On Mon, 2024-11-04 at 15:47 +1100, NeilBrown wrote:
> An NFSv4.2 COPY request can explicitly request a synchronous copy. If
> that is not requested then the server is free to perform the copy
> synchronously or asynchronously.
>
> In the Linux implementation an async copy requires more resources than a
> sync copy. If nfsd cannot allocate these resources, the best response
> is to simply perform the copy (or the first 4MB of it) synchronously.
>
Where does the copy get clamped at 4MB?
> This choice may be debatable if the unavailable resource was due to
> memory allocation failure - when memalloc fails it might be best to
> simply give up as the server is clearly under load. However in the case
> that policy prevents another kthread being created there is no benefit
> and much cost is failing with NFS4ERR_DELAY. In that case it seems
> reasonable to avoid that error in all circumstances.
>
> So change the out_err case to retry as a sync copy.
>
> Fixes: aadc3bbea163 ("NFSD: Limit the number of concurrent async COPY operations")
> Signed-off-by: NeilBrown <neilb@xxxxxxx>
> ---
> fs/nfsd/nfs4proc.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/fs/nfsd/nfs4proc.c b/fs/nfsd/nfs4proc.c
> index fea171ffed62..06e0d9153ca9 100644
> --- a/fs/nfsd/nfs4proc.c
> +++ b/fs/nfsd/nfs4proc.c
> @@ -1972,6 +1972,7 @@ nfsd4_copy(struct svc_rqst *rqstp, struct nfsd4_compound_state *cstate,
> wake_up_process(async_copy->copy_task);
> status = nfs_ok;
> } else {
> + retry_sync:
> status = nfsd4_do_copy(copy, copy->nf_src->nf_file,
> copy->nf_dst->nf_file, true);
> }
> @@ -1990,8 +1991,7 @@ nfsd4_copy(struct svc_rqst *rqstp, struct nfsd4_compound_state *cstate,
> }
> if (async_copy)
> cleanup_async_copy(async_copy);
> - status = nfserr_jukebox;
> - goto out;
> + goto retry_sync;
> }
>
> static struct nfsd4_copy *
>
> base-commit: 26e6e693936986309c01e8bb80e318d63fda4a44
You're probably right that attempting to do this synchronously is
probably best. That should avoid another RPC, though at the cost of
having to shovel the data into the pagecache.
If memory is very tight, I suppose the synchronous copy might also fail
with NFS4ERR_DELAY, in which case we're no worse off.
Reviewed-by: Jeff Layton <jlayton@xxxxxxxxxx>
