linux-kernel - Re: Clear PG_error before reading a page

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Message-ID: <20070515210124.GA23698@chiva>
Date:	Tue, 15 May 2007 23:01:24 +0200
From:	Johann Lombardi <johann@...sterfs.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Clear PG_error before reading a page

On Tue, May 15, 2007 at 10:11:44AM -0700, Andrew Morton wrote:
> We need to make sure that this page has PG_uptodate cleared, so
> that a re-read is forced.  And the affected buffer_head, if any, should have
> buffer_uptodate() cleared.

ok.

> This change might have horrid interactions with readahead and various
> application access patterns: if for some reason a dud sector takes 30
> seconds of driver futzing to return -EIO and someone (application or
> kernel) tries to read the same sector tens or hundreds of times, suckiness
> ensues.  This will be hard to test for.

Yes, I agree. So according to you, what's the best way to address the
initial problem (i.e. PG_error never cleared)?

> Most reads don't (or shouldn't) go through block_read_full_page().
> mpage_readpages() does the heavy lifting.

Yes, indeed. However, as soon as a call to get_block() fails,
do_mpage_readpage() will call block_read_full_page() which will attach
buffers to this page.
Consequently, all subsequent reads will go through block_read_full_page().

Johann
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