Re: add a LRU for delegations

[Christoph Hellwig <hch@lst.de>]

Any comments on this series except how to make it more complicated? :)

On Wed, Jan 07, 2026 at 08:26:51AM +0100, Christoph Hellwig wrote:
> Hi all,
> 
> currently the NFS client is rather inefficient at managing delegations
> not associated with an open file.  If the number of delegations is above
> the watermark, the delegation for a free file is immediately returned,
> even if delegations that were unused for much longer would be available.
> Also the periodic freeing marks delegations as not referenced for return,
> even if the file was open and thus force the return on close.
> 
> This series reworks the code to introduce an LRU and return the least
> used delegations instead.
> 
> For a workload simulating repeated runs of a python program importing a
> lot of modules, this leads to a 97% reduction of on-the-wire operations,
> and ~40% speedup even for a fast local NFS server.  A reproducer script
> is attached.
> 
> You'll want to make sure the dentry caching fix posted by Anna in reply
> to the 6.19 NFS pull is included for testing, even if the patches apply
> without it.  Note that with this and also with the follow on patches the
> baselines will still crash in some tests, and this series does not fix
> that.
> 
> Changes since v1:
>  - fix the nfsv4.0 hang
> 
> Diffstat:
>  fs/nfs/callback_proc.c    |   13 -
>  fs/nfs/client.c           |    3 
>  fs/nfs/delegation.c       |  544 +++++++++++++++++++++++-----------------------
>  fs/nfs/delegation.h       |    4 
>  fs/nfs/nfs4proc.c         |   82 +++---
>  fs/nfs/nfs4trace.h        |    2 
>  fs/nfs/super.c            |   14 -
>  include/linux/nfs_fs_sb.h |    8 
>  8 files changed, 342 insertions(+), 328 deletions(-)
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