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

[Jeff Layton <jlayton@kernel.org>]

On Mon, 2025-07-07 at 14:46 -0400, Benjamin Coddington wrote:
> 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(-)

Looks like a nice simple solution, and the workqueue helper seems
reasonable.

Reviewed-by: Jeff Layton <jlayton@kernel.org>