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AJvYcCWbMqd1g1wE4XmcJXGlzUyijAx+mhmawvSgjG59y0C+bvZlXqoh03BqyzSG1nlXnrRDn85piwIHWw==@kvack.org X-Gm-Message-State: AOJu0YyxK2Gflbv8OIYHAWhQSLybLBkC01700IOL6ASFybA/VXm3Uica VThfX6XHWJdmnx2m+giF6x+gCP7+9ik3vwEeqvp2rgigMyeyxS/BEvijy7BwZG621A+31Lab93e YryzEsQj3hdXok2vnlF0aBdu+aUFXn+s= X-Gm-Gg: ASbGnctpFrtbuULmspukDTZ3IKYUPW3Yc5F836HuTtcgu2nuxY8+OOpLsJF/V137Jn3 3oNkpf52DaoLbVLBIwZu7xQRtDBJYjcWyaiWqMJHQoNh+MUD4xF67O6ghU7fxIBYRIphz+qVowj nUHxVor6uTNPy7mESjeZ7aZj4I1t60F1f/NLbVimJQ9RZ+uudypTApOtXVfwmr7wh7oXkXsoA8m 4mvOh3D2puLmEck1s5JVrtgQe/JpUwwejRcNy12Ro2/QtY= X-Google-Smtp-Source: AGHT+IEj5hCxNC661gWndefrK0QFWrEBg3qrCVagAwTeaWebQwq+mCJEkQC4fnaq2I7BeFFFGHNJ6BjP9R+zaCJ8vC4= X-Received: by 2002:a05:600c:1f13:b0:45d:d79c:7503 with SMTP id 5b1f17b1804b1-45f2128cc58mr44251025e9.12.1757714359396; Fri, 12 Sep 2025 14:59:19 -0700 (PDT) MIME-Version: 1.0 References: <20250909010007.1660-1-alexei.starovoitov@gmail.com> <20250909010007.1660-6-alexei.starovoitov@gmail.com> In-Reply-To: From: Alexei Starovoitov Date: Fri, 12 Sep 2025 14:59:08 -0700 X-Gm-Features: AS18NWB0F8pNvh3GzUKMDxADrrXIaRl6ltZK30yA_21OC0u6XClr7XbCRoVpX0s Message-ID: Subject: Re: [PATCH slab v5 5/6] slab: Reuse first bit for OBJEXTS_ALLOC_FAIL To: Shakeel Butt 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 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Stat-Signature: 9jw3ob7hwz1nes9ng34qrdqbnw7mw5j5 X-Rspamd-Queue-Id: E96FA180003 X-Rspam-User: X-Rspamd-Server: rspam03 X-HE-Tag: 1757714360-866783 X-HE-Meta: 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 Dj5WXvIC PF+WNDI4Ev3rQz0kKBS/sRsGRLEzqwm9/RCLGi2On/lVxNp4YspqLBqrX+RYZW0k4DKDvRwB+HwugIwoplh+vgV1ILxQHxs8FCBAibdkWovwM0daEiCh9YN0hjoeGynoy5m38zv/Qu8rSQC+Hmhi9E4qrgW6GnWybkYGKqGprdm7ENHth1ASvLxiDvAOhjTwMpKAtmYC30W3qbAGi3vI9y3Ba/ZgAlwrkXY5LJs1WJPgBdBwVF0H1nskeu/yZNK8a8QENumFFd0QGMpPK0gyOPWVoh91k2SCnrWtpBSUktYO5oGmjmJxzbC6wfo2e8Yu66bBePLgeSE2kyDFGPSiHuDnfQriYm5KdPK4m/iId0sHdTpB6XQcq310O5+6oDk+vqCZyxd9F/KjnqPNLffsMYA4cn73x/y9n72P64A6swk0IvBIHk1IN1vbbykFC5Ok2XMLg31MxgY7VwGKo457n824nBKw1xHnNI3wejdPjiH57N+8= 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 2:44=E2=80=AFPM Shakeel Butt wrote: > > On Fri, Sep 12, 2025 at 02:31:47PM -0700, Alexei Starovoitov wrote: > > On Fri, Sep 12, 2025 at 2:29=E2=80=AFPM Shakeel Butt wrote: > > > > > > On Fri, Sep 12, 2025 at 02:24:26PM -0700, Alexei Starovoitov wrote: > > > > On Fri, Sep 12, 2025 at 2:03=E2=80=AFPM Suren Baghdasaryan wrote: > > > > > > > > > > On Fri, Sep 12, 2025 at 12:27=E2=80=AFPM Shakeel Butt wrote: > > > > > > > > > > > > +Suren, Roman > > > > > > > > > > > > On Mon, Sep 08, 2025 at 06:00:06PM -0700, Alexei Starovoitov wr= ote: > > > > > > > From: Alexei Starovoitov > > > > > > > > > > > > > > Since the combination of valid upper bits in slab->obj_exts w= ith > > > > > > > OBJEXTS_ALLOC_FAIL bit can never happen, > > > > > > > use OBJEXTS_ALLOC_FAIL =3D=3D (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 h= ave > > > > > > 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 wit= h the > > > > > same field (page->memcg_data and slab->obj_exts are aliases). Eve= n 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 ag= ainst > > > > > 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 th= at > > > in the commit message. > > > > Disgaree. It's not a job of a small commit to summarize all options > > that were discussed on the list. That's what the cover letter is for > > and there there are links to all previous threads. > > Currently the commit message is only telling what the patch is doing and > is missing the 'why' part and I think adding the 'why' part would make it > better for future readers i.e. less effort to find why this is being > done this way. (Anyways this is just a nit from me) I think 'why' here is obvious. Free the bit to use it later. >From time to time people add a sentence like "this bit will be used in the next patch", but I never do this and sometimes remove it from other people's commits, since "in the next patch" is plenty ambiguous and not helpful.