Re: [PATCH v3 2/3] iomap: Don't create iomap_page objects for inline files

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

On Wed, Jul 07, 2021 at 03:28:47PM +0100, Matthew Wilcox wrote:
> On Wed, Jul 07, 2021 at 01:55:23PM +0200, Andreas Gruenbacher wrote:
> > @@ -252,6 +253,7 @@ iomap_readpage_actor(struct inode *inode, loff_t pos, loff_t length, void *data,
> >  	}
> >  
> >  	/* zero post-eof blocks as the page may be mapped */
> > +	iop = iomap_page_create(inode, page);
> >  	iomap_adjust_read_range(inode, iop, &pos, length, &poff, &plen);
> >  	if (plen == 0)
> >  		goto done;
> 
> I /think/ a subsequent patch would look like this:
> 
> +	/* No need to create an iop if the page is within an extent */
> +	loff_t page_pos = page_offset(page);
> +	if (pos > page_pos || pos + length < page_pos + page_size(page))
> +		iop = iomap_page_create(inode, page);
> 
> but that might miss some other reasons to create an iop.

I was under the impression that for blksize<pagesize filesystems, the
page always had to have an iop attached.  In principle I think you're
right that we don't need one if all i_blocks_per_page blocks have the
same uptodate state, but someone would have to perform a close reading
of buffered-io.c to make it drop them when unnecessary and re-add them
if it becomes necessary.  That might be more cycling through kmem_alloc
than we like, but as I said, I have never studied this idea.

--D