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Date:   Thu, 8 Jul 2021 21:27:37 -0700
From:   "Darrick J. Wong" <djwong@kernel.org>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Andreas Gruenbacher <agruenba@redhat.com>,
        Christoph Hellwig <hch@infradead.org>,
        linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        cluster-devel@redhat.com
Subject: Re: [PATCH v3 2/3] iomap: Don't create iomap_page objects for inline
 files
Message-ID: <20210709042737.GT11588@locust>
References: <20210707115524.2242151-1-agruenba@redhat.com>
 <20210707115524.2242151-3-agruenba@redhat.com>
 <YOW6Hz0/FgQkQDgm@casper.infradead.org>
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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