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Wed, 07 May 2025 05:39:00 -0700 (PDT) MIME-Version: 1.0 References: <20250506-fork-fixes-v1-0-bd35b63f0f1b@linaro.org> <20250506-fork-fixes-v1-3-bd35b63f0f1b@linaro.org> <20250506185322.cd87ba823030024061b5ae77@linux-foundation.org> In-Reply-To: <20250506185322.cd87ba823030024061b5ae77@linux-foundation.org> From: Linus Walleij Date: Wed, 7 May 2025 14:38:47 +0200 X-Gm-Features: ATxdqUGSt_HwpVhMJ9lvyKbvUoH8KegjIyX1JMZ47lmGfUG1h6jo7U746E63no4 Message-ID: Subject: Re: [PATCH 3/5] fork: Remove assumption that vm_area->nr_pages equals to THREAD_SIZE To: Andrew Morton Cc: linux-mm@kvack.org, Pasha Tatashin Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspam-User: X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: CB10118000D X-Stat-Signature: y1tkcn3j7hd3jy6m7eswusys8zhteqs9 X-HE-Tag: 1746621542-757764 X-HE-Meta: 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 mCm1g7lF DtwljG6dwrDSEopRZPWuqC2K62pK28z7BEUNg0e2QPpV7+D6qFkCqlT3hUJ7JaDatvx+XUpOuBjO7UrJPipfgBmsREBddgDtmvVnNc5xrWmgUuUuqcqb18Cc8HIXRHOrQcIC7L6AQwSQ/hn2rzZqV7s44Qfsv8dTpvo6IzbbD2KHpfLeYzJlQZI9n5avUpZDz9+3JOV5AsQuKfIG5wX0m3GrODrqTGkVLZa+ZBhy18Rm/pxzLZrXRxJ+xglLAhOHdO0fhqSpWV3/rFNQO07HNvwp1648yYobXSwEvozHdnQBR+eg= 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, May 7, 2025 at 3:53=E2=80=AFAM Andrew Morton wrote: > > - BUG_ON(vm->nr_pages !=3D THREAD_SIZE / PAGE_SIZE); > > - > > - for (i =3D 0; i < THREAD_SIZE / PAGE_SIZE; i++) { > > + for (i =3D 0; i < vm_area->nr_pages; i++) { > > Well, constants are fast and small. I assume this will add a few > instructions and a bit of runtime overhead? That is true. Pashas original patch has a local variable instead of comparing vm_area->nr_pages, so I went back to that to ascertain we are not dereferencing the struct on every iteration of the loop, but will instead use a register. For this first instance the assignment of that variable should be no more time consuming than the deleted BUG_ON() dereference so it is +/- 0. For the other two instances I like the readability this gives, the local variable restored there too. Maybe those two should have also has BUG_ON() guards actually. vm_area->nr_pages should be the trusted, run-time value. I'll resend what I have so you can decide. Yours, Linus Walleij