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Fri, 27 Mar 2026 01:07:33 -0700 (PDT) MIME-Version: 1.0 References: <20260327055846.248829-1-mikhail.v.gavrilov@gmail.com> In-Reply-To: From: Mikhail Gavrilov Date: Fri, 27 Mar 2026 13:07:21 +0500 X-Gm-Features: AQROBzCzB3coDtODs6w7gt0-QdNGc5pwWqkx1wDss66ksHdICPpNIJLEWH3TZ30 Message-ID: Subject: Re: [PATCH] mm/slab: align kmalloc to cacheline when DMA API debugging is active To: "Harry Yoo (Oracle)" Cc: vbabka@kernel.org, akpm@linux-foundation.org, hao.li@linux.dev, cl@gentwo.org, rientjes@google.com, roman.gushchin@linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-usb@vger.kernel.org, stern@rowland.harvard.edu, linux@roeck-us.net, andy.shevchenko@gmail.com, hch@lst.de, Jeff.kirsher@gmail.com, Catalin Marinas Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspam-User: X-Stat-Signature: hju5keo1396m6mk81swm9ye9hydcunjr X-Rspamd-Queue-Id: 32F451C0003 X-Rspamd-Server: rspam09 X-HE-Tag: 1774598854-720095 X-HE-Meta: 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 6XWv4Tdm 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Fri, Mar 27, 2026 at 1:00=E2=80=AFPM Harry Yoo (Oracle) wrote: > > But dma_get_cache_alignment() < L1_CACHE_BYTES means the architecture > actually allows overlapping cachelines, no? Hi Harry, On x86_64, dma_get_cache_alignment() returns L1_CACHE_BYTES (both are 64). The condition (dma_get_cache_alignment() < L1_CACHE_BYTES) would be false, so the check wouldn't suppress the warning. The problem isn't that the architecture allows overlapping -- it's that kmalloc returns 8-byte aligned buffers that happen to land in the same 64-byte cacheline. The DMA debug code correctly identifies that two DMA mappings share a cacheline, but on coherent platforms this is harmless. Adding a dev_is_dma_coherent() check in dma-debug would fix x86 but would also silence the warning for any coherent device, including ones behind IOMMUs that might have non-coherent paths. That's why Alan's conclusion was that fixing the allocator side is safer -- it doesn't weaken any debug checks, it just ensures the situation never arises. --=20 Best Regards, Mike Gavrilov.