Re: [PATCH 3/3] xfs: measure all contiguous previous extents for prealloc size

[Christoph Hellwig <hch@infradead.org>]

On Wed, May 20, 2020 at 02:17:16PM -0700, Darrick J. Wong wrote:
> On Tue, May 19, 2020 at 05:54:37AM -0700, Christoph Hellwig wrote:
> > The actual logic looks good, but I think the new helper and another
> > third set of comment explaining what is going on makes this area even
> > more confusing.  What about something like this instead?
> 
> This seems reasonable, but the callsite cleanups ought to be a separate
> patch from the behavior change.

Do you want me to send prep patches, or do you want to split it our
yourself?

> > +	if (eof && offset + count > XFS_ISIZE(ip)) {
> > +		/*
> > +		 * Determine the initial size of the preallocation.
> > + 		 * We clean up any extra preallocation when the file is closed.
> > +		 */
> > +		if (mp->m_flags & XFS_MOUNT_ALLOCSIZE)
> > +			prealloc_blocks = mp->m_allocsize_blocks;
> > +		else
> > +			prealloc_blocks = xfs_iomap_prealloc_size(ip, allocfork,
> > +						offset, count, &icur);
> 
> I'm not sure how much we're really gaining from moving the
> MOUNT_ALLOCSIZE check out to the caller, but I don't feel all that
> passionate about this.

From the pure code stats point of view it doensn't matter.  But from the
software architecture POV it does - now xfs_iomap_prealloc_size contains
the dynamic prealloc size algorithm, while the hard coded case is
handled in the caller.