A FUSE request that names a nodeid and nothing else is only sent for an inode the client has looked up and holds a reference to, and the server owes the client that inode until it is sent FUSE_FORGET. When a server answers such a request with ENOENT it is describing a handle it was obliged to honour, not a file that has gone away, and the client passes that ENOENT to the caller unchanged: a file that never stopped existing is reported missing.
This series reports that case as stale instead, so the VFS retries the lookup rather than handing a spurious ENOENT to userspace. The path opens say EOPENSTALE, which is what an open says when the cached information it started from has gone stale and which path_openat() resolves into ECHILD under LOOKUP_RCU or ESTALE otherwise; everything else says ESTALE, which retry_estale() answers by repeating the lookup once under LOOKUP_REVAL. NFS reports its own stale opens the same way. The conversion is written twice, once in fs/fuse for any transport and once in fs/fuse/virtio_fs.c at reply completion, and each one fixes the symptom without the other. They are sent together because they were found together; if only one is wanted, patch 3 is the general one and patch 5 is the virtio-specific one, and either can be dropped. Why it matters, and how it was found: a server that releases an inode as soon as a rename displaces the name it was looked up by will refuse opens for inodes the client still holds. On such a server, roughly one open in eight during a rename race is refused while stat continues to describe the file. That reaches real programs. git's t5318 fails 52 to 53 subtests per round against such a mount and 0 against local disk on the same machine in the same alternating run, and the mechanism is visible underneath it: .git/index opens ending ENOENT are 49 of 116 on the affected mount against 9 of 157 on local disk, where those 9 are the legitimate ones a fresh repository makes before an index exists. The user-visible failure is a destroyed git index. Measurements. All four arms are one kernel source, 6.18.5, with one config, built from trees that differ only in whether these patches are applied, run against the same server with the same probe for the same duration: arm fs/fuse virtio_fs opens refused A stock stock 323076 45201 (14.0%) B stock patched 54378 0 C patched stock 323776 0 D patched patched 315114 0 Every refusal on arm A is the contradiction: open said ENOENT while stat on the same path still described the file. Arms C and A carry almost identical open counts against the same server, so the pair differing only in fs/fuse is 45201 refusals against none. Arms B and C are why the two conversions are sent together but either can be dropped: each removes the symptom without the other. For the record, the same fault measured 12.9% on a 6.8 distro kernel three weeks earlier, so nothing between 6.8 and 6.18 addressed it, which is consistent with these patches applying to current mainline with only hunk-offset movement. The first two patches are selftest housekeeping that patch 4 needs: a build artifact that was not ignored, and a rename of the libfuse3 pkg-config variables so a second test can share the same guard. Patch 4 is the test itself, which mounts a libfuse3 server that refuses one request for a live nodeid and asserts the caller does not see ENOENT. It skips where libfuse3 or fusermount3 is unavailable. checkpatch is clean apart from two warnings I believe are false positives, and I would rather name them than leave them to be rediscovered: MAINTAINERS already carries F: tools/testing/selftests/filesystems/fuse/ under FUSE FILESYSTEM [CORE], so the new selftest needs no MAINTAINERS change; and the char *fuse_argv[] in the test cannot be static const, because struct fuse_args carries a plain char **argv that libfuse may modify. Aaron Paterson (5): selftests/fuse: ignore the built acl cache test selftests/fuse: name the libfuse3 flags for the library fuse: report a request refused for a live nodeid as stale selftests/fuse: cover a request refused for a live nodeid virtiofs: report a request refused for a live nodeid as stale fs/fuse/dir.c | 10 + fs/fuse/file.c | 11 + fs/fuse/fuse_i.h | 19 + fs/fuse/inode.c | 2 +- fs/fuse/virtio_fs.c | 62 +++ .../selftests/filesystems/fuse/.gitignore | 2 + .../selftests/filesystems/fuse/Makefile | 16 +- .../filesystems/fuse/fuse_estale_test.c | 450 ++++++++++++++++++ 8 files changed, 565 insertions(+), 7 deletions(-) create mode 100644 tools/testing/selftests/filesystems/fuse/fuse_estale_test.c -- 2.55.0.553.g4ad8c266be
