[PATCH 0/2] Fix loopback mounted filesystems on NFS

[Benjamin Coddington <bcodding@redhat.com>]

We've been investigating new reports of filesystem corruption on
loopback images on NFS clients.  It appears that during writeback the
loopback driver encounters allocation failures in NFS and fails to write
dirty pages to the backing file.

We believe the problem is due to the loopback driver performing writeback
from a workqueue (so PF_WQ_WORKER is set), however ever since work to
improve NFS' memory allocation strategies [1] its possible that NFS
incorrectly assumes that if PF_WQ_WORKER is set then the writeback context
is nfsiod.  To make things worse, NFS does not expect PF_WQ_WORKER to be set
along with other PF_ flags such as PF_MEMALLOC_NOIO, but cannot really know
(without checking them all) which other allocation flags are set should
writeback be entered from a NFS-external workqueue worker.

To fix this, I'd like to introduce a way to check which specific workqueue
is being served by a worker (in patch 1), so that NFS can ensure that it
sets certain allocation flags only for the nfsiod workqueue workers (in
patch 2).

[1]: https://lore.kernel.org/linux-nfs/20220322011618.1052288-1-trondmy@kernel.org/

Benjamin Coddington (2):
  workqueue: Add a helper to identify current workqueue
  NFS: Improve nfsiod workqueue detection for allocation flags

 fs/nfs/internal.h         | 12 +++++++++++-
 include/linux/workqueue.h |  1 +
 kernel/workqueue.c        | 18 ++++++++++++++++++
 3 files changed, 30 insertions(+), 1 deletion(-)

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2.47.0