Re: [PATCH 4/4] xfs: report the correct read/write dio alignment for reflinked inodes

["Darrick J. Wong" <djwong@kernel.org>]

On Tue, Jan 07, 2025 at 07:10:12AM +0100, Christoph Hellwig wrote:
> On Mon, Jan 06, 2025 at 06:37:06PM +0000, John Garry wrote:
> >> +	/*
> >> +	 * On COW inodes we are forced to always rewrite an entire file system
> >> +	 * block or RT extent.
> >> +	 *
> >> +	 * Because applications assume they can do sector sized direct writes
> >> +	 * on XFS we fall back to buffered I/O for sub-block direct I/O in that
> >> +	 * case.  Because that needs to copy the entire block into the buffer
> >> +	 * cache it is highly inefficient and can easily lead to page cache
> >> +	 * invalidation races.
> >> +	 *
> >> +	 * Tell applications to avoid this case by reporting the natively
> >> +	 * supported direct I/O read alignment.
> >
> > Maybe I mis-read the complete comment, but did you really mean "natively 
> > supported direct I/O write alignment"? You have been talking about writes 
> > only, but then finally mention read alignment.
> 
> No, this is indeed intended to talk about the different (smaller) read
> alignment we are now reporting.  But I guess the wording is confusing
> enough that I should improve it?

How about:

/*
 * For COW inodes, we can only perform out of place writes of entire
 * file allocation units (clusters).  For a sub-cluster directio write,
 * we must fall back to buffered I/O to perform the RMW.  At best this
 * is highly inefficient; at worst it leads to page cache invalidation
 * races.  Tell applications to avoid this by reporting separately the
 * read and (larger) write alignments.
 */

--D