[0/2] virtiofs submounts forgotten after client / guest memory pressure

Message

Krister Johansen Sept. 11, 2023, 6:38 p.m. UTC

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(-)