On Mon, Nov 23, 2020 at 10:07 PM Ken Schalk <kschalk@xxxxxxxxxx> wrote: > > On Thu, Oct 1, 2020 Miklos Szeredi <miklos@xxxxxxxxxx> wrote: > > On Fri, Aug 28, 2020 at 11:01 PM Ken Schalk <kschalk@xxxxxxxxxx> wrote: > > > > Thanks very much for your help. The patch you provided does solve > > > > the problem in the O_CREAT|O_EXCL case (by making a lookup call to > > > > re-validate the entry of the since deleted file), but not in the > > > > O_CREAT case. (In that case the kernel still winds up making a FUSE > > > > open request rather than a FUSE create request.) I'd like to > > > > suggest the slightly different attached patch instead, which > > > > triggers re-validation in both cases. > > > Which is a problem, because that makes O_CREAT on existing files (a > > fairly common case) add a new synchronous request, possibly > > resulting in a performance regression. > > > I don't see an easy way this can be fixed, and I'm not sure this > > needs to be fixed. > > > Are you seeing a real issue with just O_CREAT? > > Yes, we definitely do see issues with just O_CREAT. The specific > sequence that involves O_CREAT without O_EXCL is: > > 1. A file exists and is accessed through our FUSE distributed > filesystem on host X. The kernel on host X caches the directory > entry for the file. > > 2. The file is unlinked through our FUSE distributed filesystem on > host Y. > > 3. An open(2) call with O_CREAT for the file occurs on host X. > Because the kernel has a cached dentry for the now deleted file, it > makes a FUSE open request to our filesystem (rather than a FUSE > create request). > > 4. Our filesystem's open handler finds that the file does not exist > (because it was unlinked in step 2) and replies to the open request > with ENOENT. ESTALE is the correct error value here, not ENOENT. I agree this is subtle and not well documented, but it's quite logical: the cache contains stale lookup information and hence open finds the file non-existent. In no case shall the OPEN request return ENOENT, that's up to the LOOKUP request. Exclusive create is a different matter. It must not open an existing file, and so it must never send an OPEN request. Thanks, Miklos
