Hi, I recently ran into a situation where a virtiofs client began encountering EBADF after the client / guest system had an OOM. After reproducing the issue and debugging, it appears that the problem is caused by a virtiofsd submount being forgotten once the dentry referencing that submount is killed by the shrinker. In this particular case, the submount had been bind mounted into a container's mount namespace. The reference count on the original dentry was 0, making it eligible for eviction. However, because this dentry was also the last reference the client knew it had, it sent a forget message to the server. This caused all future references to the FUSE node-id from virtiofsd perspective to become invalid. Subsequent attempts to used the node-id received an EBADF from the server. This pair of patches modifies the virtiofs submount code to perform a lookup on the nodeid that forms the root of the submount. The patch before this pulls the revalidate lookup code into a helper function that can be used both in revalidate and submount superblock fill. Tested via: - fstests for virtiofs - fstests for fuse (against passthrough_ll) - manual testing to watch how refcounts change between client and server in response to filesytem access, umount, and eviction by the shrinker. Changes since RFC: - Modified fuse_fill_super_submount to always fail if dentry cannot be revalidated. (Feedback from Bernd Schubert) - Fixed up an edge case where looked up but subsequently declared invalid dentries were not correctly tracking nlookup. (Error was introduced in my RFC). Thanks, -K Krister Johansen (2): fuse: revalidate: move lookup into a separate function fuse: ensure that submounts lookup their root fs/fuse/dir.c | 87 +++++++++++++++++++++++++++++++++--------------- fs/fuse/fuse_i.h | 6 ++++ fs/fuse/inode.c | 43 ++++++++++++++++++++---- 3 files changed, 103 insertions(+), 33 deletions(-) -- 2.25.1
