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

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Message-ID: <20090602132538.GK1065@one.firstfloor.org>
Date:	Tue, 2 Jun 2009 15:25:38 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Nick Piggin <npiggin@...e.de>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Wu Fengguang <fengguang.wu@...el.com>,
	"hugh@...itas.com" <hugh@...itas.com>,
	"riel@...hat.com" <riel@...hat.com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"chris.mason@...cle.com" <chris.mason@...cle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH] [13/16] HWPOISON: The high level memory error handler in the VM v3

On Tue, Jun 02, 2009 at 02:57:13PM +0200, Nick Piggin wrote:
> > > not a big deal and just avoids duplicating code. I attached an
> > > (untested) patch.
> > 
> > Thanks. But the function in the patch is not doing the same what
> > the me_pagecache_clean/dirty are doing. For once there is no error
> > checking, as in the second try_to_release_page()
> > 
> > Then it doesn't do all the IO error and missing mapping handling.
> 
> Obviously I don't mean just use that single call for the entire
> handler. You can set the EIO bit or whatever you like. The
> "error handling" you have there also seems strange. You could
> retain it, but the page is assured to be removed from pagecache.

The reason this code double checks is that someone could have 
a reference (remember we can come in any time) we cannot kill immediately.

> > The page_mapped() check is useless because the pages are not 
> > mapped here etc.
> 
> That's OK, it is a core part of the protocol to prevent
> truncated pages from being mapped, so I like it to be in
> that function.
> 
> (you are also doing extraneous page_mapped tests in your handler,
> so surely your concern isn't from the perspective of this
> error handler code)

We do page_mapping() checks, not page_mapped checks.

I know details, but ...

> 
> 
> > We could probably call truncate_complete_page(), but then
> > we would also need to duplicate most of the checking outside
> > the function anyways and there wouldn't be any possibility
> > to share the clean/dirty variants. If you insist I can
> > do it, but I think it would be significantly worse code
> > than before and I'm reluctant to do that.
> 
> I can write you the patch for that too if you like.

Ok I will write it, but I will add a comment saying that Nick forced
me to make the code worse @)

It'll be fairly redundant at least.

> > > if you already have other large ones.
> > 
> > That's unclear too.
> 
> You can't do much about most kernel pages, and dirty metadata pages
> are both going to cause big problems. User pagetable pages. Lots of
> stuff.

User page tables was on the todo list, these are actually relatively
easy. The biggest issue is to detect them.

Metadata would likely need file system callbacks, which I would like to 
avoid at this point.

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only.
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