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Fri, 21 Mar 2025 09:23:18 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id sOGEIAYw3WeGGAAAD6G6ig (envelope-from ); Fri, 21 Mar 2025 09:23:18 +0000 Message-ID: Date: Fri, 21 Mar 2025 10:23:18 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 0/2] Fix parameter passed to page_mapcount_is_type() To: Gavin Shan , linux-mm@kvack.org Cc: linux-kernel@vger.kernel.org, akpm@linux-foundation.org, willy@infradead.org, david@redhat.com, osalvador@suse.de, gehao@kylinos.cn, shan.gavin@gmail.com References: <20250321053148.1434076-1-gshan@redhat.com> Content-Language: en-US From: Vlastimil Babka In-Reply-To: <20250321053148.1434076-1-gshan@redhat.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Action: no action X-Rspam-User: X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: CF5C5140009 X-Stat-Signature: py676ygtr4qrmkgap631ptbhgmdyp3ar X-HE-Tag: 1742549000-78457 X-HE-Meta: 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 xVgXXv33 vgMnawVyGDHyyrTnMFhsqah+i1Y0hG2YAV1Q1HjnxENqU8Y7mk9ld7/OUWREA+u5vTUHIPsg9Tl6aG6ul6ItuZoyr0yeocAOg4piIqqjiVggulrWz94s/WAWeLg37H5dtTncyx8ZL0tFXTmHiA+vLvuQlwvj0WIygeUrQAr+6CFb/7W0srfjcC3D06gV+ykvsI4sPMb8U8ANAZ3oGc/CevaF4eM2hg1FXSOLbxc5FnsK8wOKLS1xay3OzyRc7gJFpfgHLGuYuGY68mPpw8JDHXt23mYVc+bM0JEmelPnsy5bjANce830WSEATDliPBKpOCslxSiJdz8VjMpBWPbBrRMzP5U26rfYG/Bhj6DxZd2+v2W8sbhAUYwEkFZSgJ8K71yvewnSBxkvJiuMd4M9PMvSy7Q== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000032, 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 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? 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. > PATCH[1] fixes the parameter for (a) > PATCH[2] fixes the parameter for (b) > > Gavin Shan (2): > mm: Fix parameter passed to page_mapcount_is_type() > mm/debug: Fix parameter passed to page_mapcount_is_type() > > include/linux/page-flags.h | 2 +- > mm/debug.c | 4 ++-- > 2 files changed, 3 insertions(+), 3 deletions(-) >