Re: [PATCH 5/6] NFSD: leverage DIO alignment to selectively issue O_DIRECT reads and writes

Mike Snitzer
2025-06-11T20:51:04Z

On Wed, Jun 11, 2025 at 11:44:29AM -0400, Jeff Layton wrote:
> On Wed, 2025-06-11 at 11:11 -0400, Chuck Lever wrote:
> > On 6/11/25 11:07 AM, Jeff Layton wrote:
> > > On Wed, 2025-06-11 at 10:42 -0400, Chuck Lever wrote:
> > > > On 6/10/25 4:57 PM, Mike Snitzer wrote:
> > > > > IO must be aligned, otherwise it falls back to using buffered IO.
> > > > >
> > > > > RWF_DONTCACHE is _not_ currently used for misaligned IO (even when
> > > > > nfsd/enable-dontcache=1) because it works against us (due to RMW
> > > > > needing to read without benefit of cache), whereas buffered IO enables
> > > > > misaligned IO to be more performant.
> > > > >
> > > > > Signed-off-by: Mike Snitzer <snitzer@kernel.org>
> > > > > ---
> > > > > fs/nfsd/vfs.c | 40 ++++++++++++++++++++++++++++++++++++----
> > > > > 1 file changed, 36 insertions(+), 4 deletions(-)
> > > > >
> > > > > diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
> > > > > index e7cc8c6dfbad..a942609e3ab9 100644
> > > > > --- a/fs/nfsd/vfs.c
> > > > > +++ b/fs/nfsd/vfs.c
> > > > > @@ -1064,6 +1064,22 @@ __be32 nfsd_splice_read(struct svc_rqst *rqstp, struct svc_fh *fhp,
> > > > > return nfsd_finish_read(rqstp, fhp, file, offset, count, eof, host_err);
> > > > > }
> > > > >
> > > > > +static bool is_dio_aligned(const struct iov_iter *iter, loff_t offset,
> > > > > + const u32 blocksize)
> > > > > +{
> > > > > + u32 blocksize_mask;
> > > > > +
> > > > > + if (!blocksize)
> > > > > + return false;
> > > > > +
> > > > > + blocksize_mask = blocksize - 1;
> > > > > + if ((offset & blocksize_mask) ||
> > > > > + (iov_iter_alignment(iter) & blocksize_mask))
> > > > > + return false;
> > > > > +
> > > > > + return true;
> > > > > +}
> > > > > +
> > > > > /**
> > > > > * nfsd_iter_read - Perform a VFS read using an iterator
> > > > > * @rqstp: RPC transaction context
> > > > > @@ -1107,8 +1123,16 @@ __be32 nfsd_iter_read(struct svc_rqst *rqstp, struct svc_fh *fhp,
> > > > > trace_nfsd_read_vector(rqstp, fhp, offset, *count);
> > > > > iov_iter_bvec(&iter, ITER_DEST, rqstp->rq_bvec, v, *count);
> > > > >
> > > > > - if (nfsd_enable_dontcache)
> > > > > - flags |= RWF_DONTCACHE;
> > > > > + if (nfsd_enable_dontcache) {
> > > > > + if (is_dio_aligned(&iter, offset, nf->nf_dio_read_offset_align))
> > > > > + flags |= RWF_DIRECT;
> > > > > + /* FIXME: not using RWF_DONTCACHE for misaligned IO because it works
> > > > > + * against us (due to RMW needing to read without benefit of cache),
> > > > > + * whereas buffered IO enables misaligned IO to be more performant.
> > > > > + */
> > > > > + //else
> > > > > + // flags |= RWF_DONTCACHE;
> > > > > + }
> > > > >
> > > > > host_err = vfs_iter_read(file, &iter, &ppos, flags);
> > > > > return nfsd_finish_read(rqstp, fhp, file, offset, count, eof, host_err);
> > > > > @@ -1217,8 +1241,16 @@ nfsd_vfs_write(struct svc_rqst *rqstp, struct svc_fh *fhp,
> > > > > nvecs = xdr_buf_to_bvec(rqstp->rq_bvec, rqstp->rq_maxpages, payload);
> > > > > iov_iter_bvec(&iter, ITER_SOURCE, rqstp->rq_bvec, nvecs, *cnt);
> > > > >
> > > > > - if (nfsd_enable_dontcache)
> > > > > - flags |= RWF_DONTCACHE;
> > > > > + if (nfsd_enable_dontcache) {
> > > > > + if (is_dio_aligned(&iter, offset, nf->nf_dio_offset_align))
> > > > > + flags |= RWF_DIRECT;
> > > > > + /* FIXME: not using RWF_DONTCACHE for misaligned IO because it works
> > > > > + * against us (due to RMW needing to read without benefit of cache),
> > > > > + * whereas buffered IO enables misaligned IO to be more performant.
> > > > > + */
> > > > > + //else
> > > > > + // flags |= RWF_DONTCACHE;
> > > > > + }
> > > >
> > > > IMO adding RWF_DONTCACHE first then replacing it later in the series
> > > > with a form of O_DIRECT is confusing. Also, why add RWF_DONTCACHE here
> > > > and then take it away "because it doesn't work"?

