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[2a00:1028:83b8:1e7a:3010:3bd6:8521:caf1]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-39c3020d943sm4126180f8f.74.2025.04.04.04.12.44 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 04 Apr 2025 04:12:45 -0700 (PDT) Date: Fri, 4 Apr 2025 13:12:39 +0200 From: Petr Tesarik To: Harry Yoo Cc: Vlastimil Babka , Feng Tang , Peng Fan , Hyeonggon Yoo <42.hyeyoo@gmail.com>, David Rientjes , Christoph Lameter , "linux-mm@kvack.org" Subject: Re: slub - extended kmalloc redzone and dma alignment Message-ID: <20250404131239.2a987e58@mordecai> In-Reply-To: References: X-Mailer: Claws Mail 4.3.1 (GTK 3.24.48; x86_64-suse-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: 16A87C0015 X-Stat-Signature: fwtzqidwrzq83qim6ka15ihids6z66x7 X-HE-Tag: 1743765166-178026 X-HE-Meta: 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 ZesDmrFk 5izxXuAdon6r+4Eivc7ob1y7hyEoN2Lna3W3zy34B0UgAIDEbMvkwKl1Pj9gu2SwUKlDUYZYt1MLGUgkUFp/ZxdFEPFuzQAH3GIeOSAYpm7M3wWxbxGycriBZdP9rMQs3RuNqoIxkkp6hnynZZkDUwXXE0d2e7pKJ0aiHcI8jcllqfSUHjmhcOtGJmL2uacEjKgC5tm/a5y1EtdRghE5OxlC8cILA+JWLg2HILBMGcCNLkYj9AfR6H+XDg0blB/K4v9LNdzLMvCBWt89pLAtuiKqJyi8VYt3xLnDm7IJR1SpBuYigAW6uryDYBdcD1T4kzIIg1WN87do4sbaMmXiahfBLOVYY9W8U1UHurMYQ0mCFz4oD1HrxdNRzbh1aM5IcLw+B5HNpylxnkek= 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, 4 Apr 2025 19:30:09 +0900 Harry Yoo wrote: > On Fri, Apr 04, 2025 at 11:30:49AM +0200, Vlastimil Babka wrote: > > Hi, > > > > due to some off-list inquiry I have realized that since 946fa0dbf2d8 > > ("mm/slub: extend redzone check to extra allocated kmalloc space than > > requested") > > we might be reporting false positives due to dma writing into the redzone. > > > > It wasn't confirmed (yet) during the conversation but AFAICS it can be > > happening. We have this ARCH_DMA_MINALIGN and kmalloc() will guarantee it, > > but the redzone check doesn't take it into account. > > Sounds valid to me. I'm not sure I understand your concerns. Are you afraid that another device on the bus caches a copy of the redzone before it was poisoned, so it overwrites the redzone with stale data on a memory write operation? IMO that's buggy, because if a bus-mastering device implements such cache, it is the device driver's responsibility to flush it before starting a DMA transfer. FTR I'm not aware of any such devices, except GPUs, but there's a whole lot to do about CPU<->GPU coherency management, including device-specific ioctl's to expose some gory details all the way down to userspace. Or are you concerned about bus data word size? I would again argue that allocating a DMA buffer with a size that is not a multiple of the transfer size is a bug. IOW the driver must make sure the buffer size is a multiple of 4 if it is used for 32-bit DMA transfers, or a multiple of 8 if it is used for 64-bit DMA transfers. Confused. Petr T