On Tue, 2023-02-07 at 15:01 +0000, Chuck Lever III wrote:
>
> > On Feb 7, 2023, at 9:50 AM, Jeff Layton <jlayton@xxxxxxxxxx> wrote:
> >
> > Most of the time, NFSv4 clients issue a COMMIT before the final CLOSE of
> > an open stateid, so with NFSv4, the fsync in the nfsd_file_free path is
> > usually a no-op and doesn't block.
> >
> > We have a customer running knfsd over very slow storage (XFS over Ceph
> > RBD). They were using the "async" export option because performance was
> > more important than data integrity for this application. That export
> > option turns NFSv4 COMMIT calls into no-ops. Due to the fsync in this
> > codepath however, their final CLOSE calls would still stall (since a
> > CLOSE effectively became a COMMIT).
> >
> > I think this fsync is not strictly necessary. We only use that result to
> > reset the write verifier. Instead of fsync'ing all of the data when we
> > free an nfsd_file, we can just check for writeback errors when one is
> > acquired and when it is freed.
> >
> > If the client never comes back, then it'll never see the error anyway
> > and there is no point in resetting it. If an error occurs after the
> > nfsd_file is removed from the cache but before the inode is evicted,
> > then it will reset the write verifier on the next nfsd_file_acquire,
> > (since there will be an unseen error).
> >
> > The only exception here is if something else opens and fsyncs the file
> > during that window. Given that local applications work with this
> > limitation today, I don't see that as an issue.
> >
> > Reported-and-Tested-by: Pierguido Lambri <plambri@xxxxxxxxxx>
> > Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
>
> Seems sensible and clean.
>
> I would like to queue this in the filecache topic branch, but
> that means it won't get merged until v6.4 at the earliest. Is
> that OK?
>
>
Thanks! v6.4 would be a little late. Can we get it in for v6.3?
Exporting with -o async is (unfortunately) quite common. I suspect we're
going to see other bug reports due to this. Waiting out a whole cycle
means wading through a bunch of those (and telling those folks to use
older kernels until we can get it in).
> > ---
> > fs/nfsd/filecache.c | 48 ++++++++++++++-------------------------------
> > fs/nfsd/trace.h | 31 -----------------------------
> > 2 files changed, 15 insertions(+), 64 deletions(-)
> >
> > diff --git a/fs/nfsd/filecache.c b/fs/nfsd/filecache.c
> > index 136e543ae44b..fcd16ffbf9ad 100644
> > --- a/fs/nfsd/filecache.c
> > +++ b/fs/nfsd/filecache.c
> > @@ -233,37 +233,27 @@ nfsd_file_alloc(struct net *net, struct inode *inode, unsigned char need,
> > return nf;
> > }
> >
> > +/**
> > + * nfsd_file_check_write_error - check for writeback errors on a file
> > + * @nf: nfsd_file to check for writeback errors
> > + *
> > + * Check whether a nfsd_file has an unseen error. Reset the write
> > + * verifier if so.
