNYppNE0UvzUa9fMmCOi5RKU5RZlodd77qXEPTkd8RYf1g53tasHi51hyvno1FFwzZRz/GpTy/VCr3pOnUMvqtJHVMT16+UJLBjo4qVS8mTPtZEA7QlCtjcJPxnkfNhMfRe8FHisiOB6f8CCoOi1JPah4CvB3EhAfXsQeyhlrB/iFgu2ZRxbKfS8BJN/R0okBqTfjfG2HrkQnahQ3IgSolwbdvsYVwxzEWyfmin+8pLUFF90NxOrhMlqpvr13CC69JHJWKmnZ6hvqxUZN99ZAo8G/m+CUtmJVyWEcsN51 rhpU0UUF tw9zKWWybsW+6yUhI/CPYDp5QblZ6i8PbD5qKJ6Ckv2tma/NIbilxqfj3OIUYLe85lu5BR2WNXLgb9dPeP5dWT+Gn84cGxAbwWl9YIRjuWAahFw/tn2k/eAJe2JhKJPliMD4LX1y9Ln0AaX5w16IkHdoswv5hCS6a/u6bvPtE4O5dp843IgTrZSguTwDjXd9U8x6DW+vBqaVONotTzCHrHQ2Xn5yDmmkDcFic/hdQIz0L7qPwmbLnLU0m66081hkj7BZD2+3TtIA8Ee7PCAKYkYPAwufd6rvlts56VHVlh9AmLawSZTpd1jnEvsI5QlBHs8Sn7Gmy6hkN1UzS/wXotYTfWv9zVjOjrLwdmhkDkdDGeVJdOCUhW7J5J3yD8QtWzNRjPo01WAhkdMTkVf1ruD8TBQbH4VzuzaOI93jZ8Lx4P6ijw/+ziPEh3o4dfgW9ega8TrLgvT9vbF6jZYi87XqnciGyCBPUhsu5/vjJKnRwEp0= 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 12:40, 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. 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? -- Cheers, David / dhildenb