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From: Shakeel Butt To: Alexei Starovoitov Cc: Suren Baghdasaryan , bpf , linux-mm , Vlastimil Babka , Harry Yoo , Michal Hocko , Sebastian Sewior , Andrii Nakryiko , Kumar Kartikeya Dwivedi , Andrew Morton , Peter Zijlstra , Steven Rostedt , Johannes Weiner , Roman Gushchin Subject: Re: [PATCH slab v5 5/6] slab: Reuse first bit for OBJEXTS_ALLOC_FAIL Message-ID: References: <20250909010007.1660-1-alexei.starovoitov@gmail.com> <20250909010007.1660-6-alexei.starovoitov@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Migadu-Flow: FLOW_OUT X-Rspamd-Queue-Id: 9FA874000D X-Stat-Signature: sz3ys93np79kwqwfuazfh6y7m4hmbd66 X-Rspam-User: X-Rspamd-Server: rspam09 X-HE-Tag: 1757712593-476930 X-HE-Meta: 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 utqyiFFj WWryJ/8SOAEUHIvlNNouNV6yz2xskucJ5dztFo7gVcj4s8uVK0KRKNIAu3/1nmwIE7b6KLDYXgoMSvTA7HsEjwo4369CAkfOFAksMdVJGffNEpU1t8VRx6aFXsdSbm/vxjVsRw3c4AryIbHZZdBP6zEpTbSKK7O3Jegza3wIShRNDHVICoqX0vgswTMizConZnx7dOux+yjc4ybQj7ET82AQBRIFtHFD6ZqXF/LvH4EGbx2HJar6hOlR9qaUxL5jflsHebZwuICR32eINzk102TTLz6Royt+FWhV5fcGVz40Q1Jo16L5vFrj3UaS+IJyRGcStqTIUI41OzuilUXH1BrbETXFMIWJ8ET5rZ/GcoF5y0CDzmWChmcZewiqYCOF/uE4KVYqx46OiZhE= 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 Fri, Sep 12, 2025 at 02:24:26PM -0700, Alexei Starovoitov wrote: > On Fri, Sep 12, 2025 at 2:03 PM Suren Baghdasaryan wrote: > > > > On Fri, Sep 12, 2025 at 12:27 PM Shakeel Butt wrote: > > > > > > +Suren, Roman > > > > > > On Mon, Sep 08, 2025 at 06:00:06PM -0700, Alexei Starovoitov wrote: > > > > From: Alexei Starovoitov > > > > > > > > Since the combination of valid upper bits in slab->obj_exts with > > > > OBJEXTS_ALLOC_FAIL bit can never happen, > > > > use OBJEXTS_ALLOC_FAIL == (1ull << 0) as a magic sentinel > > > > instead of (1ull << 2) to free up bit 2. > > > > > > > > Signed-off-by: Alexei Starovoitov > > > > > > Are we low on bits that we need to do this or is this good to have > > > optimization but not required? > > > > That's a good question. After this change MEMCG_DATA_OBJEXTS and > > OBJEXTS_ALLOC_FAIL will have the same value and they are used with the > > same field (page->memcg_data and slab->obj_exts are aliases). Even if > > page_memcg_data_flags can never be used for slab pages I think > > overlapping these bits is not a good idea and creates additional > > risks. Unless there is a good reason to do this I would advise against > > it. > > Completely disagree. You both missed the long discussion > during v4. The other alternative was to increase alignment > and waste memory. Saving the bit is obviously cleaner. > The next patch is using the saved bit. I will check out that discussion and it would be good to summarize that in the commit message.