Re: [PATCH v7 06/14] NFSD: Always set IOCB_SYNC in direct write path

[Christoph Hellwig <hch@infradead.org>]

On Fri, Oct 24, 2025 at 10:42:58AM -0400, Chuck Lever wrote:
> +	/*
> +	 * IOCB_SYNC + IOCB_DIRECT requests that iter_write should persist
> +	 * both written data and dirty time stamps.
> +	 *
> +	 * When falling back to buffered I/O or handling the unaligned
> +	 * first and last segments, the data and time stamps must be
> +	 * durable before nfsd_vfs_write() returns to its caller, matching
> +	 * the behavior of direct I/O.

I still haven't understood why we need for for sync writes with direct
I/O.  The comments suggest it has something to do with the buffered write
fallback, but even for that I don't really understand it. But for pure
direct I/O writes that are properly aligned there definitively should be
no need.  If there is a need for the fallback we really need to explain
it, as it's non-obvious and a performance issue.