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b=Twq5MyQSg58Egx5BZr0+ls7uvN84oPSehGSSuE46oaLtK6++g3ZwUTDbAWifQ2nhI T0nNCPjQv9YHvL7DPx5j7x/EZt0sWxlkIZYMMlAp+LOUh0ItzOQw+izPoZZaXQVTf1 eFeu6hWYJStwAzWoRWfoXr7jXAPHaqv8FCIzMKRyGh8x3huVr4rjPvmIg2qXzWREua oTiRVKin9vO1RSpt8Fn3F/BWKxaYHEsvKGcL+sa9Jvoh7IMbk8Z5IvACVidC9qR9Xc uh1Jz7g4P0rnvKOBPzenGEDoAjJq6cWrTspuTVxyBkj3J/p1BA8n7pAUdnLCej1qJ0 21z2oGIdTbSlg== Date: Fri, 27 Mar 2026 15:37:57 +0900 From: "Harry Yoo (Oracle)" To: Mikhail Gavrilov 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 Subject: Re: [PATCH] mm/slab: align kmalloc to cacheline when DMA API debugging is active Message-ID: References: <20260327055846.248829-1-mikhail.v.gavrilov@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260327055846.248829-1-mikhail.v.gavrilov@gmail.com> X-Rspamd-Queue-Id: 1EA2AA0007 X-Stat-Signature: 6113ob848pd5t8j4bah1piruajs3rud6 X-Rspam-User: X-Rspamd-Server: rspam08 X-HE-Tag: 1774593480-833984 X-HE-Meta: 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 Ak5WMFkl SzNZbEKUzm0KpavoHeKS9ONBu9KnYBahotTbj1IXnw/THUbCOt4oLXTglIluh9n4inBeYBMSsUk+79CxFCwJbO4+gg6p90Zqxj4E6b+Pe9T3jiz6c57sSS73KbCkE13M6vHvg4hNpOvimMVcGKtcjOTnmtRBA0Ka/unvUArIkIV/pqOLaSzxzluo0GU/QmiZBJ1b53TXFOrj2GCcGNxgiNXXShvOHrB6SGAEQvNpHY606RPfA5MqDeOtqpDB4YQtFwfAs0BgYGmVDC85rgLaEjKt0vNiZTlDh+YqaDreWdmBz6g2a2Lsr+bCQj9eJjPokC1DZsFkR9/un2KUDFwm9x2PmmCLQCURNE7enwjVXpG/tj8Qps+7MAoL0hgYMlKLd30OYTMhb0aWa+2Euv6b5WUuMDzZceTilRWK+djMkzwbtuEU= 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 10:58:46AM +0500, Mikhail Gavrilov wrote: > When CONFIG_DMA_API_DEBUG is enabled, the DMA debug infrastructure > tracks active mappings per cacheline and warns if two different DMA > mappings share the same cacheline ("cacheline tracking EEXIST, > overlapping mappings aren't supported"). > > On x86_64, ARCH_KMALLOC_MINALIGN defaults to 8, so small kmalloc > allocations (e.g. the 8-byte hub->buffer and hub->status in the USB > hub driver) frequently land in the same 64-byte cacheline. When both > are DMA-mapped, this triggers a false positive warning. Is it feasible to suppress the warning if dma_get_cache_alignment() is smaller than L1_CACHE_BYTES? > This has been reported repeatedly since v5.14 (when the EEXIST check > was added) across various USB host controllers and devices including > xhci_hcd with USB hubs, USB audio devices, and USB ethernet adapters. > > Raise ARCH_KMALLOC_MINALIGN to L1_CACHE_BYTES when CONFIG_DMA_API_DEBUG > is enabled, ensuring each kmalloc allocation occupies its own cacheline > and eliminating the false positive. > > Verified with a kernel module reproducer that performs two kmalloc(8) > allocations back-to-back and DMA-maps both: > > Before: allocations share a cacheline, EEXIST fires within ~50 pairs > After: 64 pairs allocated, all in separate cachelines, no warning > > Fixes: 2b4bbc6231d7 ("dma-debug: report -EEXIST errors in add_dma_entry") > Link: https://bugzilla.kernel.org/show_bug.cgi?id=215740 > Suggested-by: Alan Stern > Suggested-by: Guenter Roeck > Tested-by: Jeff Kirsher > Tested-by: Mikhail Gavrilov > Signed-off-by: Mikhail Gavrilov -- Cheers, Harry / Hyeonggon