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Fri, 21 Mar 2025 04:25:33 -0700 (PDT) X-Google-Smtp-Source: AGHT+IFhnSanbQkjja3u7vjcfqJx/EYbJW2GJpG5lYb8dvzNwA2Xr5Zjdg+wi9FI4uk1RPAbUvu8AQ== X-Received: by 2002:a17:90b:4d08:b0:2fa:f8d:65de with SMTP id 98e67ed59e1d1-3030fefd8bcmr3349300a91.22.1742556333271; Fri, 21 Mar 2025 04:25:33 -0700 (PDT) Received: from [192.168.68.55] ([180.233.125.167]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-301bf59e2desm5742121a91.23.2025.03.21.04.25.29 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 21 Mar 2025 04:25:32 -0700 (PDT) Message-ID: <5ec97dd6-6561-4687-ac94-41c63ffc82cf@redhat.com> Date: Fri, 21 Mar 2025 21:25:27 +1000 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 0/2] Fix parameter passed to page_mapcount_is_type() To: David Hildenbrand , Vlastimil Babka , linux-mm@kvack.org Cc: linux-kernel@vger.kernel.org, akpm@linux-foundation.org, willy@infradead.org, osalvador@suse.de, gehao@kylinos.cn, shan.gavin@gmail.com References: <20250321053148.1434076-1-gshan@redhat.com> <0d096764-302f-4b80-a867-22f5302b8045@redhat.com> From: Gavin Shan In-Reply-To: <0d096764-302f-4b80-a867-22f5302b8045@redhat.com> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: -xvW3cY3WOe4kAAhDhUmUi7Pf1VWAz3SiH0VYAFLffg_1742556334 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: rspam07 X-Rspamd-Queue-Id: 72722C0018 X-Stat-Signature: h1tws38ipquwtqko3poqgigjgg8dqbcm X-Rspam-User: X-HE-Tag: 1742556339-272015 X-HE-Meta: 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 jkaaNHOt 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000758, 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 3/21/25 8:11 PM, David Hildenbrand wrote: > On 21.03.25 10:23, Vlastimil Babka wrote: >> On 3/21/25 06:31, Gavin Shan wrote: >>> Found by code inspection. There are two places where the parameter >>> passed to page_mapcount_is_type() is (page->__mapcount), which is >>> correct since it should be one more than the value, as explained in >>> the comments to page_mapcount_is_type(): (a) page_has_type() in >>> page-flags.h (b) __dump_folio() in mm/debug.c >> >> IIUC you are right. Luckily thanks to the the PGTY_mapcount_underflow limit, >> this off-by-one error doesn't currently cause visible issues i.e. >> misclassifications legitimate mapcount as page type and vice versa, right? >> We'd have to have a mapcount underflown severely right to the limit to make >> that off-by-one error cross it? > > Agreed. Likely not stable material because it isn't actually fixing anything (because of the safety gaps). > Yes, it shouldn't cause any visible impacts so far due to the gap. I just found the issue by code inspection. Lets drop the fix tags in v2. >> >> I wonder if a more future-proof solution would be to redefine >> page_mapcount_is_type() instead to not subtract. But I'll leave that to willy. > > With upcoming changes around that, likely best to leave that alone. I expect page_mapcount_is_type() to completely vanish. > +1 to remove page_mapcount_is_type(). After Willy confirms, I can post an extra series to do it if needed. Thanks, Gavin