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[82.132.213.26]) by smtp.gmail.com with ESMTPSA id 4fb4d7f45d1cf-6aa67e0d3c5sm4446691a12.22.2026.09.21.06.57.09 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 21 Sep 2026 06:57:14 -0700 (PDT) Message-ID: Date: Mon, 21 Sep 2026 14:57:05 +0100 Precedence: bulk X-Mailing-List: nvdimm@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 01/14] dma-buf: introduce initial file I/O infrastructure To: =?UTF-8?Q?Christian_K=C3=B6nig?= , Jens Axboe , Keith Busch , Christoph Hellwig , Sagi Grimberg , linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, linux-nvme@lists.infradead.org, linux-fsdevel@vger.kernel.org, io-uring@vger.kernel.org, linux-media@vger.kernel.org, dri-devel@lists.freedesktop.org, linaro-mm-sig@lists.linaro.org Cc: Alexander Viro , Christian Brauner , Andrew Morton , Sumit Semwal , Nitesh Shetty , Kanchan Joshi , Anuj Gupta , Tushar Gohad , William Power , Phil Cayton , Jason Gunthorpe , Damien Le Moal , Alasdair Kergon , Mike Snitzer , Mikulas Patocka , Benjamin Marzinski , Vishal Verma , David Sterba , Ilya Dryomov , dm-devel@lists.linux.dev, nvdimm@lists.linux.dev, linux-btrfs@vger.kernel.org, ceph-devel@vger.kernel.org References: <181b08be-04bc-4ae9-bb88-15321a82440f@amd.com> <8285ede8-bebb-403a-8a37-b9987cd37a8f@gmail.com> <0113c49a-a364-4df6-9ab4-681c789d9d3b@amd.com> Content-Language: en-US From: Pavel Begunkov In-Reply-To: <0113c49a-a364-4df6-9ab4-681c789d9d3b@amd.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 8/6/26 08:29, Christian König wrote: > On 8/5/26 12:59, Pavel Begunkov wrote: >> On 8/5/26 09:27, Christian König wrote: > >>>> + >>>> +    dma_resv_lock(dmabuf->resv, NULL); >>>> +    ctx->dev_ops->unmap(ctx, map); >>>> +    dma_resv_unlock(dmabuf->resv); >>>> + >>>> +    dma_fence_put(&fence->base); >>> >>> You should probably set map->fence to NULL after that. >> >> The map is freed two lines below, but I can add it as >> a defensive measure. > > In that case it's ok, I've just haven't seen the kfree(map) below. > >> ... >>>> +    ret = dma_resv_reserve_fences(dmabuf->resv, 1); >>>> +    if (WARN_ON_ONCE(ret)) { >>>> +        struct dma_fence *fence = &map->fence->base; >>>> + >>>> +        dma_fence_get(fence); >>>> +        percpu_ref_kill(&map->refs); >>>> +        dma_fence_wait(fence, false); >>>> +        dma_fence_put(fence); >>>> +        return; >>>> +    } >>>> + >>>> +    dma_resv_add_fence(dmabuf->resv, &map->fence->base, >>>> +               DMA_RESV_USAGE_KERNEL); >>> >>> That sequence is clearly incorrect! >>> >>> The fence must be created after dma_resv_reserve_fences(), otherwise you definately have an illegal memory operation here. >> >> I'm not sure what you mean, can you elaborate? I only cared about >> pre-allocating it to avoid allocations here. We add / signal the fence >> only once, no reuse. The map is going to be killed here, and if we >> create a new map, it'll have its own fence. >> >> I can move the dma_fence_init() call here if that makes a difference? > > Yeah that is a good start, but you might need a bit more. > > Here is a summary of the usual procedure you need to follow when implementing a dma_fence backend: > > 1. Allocate your operation object, in this case here it's your mapping I think. > 2. Prepare your operation, including all memory allocations. > 3. Call dma_resv_reserve_fences() to reserve a fence slot. > 4. Allocate and init your dma_fence object. Looks I forgot it reply here, sorry for that. I killed dma fences in v6 in favour of synchronous waiting, it turned to be less trouble even outside of these fencing rules as I can rely on dma-resv lock less in the code. -- Pavel Begunkov