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[99.254.121.117]) by smtp.gmail.com with ESMTPSA id d16-20020a0cfe90000000b00677f3081deesm126346qvs.60.2023.11.23.09.59.29 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 23 Nov 2023 09:59:30 -0800 (PST) Date: Thu, 23 Nov 2023 12:59:27 -0500 From: Peter Xu To: Christoph Hellwig Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Mike Kravetz , "Kirill A . Shutemov" , Lorenzo Stoakes , Axel Rasmussen , Matthew Wilcox , John Hubbard , Mike Rapoport , Hugh Dickins , David Hildenbrand , Andrea Arcangeli , Rik van Riel , James Houghton , Yang Shi , Jason Gunthorpe , Vlastimil Babka , Andrew Morton Subject: Re: [PATCH RFC 05/12] mm/gup: Fix follow_devmap_p[mu]d() to return even if NULL Message-ID: References: <20231116012908.392077-1-peterx@redhat.com> <20231116012908.392077-6-peterx@redhat.com> 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: rspam06 X-Rspamd-Queue-Id: AE3A4100019 X-Stat-Signature: 4j94bidpzt4efg9c41yqep3yfxdkxf6j X-HE-Tag: 1700762375-784309 X-HE-Meta: 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 wCHGzGY6 QbLQewEi1q8Pt3Z/mp+kSbIrKVKhGxX4yr8Dzl3FNffLMGLL/07zctiVoFK5mG5YaOUKSWy4amPKk/QBkQmptNviCX0w58ex5R8xEiSU+9oWUIkdwOOxkVXAqKOxySlS5GPCEe+WRxj/oSnFQMa3FmR67yZR3+LeKhJ+fVvsKoebU5FkRZJ55DtVMTS8ErsJ0p5PYCAE6hKZH6qy8BVHeriXIPUXZDQtgnExvWpR6qbIbHjNqDA6/lG32D96WxQnnHQH/ZfEsgdMWmkI7eyjtUCKPO+fIes+HZ5lHUgi/Vov3+QZGO6vBXhCpYQ0KArjzOO8QsNyubi6qgPDV86apSHfR/WYCG9HOpuyojylkJw5iWBlfUMAvENOQsdkxrcbyy4Em2pQJe/R9/SR0KTe2ht8WjzhIVIDncf5Vfazo56yain9px4cg3oYg0A== 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 Wed, Nov 22, 2023 at 11:25:28PM -0800, Christoph Hellwig wrote: > On Wed, Nov 15, 2023 at 08:29:01PM -0500, Peter Xu wrote: > > This seems to be a bug not by any report but by code observations. > > > > When GUP sees a devpmd or devpud, it should return whatever value returned > > from follow_devmap_p[mu]d(). If page==NULL returned, it means a fault is > > probably required. Skipping return the NULL should allow the code to fall > > through, which can cause unexpected behavior. > > > > It was there at least before the follow page rework (080dbb618b) in 2017, > > so 6 years. Not yet digging for a Fixes, assuming it can hardly trigger > > even if the logical bug does exist. > > > > Signed-off-by: Peter Xu > > I'd still add a fixes tag and send if off to Linux for 6.7-rc instead > of letting it linger. Will do. > > Otherwise looks good: > > Reviewed-by: Christoph Hellwig When rethinking, "return page" is correct for devmap, but it should be better to always use no_page_table() instead for page==NULL when trying to return NULL for follow_page(), even if no_page_table() should constantly return NULL for devmap, afaict, so it should be the same. So I'll make it slightly different when reposting separately, please feel free to have another look. Thanks, -- Peter Xu