linux-ext4 - RE: [PATCH V4 4/8] Cleancache: VFS hooks for cleancache

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Date:	Mon, 30 Aug 2010 17:48:17 -0700 (PDT)
From:	Dan Magenheimer <dan.magenheimer@...cle.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Chris Mason <chris.mason@...cle.com>, viro@...iv.linux.org.uk,
	akpm@...ux-foundation.org, adilger@....com, tytso@....edu,
	mfasheh@...e.com, Joel Becker <joel.becker@...cle.com>,
	matthew@....cx, linux-btrfs@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-ext4@...r.kernel.org, ocfs2-devel@....oracle.com,
	linux-mm@...ck.org, ngupta@...are.org, JBeulich@...ell.com,
	Kurt Hackel <kurt.hackel@...cle.com>, npiggin@...nel.dk,
	Dave Mccracken <dave.mccracken@...cle.com>, riel@...hat.com,
	avi@...hat.com, Konrad Wilk <konrad.wilk@...cle.com>,
	mel@....ul.ie, yinghan@...gle.com, gthelen@...gle.com
Subject: RE: [PATCH V4 4/8] Cleancache: VFS hooks for cleancache

> > +		if (s->cleancache_poolid >= 0) {
> > +			int cleancache_poolid = s->cleancache_poolid;
> 
> That's a lot of characters for a local in a 3-line scope.

:-) OK.
 
> > +			s->cleancache_poolid = -1; /* avoid races */
> 
> Races with what?  Something else sneaking something into the pool after
> the flush?  Is the filesystem dead at this stage or not?

If there are any inodes in the page cache that point to
this superblock, a cleancache_put_page may happen asynchronously
that grabs from page->...->i_sb->cleancache_poolid.
If the cleancache_flush_fs races with it and another
cleancache_init_fs happens after the cleancache_put_page
gets the poolid, the poolid could be stale.

Highly unlikely, but I thought it best to be safe.

> > +	/* 99% of the time, we don't need to flush the cleancache on the
> bdev.
> > +	 * But, for the strange corners, lets be cautious
> > +	 */
> 
> This comment-style is... unconventional for the kernel.

Yeah, I decided to defer to Chris Mason's wisdom and left it. :-)

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