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

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Message-ID: <4CDDA8E7.2090003@redhat.com>
Date:	Fri, 12 Nov 2010 14:51:51 -0600
From:	Eric Sandeen <esandeen@...hat.com>
To:	Rik van Riel <riel@...hat.com>
CC:	Zan Lynx <zlynx@....org>, Jeff Layton <jlayton@...hat.com>,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	jmoyer@...hat.com, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] clear PageError bit in msync & fsync

On 11/09/2010 03:24 PM, Rik van Riel wrote:
> On 11/09/2010 04:21 PM, Zan Lynx wrote:
>> On 11/9/10 12:33 PM, Rik van Riel wrote:
>>> 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...
>>
>> Just an off the top of my head crazy idea...
>>
>> Could you leave the error bit set on the page and treat it as a dirty
>> bit during a future msync, clearing the error bit at that point.
>>
>> The general idea would be to leave the error set unless an explicit
>> write was requested.
> 
> The problem with that is that the page will be unreclaimable,
> and the VM could get filled with PageError pages and be unable
> to make further progress (if the IO path does not come back).

As a further crazy idea ;)  what if it only persisted for "X" write
attempts?  Maybe (sigh) a tunable?

That way several fsyncs get the chance to see it, but eventually
enough writebacks will go off to give up and clear it.  Hacky,
but an idea ...

-Eric
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