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(e.d.c.5.4.7.0.e.f.a.2.f.e.2.0.1.4.d.3.1.6.b.8.0.0.b.8.0.1.0.0.2.ip6.arpa. [2001:8b0:8b6:13d4:102e:f2af:e074:5cde]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-45412820454sm149530f8f.6.2026.05.07.09.09.59 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 07 May 2026 09:10:00 -0700 (PDT) Message-ID: <2a75c858-5dfe-4b93-98fc-909a2ee5558d@meta.com> Date: Thu, 7 May 2026 17:09:59 +0100 Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 7/9] vfio/pci: Support mmap() of a VFIO DMABUF Content-Language: en-GB To: Jason Gunthorpe Cc: Alex Williamson , Leon Romanovsky , Alex Mastro , =?UTF-8?Q?Christian_K=C3=B6nig?= , Mahmoud Adam , David Matlack , =?UTF-8?B?QmrDtnJuIFTDtnBlbA==?= , Sumit Semwal , Kevin Tian , Ankit Agrawal , Pranjal Shrivastava , Alistair Popple , Vivek Kasireddy , linux-kernel@vger.kernel.org, linux-media@vger.kernel.org, dri-devel@lists.freedesktop.org, linaro-mm-sig@lists.linaro.org, kvm@vger.kernel.org References: <20260416131815.2729131-1-mattev@meta.com> <20260416131815.2729131-8-mattev@meta.com> <20260424183006.GI3444440@nvidia.com> From: Matt Evans In-Reply-To: <20260424183006.GI3444440@nvidia.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwNTA3MDE2MSBTYWx0ZWRfX5hKDtpjgoD9X t4kfrMgsLrqfeO15hCnfsWlyq1SA4W3rj+OAz6bE0q+rGJ1PU9G+rDxwqLJoWt7/OY50MqISehu c2j/1zV9EkBIlDkii8LQrvrvEdaIsCYzHtp45rt5pYwj30Lo+Qo8oR7dn8iLgf+xBE3AzAZT3Na sTamYH5RFuBCe1qvZhDRflf0xDH8ClWk+17AMcfAbLsuU6FGj1AfZ+EtU4c8Fzf6JldaQ4KjOa9 fTg4pMqZ1lLUnyi68cOx8AjIYYNvd27JbAs4HRssg4ZmW7b4f9PkiNUWw3wXWe52DKNHw2SlqFD FcwAA+2yzLy5m4tIGoHoz4GdGJ9QYatiM9R64tIrYoHBsE6gAevSEqILyynLtBBHRn52rxCCIHf T7U4sqEHb4J9FjAZykSRS6dHyb/WFCSg9RGpZaT3bJWXKLVusZb+tYqVkWqNMew2S+kZsM2STgC U/40mKsUwvjfdxZp1mQ== X-Proofpoint-ORIG-GUID: j4XdOVXnZuT60TlGatdHHpcHFxLsG9b5 X-Proofpoint-GUID: j4XdOVXnZuT60TlGatdHHpcHFxLsG9b5 X-Authority-Analysis: v=2.4 cv=F7dnsKhN c=1 sm=1 tr=0 ts=69fcb95b cx=c_pps a=pX1sPZUoWPGeQgUnKQNIgA==:117 a=xqWC_Br6kY4A:10 a=IkcTkHD0fZMA:10 a=NGcC8JguVDcA:10 a=VkNPw1HP01LnGYTKEx00:22 a=7x6HtfJdh03M6CCDgxCd:22 a=tpM8CJlwf7uhpglF1g9U:22 a=bfFL6uTa8VbwhnMIod8A:9 a=QEXdDO2ut3YA:10 a=XGGHjNwGSpTZ5T66x7Ix:22 X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1143,Hydra:6.1.51,FMLib:17.12.100.49 definitions=2026-05-07_01,2026-05-06_01,2025-10-01_01 Hi Jason, On 24/04/2026 19:30, Jason Gunthorpe wrote: > > On Thu, Apr 16, 2026 at 06:17:50AM -0700, Matt Evans wrote: >> + >> + dma_resv_lock(priv->dmabuf->resv, NULL); >> vdev = READ_ONCE(priv->vdev); >> >> + if (READ_ONCE(priv->revoked) || !vdev) { > > Why is this read once? It is inside the resv lock so it is stable? > >> + pr_debug_ratelimited("%s VA 0x%lx, pgoff 0x%lx: DMABUF revoked/cleaned up\n", >> + __func__, vmf->address, vma->vm_pgoff); >> + dma_resv_unlock(priv->dmabuf->resv); >> + return VM_FAULT_SIGBUS; >> + } >> + /* vdev is usable */ >> + >> + if (!vfio_device_try_get_registration(&vdev->vdev)) { >> + /* >> + * If vdev != NULL (above), the registration should >> + * already be >0 and so this try_get should never >> + * fail. >> + */ >> + dev_warn(&vdev->pdev->dev, "%s: Unexpected registration failure\n", >> + __func__); >> + dma_resv_unlock(priv->dmabuf->resv); >> + return VM_FAULT_SIGBUS; >> + } >> + dma_resv_unlock(priv->dmabuf->resv); >> + >> scoped_guard(rwsem_read, &vdev->memory_lock) { >> - if (!priv->revoked) { >> + if (!READ_ONCE(priv->revoked)) { > > Same here, it is not read once since you hold the memory_lock it is > stable. I used them more as an 'eyecatcher' to complement the comment. Although they're not strictly required (compiler barriers at the lock/unlocks), they stand out to say "something's going on here". Revoked/status is read first holding resv, and _must be read a second time_ once that's released and after memory_lock is taken, so two READ_ONCEs seemed appropriate to show this. But if you feel strongly, sure, I can remove it (though I'd add, say, a /* Re-read status value */ comment). Matt