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

[Chuck Lever <cel@kernel.org>]

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?


-- 
Chuck Lever