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[76.182.20.124]) by smtp.gmail.com with ESMTPSA id af79cd13be357-7a7efabb7a5sm34579585a.55.2024.08.27.10.04.59 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 27 Aug 2024 10:04:59 -0700 (PDT) Date: Tue, 27 Aug 2024 13:04:58 -0400 From: Josef Bacik To: Han-Wen Nienhuys Cc: linux-fsdevel@vger.kernel.org Subject: Re: FUSE passthrough: fd lifetime? Message-ID: <20240827170458.GD2466167@perftesting> References: Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Tue, Aug 27, 2024 at 11:41:04AM +0200, Han-Wen Nienhuys wrote: > Hi folks, > > I am implementing passthrough support for go-fuse. It seems to be > working now (code: > https://review.gerrithub.io/c/hanwen/go-fuse/+/1199984 and > predecessors), but I am unsure of the correct lifetimes for the file > descriptors. > > The passthrough_hp example seems to do: > > Open: > backing_fd = .. > backing_id = ioctl(fuse_device_fd, > FUSE_DEV_IOC_BACKING_OPEN, backing_fd) > > Release: > ioctl(fuse_device_fd, > FUSE_DEV_IOC_BACKING_CLOSE, backing_id) > close(backing_fd) > > Is it necessary to keep backing_fd open while the backing file is > used? In the case of go-fuse, the backing_fd is managed by client > code, so I can't ensure it is kept open long enough. I can work around > this by doing > > Open: > new_backing_fd = ioctl(backing_fd, DUP_FD, 0) > backing_id = ioctl(fuse_device_fd, > FUSE_DEV_IOC_BACKING_OPEN, new_backing_fd) > > Release: > ioctl(fuse_device_fd, > FUSE_DEV_IOC_BACKING_CLOSE, backing_id) > close(new_backing_fd) > > but it would double the number of FDs the process uses. > > I tried simply closing the backing FD right after obtaining the > backing ID, and it seems to work. Is this permitted? Yes that's permitted, we're holding a reference to the file in the kernel. The dup thing works too, but as you say that's a lot more fd's. Thanks, Josef