linux-kernel - Re: [PATCH] [13/16] HWPOISON: The high level memory error handler in the VM v3

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Message-ID: <20090602123720.GF1392@wotan.suse.de>
Date:	Tue, 2 Jun 2009 14:37:20 +0200
From:	Nick Piggin <npiggin@...e.de>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	hugh@...itas.com, riel@...hat.com, akpm@...ux-foundation.org,
	chris.mason@...cle.com, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, fengguang.wu@...el.com
Subject: Re: [PATCH] [13/16] HWPOISON: The high level memory error handler in the VM v3

On Tue, Jun 02, 2009 at 02:34:50PM +0200, Andi Kleen wrote:
> On Tue, Jun 02, 2009 at 02:10:31PM +0200, Nick Piggin wrote:
> > > It's not, there are various differences (like the reference count)
> > 
> > No. If there are, then it *really* needs better documentation. I
> > don't think there are, though.
> 
> Better documentation on what? You want a detailed listing in a comment
> how it is different from truncate?
> 
> To be honest I have some doubts of the usefulness of such a comment
> (why stop at truncate and not list the differences to every other
> page cache operation? @) but if you're insist (do you?) I can add one.

Because I don't see any difference (see my previous patch). I
still don't know what it is supposed to be doing differently.
So if you reinvent your own that looks close enough to truncate
to warrant a comment to say /* this is close to truncate but
not quite */, then yes I insist that you say exactly why it is
not quite like truncate ;)

 
> > I'm suggesting that EIO is traditionally for when the data still
> > dirty in pagecache and was not able to get back to backing
> > store. Do you deny that?
> 
> Yes. That is exactly the case when memory-failure triggers EIO
> 
> Memory error on a dirty file mapped page.

But it is no longer dirty, and the problem was not that the data
was unable to be written back.


> > And I think the application might try to handle the case of a
> > page becoming corrupted differently. Do you deny that?
> 
> You mean a clean file-mapped page? In this case there is no EIO,
> memory-failure just drops the page and it is reloaded.
> 
> If the page is dirty we trigger EIO which as you said above is the 
> right reaction.

No I mean the difference between the case of dirty page unable to
be written to backing sotre, and the case of dirty page becoming
corrupted.


> > OK, given the range of errors that APIs are defined to return,
> > then maybe EIO is the best option. I don't suppose it is possible
> > to expand them to return something else?
> 
> Expand the syscalls to return other errnos on specific
> kinds of IO error?
>  
> Of course that's possible, but it has the problem that you 
> would need to fix all the applications that expect EIO for
> IO error. The later I consider infeasible.

They would presumably exit or do some default thing, which I
think would be fine. Actually if your code catches them in the
act of manipulating a corrupted page (ie. if it is mmapped),
then it gets a SIGBUS.
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