> > + */
> > static void
> > -nfsd_file_fsync(struct nfsd_file *nf)
> > -{
> > - struct file *file = nf->nf_file;
> > - int ret;
> > -
> > - if (!file || !(file->f_mode & FMODE_WRITE))
> > - return;
> > - ret = vfs_fsync(file, 1);
> > - trace_nfsd_file_fsync(nf, ret);
> > - if (ret)
> > - nfsd_reset_write_verifier(net_generic(nf->nf_net, nfsd_net_id));
> > -}
> > -
> > -static int
> > nfsd_file_check_write_error(struct nfsd_file *nf)
> > {
> > struct file *file = nf->nf_file;
> >
> > - if (!file || !(file->f_mode & FMODE_WRITE))
> > - return 0;
> > - return filemap_check_wb_err(file->f_mapping, READ_ONCE(file->f_wb_err));
> > + if ((file->f_mode & FMODE_WRITE) &&
> > + filemap_check_wb_err(file->f_mapping, READ_ONCE(file->f_wb_err)))
> > + nfsd_reset_write_verifier(net_generic(nf->nf_net, nfsd_net_id));
> > }
> >
> > static void
> > nfsd_file_hash_remove(struct nfsd_file *nf)
> > {
> > trace_nfsd_file_unhash(nf);
> > -
> > - if (nfsd_file_check_write_error(nf))
> > - nfsd_reset_write_verifier(net_generic(nf->nf_net, nfsd_net_id));
> > rhltable_remove(&nfsd_file_rhltable, &nf->nf_rlist,
> > nfsd_file_rhash_params);
> > }
> > @@ -289,22 +279,13 @@ nfsd_file_free(struct nfsd_file *nf)
> > this_cpu_add(nfsd_file_total_age, age);
> >
> > nfsd_file_unhash(nf);
> > -
> > - /*
> > - * We call fsync here in order to catch writeback errors. It's not
> > - * strictly required by the protocol, but an nfsd_file could get
> > - * evicted from the cache before a COMMIT comes in. If another
> > - * task were to open that file in the interim and scrape the error,
> > - * then the client may never see it. By calling fsync here, we ensure
> > - * that writeback happens before the entry is freed, and that any
> > - * errors reported result in the write verifier changing.
> > - */
> > - nfsd_file_fsync(nf);
> > -
> > if (nf->nf_mark)
> > nfsd_file_mark_put(nf->nf_mark);
> > - if (nf->nf_file)
> > +
> > + if (nf->nf_file) {
> > + nfsd_file_check_write_error(nf);
> > filp_close(nf->nf_file, NULL);
> > + }
> >
> > /*
> > * If this item is still linked via nf_lru, that's a bug.
> > @@ -1080,6 +1061,7 @@ nfsd_file_do_acquire(struct svc_rqst *rqstp, struct svc_fh *fhp,
> > out:
> > if (status == nfs_ok) {
> > this_cpu_inc(nfsd_file_acquisitions);
> > + nfsd_file_check_write_error(nf);
> > *pnf = nf;
> > }
> > put_cred(cred);
> > diff --git a/fs/nfsd/trace.h b/fs/nfsd/trace.h
> > index 8f9c82d9e075..4183819ea082 100644
> > --- a/fs/nfsd/trace.h
> > +++ b/fs/nfsd/trace.h
> > @@ -1202,37 +1202,6 @@ TRACE_EVENT(nfsd_file_close,
> > )
> > );
> >
> > -TRACE_EVENT(nfsd_file_fsync,
> > - TP_PROTO(
> > - const struct nfsd_file *nf,
> > - int ret
> > - ),
> > - TP_ARGS(nf, ret),
> > - TP_STRUCT__entry(
> > - __field(void *, nf_inode)
> > - __field(int, nf_ref)
> > - __field(int, ret)
> > - __field(unsigned long, nf_flags)
> > - __field(unsigned char, nf_may)
> > - __field(struct file *, nf_file)
> > - ),
> > - TP_fast_assign(
> > - __entry->nf_inode = nf->nf_inode;
> > - __entry->nf_ref = refcount_read(&nf->nf_ref);
> > - __entry->ret = ret;
> > - __entry->nf_flags = nf->nf_flags;
> > - __entry->nf_may = nf->nf_may;
> > - __entry->nf_file = nf->nf_file;
> > - ),
> > - TP_printk("inode=%p ref=%d flags=%s may=%s nf_file=%p ret=%d",
> > - __entry->nf_inode,
> > - __entry->nf_ref,
> > - show_nf_flags(__entry->nf_flags),
> > - show_nfsd_may_flags(__entry->nf_may),
> > - __entry->nf_file, __entry->ret
> > - )
> > -);
> > -
> > #include "cache.h"
> >
> > TRACE_DEFINE_ENUM(RC_DROPIT);
> > --
> > 2.39.1
> >
>
> --
> Chuck Lever
>
>
>
--
Jeff Layton <jlayton@xxxxxxxxxx>
