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([2a09:80c0:192:0:5dac:bf3d:c41:c3e7]) by smtp.gmail.com with ESMTPSA id b13-20020adfe30d000000b003113ed02080sm1367445wrj.95.2023.08.10.01.37.05 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 10 Aug 2023 01:37:05 -0700 (PDT) Message-ID: <7e31254d-8889-7e79-50e1-2630bd493d59@redhat.com> Date: Thu, 10 Aug 2023 10:37:04 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.13.0 To: Matthew Wilcox , Peter Xu Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-doc@vger.kernel.org, Andrew Morton , Jonathan Corbet , Mike Kravetz , Hugh Dickins , Ryan Roberts , Yin Fengwei , Yang Shi , Zi Yan References: <20230809083256.699513-1-david@redhat.com> From: David Hildenbrand Organization: Red Hat Subject: Re: [PATCH mm-unstable v1] mm: add a total mapcount for large folios In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 7351140013 X-Stat-Signature: 1nkh1n53td8etqotkdw4pqhe6cscmr4o X-Rspam-User: X-HE-Tag: 1691656630-329849 X-HE-Meta: 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 f9LurBlZ voBnV/Vr8sJwhMqCVDp/g+sqlbaTooerciwvECknxbN1i0WMwNSamEAzHBXgZKMKqbYcFgRAsCC/60Fm5iscWQcSH1j2fQfwHA82aoMNwNuKghouWSJql9LW1lwRZK0hHVI3CnjK9j7VKoRikh/hcFP5W44AurrGBcPo5un0GJYZ81umoivB2lpejYU+XPNVim5fuwtOfeNDG1EFh3ZWzUJDAUtJVQivRv7XfE0tInIzI/sSCPvwBOb9+SEMhXigHAq00iLM4ceI6oj00riSfWlsVyaKN3MX+AKJ7l7dzdVXvOETlkI1XAYWKFPiSLx4Zx7Bh2dFtpeIey/sAYHIV5dWeUUrqz+ANMp5kcLCsWIt8lJT+pdrfKy6EAweoRxkJK8CO 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 10.08.23 05:25, Matthew Wilcox wrote: > On Wed, Aug 09, 2023 at 05:23:46PM -0400, Peter Xu wrote: >> Hi, David, >> >> Some pure questions below.. >> >> On Wed, Aug 09, 2023 at 10:32:56AM +0200, David Hildenbrand wrote: >>> Let's track the total mapcount for all large folios in the first subpage. >>> >>> The total mapcount is what we actually want to know in folio_mapcount() >>> and it is also sufficient for implementing folio_mapped(). This also >>> gets rid of any "raceiness" concerns as expressed in >>> folio_total_mapcount(). >> >> Any more information for that "raciness" described here? > > UTSL. > > /* > * Add all the PTE mappings of those pages mapped by PTE. > * Limit the loop to folio_nr_pages_mapped()? > * Perhaps: given all the raciness, that may be a good or a bad idea. > */ > Yes, that comment from Hugh primarily discusses how we could possibly optimize the loop, and if relying on folio_nr_pages_mapped() to reduce the iterations would be racy. As far as I can see, there are cases where "it would be certainly a bad idea" :) In the other comment in that function, it's also made clear what the traditional behavior with PMD-mappable THP was "In the common case, avoid the loop when no pages mapped by PTE", which will no longer hold with sub-PMD THP. -- Cheers, David / dhildenb