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

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Message-ID: <20070516153919.GC2630@chiva>
Date:	Wed, 16 May 2007 17:39:19 +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 02:23:39PM -0700, Andrew Morton wrote:
> > 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().
> 
> hm, confused.  Why is get_block() failing?  That has to go and read
> metadata. 

In fact, I am referring to the first part of my test case (i.e. mount the ext3
fs, enable medium errors in scsi_debug and try to read a file from the fs).

So, when I try to read a file, ext3_get_block() needs to read metadata from the
disk. However, given that the SCSI disk simulated by scsi_debug reports medium
errors, ext3_get_block() returns EIO to the caller (i.e. do_mpage_readpage()).
That's why get_block() is failing.

Then, do_mpage_readpage() calls block_read_full_page() (via "goto confused").
block_read_full_page() attaches buffers to this page and calls ext3_get_block()
which fails for the same reason as before. Consequently, block_read_full_page()
sets the PG_error flag.
Moreover, all subsequent readpage calls will go through block_read_full_page()
because the page has now buffers attached.

Basically, my problem is that afterwards, when the device no longer returns
any errors, the PG_error flag is never cleared and, as a result, I keep
getting -EIO. That's the problem I'd like to address.

> If get_block() failed then we don't know what blocks to read to
> bring this page uptodate, so the pagecache page should remain in state
> !PageUptodate(), !PageError().  But then, we shouldn't have populated
> pagecache at that offset at all.

Yes, indeed. do_generic_mapping_read() doesn't populate the pagecache if an
error occurred.
Still, __do_page_cache_readahead()->read_pages()->ext3_readpages()->mpage_readpages()
does populate the pagecache even if the page reads failed.

> I think I'm missing something here.  I suspect you're referring to a mix
> of reading the blockdev via /dev/hda1 and then using the already-populated
> pagecache as filesystem metadata, or something?

I don't think so, unless I'm missing something.

> Is the PageError page part of an S_ISREG file, or is it part of an S_ISBLK
> file?

The PageError pages are part of a regular file and have been read through
readahead.

Johann
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