Re: [PATCH v7 14/14] NFSD: Initialize separate ki_flags

[Mike Snitzer <snitzer@kernel.org>]

On Mon, Oct 27, 2025 at 09:48:23AM -0400, Chuck Lever wrote:
> On 10/27/25 6:50 AM, Jeff Layton wrote:
> > On Mon, 2025-10-27 at 01:15 -0700, Christoph Hellwig wrote:
> >> On Fri, Oct 24, 2025 at 07:56:59PM -0400, Mike Snitzer wrote:
> >>> Really, even ignoring all the quirkiness of this: that O_DIRECT can
> >>> fallback to buffered, and we need IOCB_DSYNC|IOCB_SYNC for our use of
> >>> buffered IO when NFSD_IO_DIRECT configured to ensure data has hit
> >>> stable storage -- that's enough justification.  Bit circular but
> >>> compelling to prove the need.. albeit wordy and a lot to unpack.
> >> You always need IOCB_DSYNC for data to hit stable storage, both for
> >> buffered and direct I/O.  You need IOCB_SYNC in addition to also sync
> >> out the timestamps, which I think we now agree we need.  I still don't
> >> understand why using direct I/O implies that we want NFS stable writes
> >> and not two-stage writes, though.
> > That's certainly a possibility too. Consider the case where we have a
> > WRITE with unaligned parts at both ends. This set so far just does the
> > ends as synchronous I/Os.
> > 
> > We could do the end bits as non-synchronous writes, and follow up with
> > a vfs_fsync_range() call before returning NFS_FILE_SYNC.
> 
> What concerns me a bit is that the code that handles unaligned ends
> is careful to issue the vfs_iocb_iter_writes in file offset order. Are
> we OK to use IOCB_DSYNC for the unaligned parts but IOCB_DIRECT +
> subsequent COMMIT for the direct I/O middle segment?

LOCALIO's misaligned DIO will issue head/tail followed by O_DIRECT
middle (via AIO completion of that aligned middle).  So out of order
relative to file offset.

That has been working well with LOCALIO.. though I did just post a
patchset today that fixes some quirks of the implementation that got
flagged for KASAN use-after-free.

(But now that I look at LOCALIO misaligned DIO code, it actually isn't
even setting IOCB_DSYNC for the misaligned head/tail... needs fixing.)