Re: [PATCH 1/2] iomap: support tail packing inline read

From: Gao Xiang <hsiangkao@linux.alibaba.com>
To: Matthew Wilcox <willy@infradead.org>
Cc: "Darrick J. Wong" <djwong@kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Christoph Hellwig <hch@infradead.org>,
	Joseph Qi <joseph.qi@linux.alibaba.com>,
	Liu Bo <bo.liu@linux.alibaba.com>,
	linux-fsdevel@vger.kernel.org,
	Liu Jiang <gerry@linux.alibaba.com>,
	linux-erofs@lists.ozlabs.org
Subject: Re: [PATCH 1/2] iomap: support tail packing inline read
Date: Fri, 16 Jul 2021 21:56:23 +0800	[thread overview]
Message-ID: <YPGQB3zT4Wp4Q38X@B-P7TQMD6M-0146.local> (raw)
In-Reply-To: <YPGDZYT9OxdgNYf2@casper.infradead.org>

Hi Matthew,

On Fri, Jul 16, 2021 at 02:02:29PM +0100, Matthew Wilcox wrote:
> On Fri, Jul 16, 2021 at 01:07:23PM +0800, Gao Xiang wrote:
> > This tries to add tail packing inline read to iomap. Different from
> > the previous approach, it only marks the block range uptodate in the
> > page it covers.
> 
> Why?  This path is called under two circumstances: readahead and readpage.
> In both cases, we're trying to bring the entire page uptodate.  The inline
> extent is always the tail of the file, so we may as well zero the part of
> the page past the end of file and mark the entire page uptodate instead
> and leaving the end of the page !uptodate.
> 
> I see the case where, eg, we have the first 2048 bytes of the file
> out-of-inode and then 20 bytes in the inode.  So we'll create the iop
> for the head of the file, but then we may as well finish the entire
> PAGE_SIZE chunk as part of this iteration rather than update 2048-3071
> as being uptodate and leave the 3072-4095 block for a future iteration.

Thanks for your comments. Hmm... If I understand the words above correctly,
what I'd like to do is to cover the inline extents (blocks) only
reported by iomap_begin() rather than handling other (maybe)
logical-not-strictly-relevant areas such as post-EOF (even pages
will be finally entirely uptodated), I think such zeroed area should
be handled by from the point of view of the extent itself

         if (iomap_block_needs_zeroing(inode, iomap, pos)) {
                 zero_user(page, poff, plen);
                 iomap_set_range_uptodate(page, poff, plen);
                 goto done;
         }

The benefits I can think out are 1) it makes the logic understand
easier and no special cases just for tail-packing handling 2) it can
be then used for any inline extent cases (I mean e.g. in the middle of
the file) rather than just tail-packing inline blocks although currently
there is a BUG_ON to prevent this but it's easier to extend even further.
3) it can be used as a part for later partial page uptodate logic in
order to match the legacy buffer_head logic (I remember something if my
memory is not broken about this...)

Thanks,
Gao Xiang