Re: [PATCH v7 05/14] NFSD: @stable for direct writes is always NFS_FILE_SYNC

[Mike Snitzer <snitzer@kernel.org>]

On Mon, Oct 27, 2025 at 07:36:18AM -0700, Christoph Hellwig wrote:
> On Mon, Oct 27, 2025 at 10:31:55AM -0400, Mike Snitzer wrote:
> > > that.  High-end enough device won't have one, but a lot of devices that
> > > people NFS-export do.  For pure overwrites the file system could
> > > optimize this way by using the FUA flag, and at least the iomap direct
> > > I/O code does implementation that optimization for that particular case.
> > 
> > NFSD_IO_DIRECT isn't meant to be uniformly better for all types of
> > storage.  Any storage that has a volatile write cache is probably best
> > served by existing NFSD default (NFSD_IO_BUFFERED).
> 
> That's a very odd claim.  Also what does it have to do with the rest of
> the discussion here?

What's an odd claim?  That shitty storage shouldn't be the bad apple
that spoils the bunch?  You seem to be advocating for requiring
additional NFSD resources as part of the NFSD_IO_DIRECT
implementation.

But we could make NFSD's behavior tuneable such that it can signal to
NFS client NFS_FILE_SYNC or not.

The goal is to walk before expanding to running with other
permutations of how to tune NFSD's IO modes.

> > The client doesn't control if/when NFSD would make use of O_DIRECT
> > (other than if it sends misaligned IO and NFSD must do what it can to
> > ensure it safely hits stable storage).
> 
> Sure.
> 
> > In addition, the use of NFSD_IO_DIRECT is intended to allow for
> > systems large _and_ small to get the advantage of lower memory
> > utilization.  Buffered IO is one extreme, but even using a model where
> > NFSD were to not impose NFS_FILE_SYNC would create a situation where
> > more memory needed batch IO and then wait for client to send COMMIT.
> 
> Why?

Because NFSD will then need to hold the IO until the COMMIT operation.
That requires extra NFSD resurces right?

> > The current approach of using IOCB_DSYNC|IOCB_SYNC have performed
> > really well on modern NVMe servers.
> 
> NVMe does not implement a concept called servers.

But you're aware that servers have NVMe devices in them.. that's all I
meant.  All of these NFSD_IO_DIRECT changes have been developed and
tested in modern servers with 8 NVMe, see:
https://www.youtube.com/watch?v=tpPFDu9Nuuw

(NOTE: results covered in this session did _not_ have the benefit
of NFSD responding to client with NFS_FILE_SYNC to avoid COMMIT, the
ability to do so was discussed at Bakeathon and was acted on with
these latest NFSD Direct patchsets).