linux-kernel - Re: [PATCH] clear PageError bit in msync & fsync

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Message-ID: <4CD9A209.6070807@redhat.com>
Date:	Tue, 09 Nov 2010 14:33:29 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Jeff Layton <jlayton@...hat.com>
CC:	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	esandeen@...hat.com, jmoyer@...hat.com,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] clear PageError bit in msync & fsync

On 11/09/2010 02:21 PM, Jeff Layton wrote:

> This does leave the page in sort of a funky state. The uptodate bit
> will still probably be set, but the dirty bit won't be. The page will
> be effectively "disconnected" from the backing store until someone
> writes to it.
>
> I suppose though that this is the best that can reasonably be done in
> this situation however...

I spent a few days looking for alternatives, and indeed I found
nothing better...

There are essentially two possibilities:
1) the VM can potentially be filled up with uncleanable dirty pages, or
2) pages that hit an IO error are left in a clean state, so they can
    be reclaimed under memory pressure

Alternative 1 could cause the entire system to deadlock, while
option 2 puts the onus on userland apps to rewrite the data
from a failed msync/fsync.

Currently the VM has behaviour #2 which is preserved with my
patch.

The only difference with my patch is, we won't keep returning
-EIO on subsequent, error free, msync or fsync calls to files
that had an IO error at some previous point in the past.

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