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Thu, 04 Apr 2024 13:45:51 -0700 (PDT) MIME-Version: 1.0 References: <20240404162515.527802-1-fvdl@google.com> <20240404131549.27a454072f7e96530dce2d62@linux-foundation.org> In-Reply-To: <20240404131549.27a454072f7e96530dce2d62@linux-foundation.org> From: Frank van der Linden Date: Thu, 4 Apr 2024 13:45:40 -0700 Message-ID: Subject: Re: [PATCH 1/2] mm/cma: drop incorrect alignment check in cma_init_reserved_mem To: Andrew Morton Cc: linux-mm@kvack.org, muchun.song@linux.dev, linux-kernel@vger.kernel.org, Marek Szyprowski , David Hildenbrand Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Queue-Id: E7804100005 X-Rspam-User: X-Rspamd-Server: rspam04 X-Stat-Signature: 95ekyppxqdt98o6dksk8hy35hjhwxwe9 X-HE-Tag: 1712263552-8211 X-HE-Meta: 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 hnUzrRU2 hbrIUVHNcKQU2y7/RGcDxxOD4RjdFRF1DL9q6lvS/hNqS0nN9wmueBSdaBiMSxtn4N5ni4ulAKGC5Cy/Bwh0JtIAhNrc2S+emF9j9RYrsIG3P8IX5tompSm5GjYIyARwKJA1A77d2sfXlbkdh50EskUC93l8wp92+lWiZ5T9xg0lveMR9325M2HX007DQL2wf7fqeHd35GEF52tBvuQu9Wu2fVANQ7PaHHwzidQOwVKV6W0DnSyFAEsLZbuVTbQbdFGeBUOuzWWhFux5yGsr9obdPRHx3KIEVEjPpsDp7XFaELYyj2ujZdRQ8s1I+AzxNopmBOiYmUfQY2wEkzqNN39b+x4EkXAPTDhVr X-Bogosity: Ham, tests=bogofilter, spamicity=0.119247, 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 Thu, Apr 4, 2024 at 1:15=E2=80=AFPM Andrew Morton wrote: > > On Thu, 4 Apr 2024 16:25:14 +0000 Frank van der Linden = wrote: > > > cma_init_reserved_mem uses IS_ALIGNED to check if the size > > represented by one bit in the cma allocation bitmask is > > aligned with CMA_MIN_ALIGNMENT_BYTES (pageblock size). > > > > However, this is too strict, as this will fail if > > order_per_bit > pageblock_order, which is a valid configuration. > > > > We could check IS_ALIGNED both ways, but since both numbers are > > powers of two, no check is needed at all. > > What are the userspace visible effects of this bug? None that I know of. This bug was exposed because I made the hugetlb code correctly pass the right order_per_bit argument (see the accompanying hugetlb cma fix), which then tripped this check when I backported it to an older kernel, passing an order of 30 (1G hugetlb page) as order_per_bit. This actually won't happen for 6.9-rc, since the (intended) order_per_bit was reduced to HUGETLB_PAGE_ORDER because of hugetlb page demotion. So, no user visible effects. However, if the other fix is going to be backported, this one is a prereq. - Frank