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Tue, 5 Aug 2025 09:20:39 +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 ci90M+fMkWhuSgAAD6G6ig (envelope-from ); Tue, 05 Aug 2025 09:20:39 +0000 Message-ID: <3c6100a1-78a3-4211-a62d-eea07db91b62@suse.cz> Date: Tue, 5 Aug 2025 11:20:48 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/1] mm: Correct misleading comment on mmap_lock field in mm_struct To: Adrian Huang , Andrew Morton Cc: David Hildenbrand , Lorenzo Stoakes , Liam.Howlett@oracle.com, Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Feng Tang , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Adrian Huang References: <20250805084726.2054-1-ahuang12@lenovo.com> Content-Language: en-US From: Vlastimil Babka In-Reply-To: <20250805084726.2054-1-ahuang12@lenovo.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: A8F91120004 X-Stat-Signature: p9jjh8dbe7zgz4wwbt95oqpe7fc5jcna X-Rspam-User: X-Rspamd-Server: rspam07 X-HE-Tag: 1754385641-628906 X-HE-Meta: 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 NrbNU/dx MiymxU6WdmKPIlrsnXqFRsczIJi8HlYAJZrexUJAQDSBRtEqq93F2UXRdCMUeR+B+Cc4BY9RpYErTb88sGbeACn9jGoVt1D2IxAsar/G3ummUjpsmLwd3/nCVoyvrJm3166yiHR+zFKPe7DGPXPu6Zj7Gj6kXdagWYBVIVjN9IwisV3UWmit+1J1GCLmllRzHKJbnmEbpKIKHIdXJf5aO9n0UKaBCw/2mBaDS60YCmZ9MlR6c3kD6NhX6aNX0rfEojXZuD3u7T8pY9vUBddgNgg4s8PIbUayPNyz1gpoe2Rwhn8iObqeovLGJEw24JOtJMDlqs36qHcPvtxCKU1GmeIChndu15kaM4IU2rAL1nG86sSEVzH9wRLCf3N4ojnqcmodBj3QP5j2K6FmiiybrxE8ZB/PyeV0nn6DbpNr9qo30RVZ7uQl9GM/8WfVAskbBzkkKgmQWg+E+BTsbH8xqMXp6hw== 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 8/5/25 10:47 AM, Adrian Huang wrote: > From: Adrian Huang > > The comment previously described the offset of mmap_lock as 0x120 (hex), > which is misleading. The correct value is 120 (decimal), and using '0x120' > could confuse readers trying to understand why the count and owner fields > reside in separate cachelines. > > This change also removes an unnecessary space for improved formatting. > > Fixes: 2e3025434a6b ("mm: relocate 'write_protect_seq' in struct mm_struct") > Signed-off-by: Adrian Huang That seems all true so Acked-by: Vlastimil Babka But I wonder why we just hope this remains true "with some kernel config" and don't employ some explicit alignment to make sure it's true (except perhaps with some debug options like lockdep bloating the structures, but we don't care about perfmance in such configs). > --- > include/linux/mm_types.h | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h > index 1ec273b06691..ec90bbf22e2b 100644 > --- a/include/linux/mm_types.h > +++ b/include/linux/mm_types.h > @@ -1027,9 +1027,9 @@ struct mm_struct { > */ > /* > * With some kernel config, the current mmap_lock's offset > - * inside 'mm_struct' is at 0x120, which is very optimal, as > + * inside 'mm_struct' is at 120, which is very optimal, as > * its two hot fields 'count' and 'owner' sit in 2 different > - * cachelines, and when mmap_lock is highly contended, both > + * cachelines, and when mmap_lock is highly contended, both > * of the 2 fields will be accessed frequently, current layout > * will help to reduce cache bouncing. > *