I spoke to this in a previous reply. I can fold patches to elininate
this distraction in v2.

> > > > But OK, your series is really a proof-of-concept. Something to work out
> > > > before it is merge-ready, I guess.
> > > >
> > > > It is much more likely for NFS READ requests to be properly aligned.
> > > > Clients are generally good about that. NFS WRITE request alignment
> > > > is going to be arbitrary. Fwiw.

Correct, thankfully TCP reads don't misalign their payload like TCP
writes do. As you know, the value of patch 6 is that application IO
that generates misaligned IO (as a side-effect of misaligned read
blocksize, e.g. IOR hard's 47008 blocksize) can issue reads using
O_DIRECT.

> > > > However, one thing we discussed at bake-a-thon was what to do about
> > > > unstable WRITEs. For unstable WRITEs, the server has to cache the
> > > > write data at least until the client sends a COMMIT. Otherwise the
> > > > server will have to convert all UNSTABLE writes to FILE_SYNC writes,
> > > > and that can have performance implications.
> > > >
> > >
> > > If we're doing synchronous, direct I/O writes then why not just respond
> > > with FILE_SYNC? The write should be on the platter by the time it
> > > returns.

For v2 I'll look to formalize responding with FILE_SYNC when
'enable-dontcache' is set.

> > Because "platter". On some devices, writes are slow.
> >
> > For some workloads, unstable is faster. I have an experimental series
> > that makes NFSD convert all NFS WRITEs to FILE_SYNC. It was not an
> > across the board win, even with an NVMe-backed file system.
> >
>
> Presumably, those devices wouldn't be exported in this mode. That's
> probably a good argument for making this settable on a per-export
> basis.

Correct. This shouldn't be used by default. But if/when it makes
sense, it *really* sings.

> > > > One thing you might consider is to continue using the page cache for
> > > > unstable WRITEs, and then use fadvise DONTNEED after a successful
> > > > COMMIT operation to reduce page cache footprint. Unstable writes to
> > > > the same range of the file might be a problem, however.
> > >
> > > Since the client sends almost everything UNSTABLE, that would probably
> > > erase most of the performance win. The only reason I can see to use
> > > buffered I/O in this mode would be because we had to deal with an
> > > unaligned write and need to do a RMW cycle on a block.
> > >
> > > The big question is whether mixing buffered and direct I/O writes like
> > > this is safe across all exportable filesystems. I'm not yet convinced
> > > of that.
> >
> > Agreed, that deserves careful scrutiny.
> >
>
> Like Mike is asking though, I need a better understanding of the
> potential races here:
>
> XFS, for instance, takes the i_rwsem shared around dio writes and
> exclusive around buffered, so they should exclude each other.

> If we did all the buffered writes as RWF_SYNC, would that prevent
> corruption?

I welcome any help pinning down what must be done to ensure this
is safe ("this" being: arbitrary switching between buffered and direct
IO and associated page cache invalidation). But to be 100% clear:
NFSD exporting XFS with enable-dontcache=1 has worked very well.

Do we need to go to the extreme of each filesystem exporting support
with a new flag like FOP_INVALIDATES_BUFFERED_VS_DIRECT? And if set,
any evidence to the contrary is a bug?

And does the VFS have a role in ensuring it's safe or can we assume
vfs/mm/etc are intended to be safe and any core common code that
proves otherwise is a bug?

> In any case, for now at least, unless you're using RDMA, it's going to
> end up falling back to buffered writes everywhere. The data is almost
> never going to be properly aligned coming in off the wire. That might
> be fixable though.

Ben Coddington mentioned to me that soft-iwarp would allow use of RDMA
over TCP to workaround SUNRPC TCP's XDR handling always storing the
write payload in misaligned IO. But that's purely a stop-gap
workaround, which needs testing (to see if soft-iwap negates the win
of using O_DIRECT, etc).

But a long-term better fix is absolutely needed, to be continued (in
the subthread I need to get going)...

Mike