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

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Date:	Tue, 2 Jun 2009 15:40:24 +0200
From:	Nick Piggin <npiggin@...e.de>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	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 03:41:26PM +0200, Andi Kleen wrote:
> On Tue, Jun 02, 2009 at 03:24:41PM +0200, Nick Piggin wrote:
> > On Tue, Jun 02, 2009 at 03:25:38PM +0200, Andi Kleen wrote:
> > > The reason this code double checks is that someone could have 
> > > a reference (remember we can come in any time) we cannot kill immediately.
> > 
> > Can't kill what? The page is gone from pagecache. It may remain
> > other kernel references, but I don't see why this code will
> > consider this as a failure (and not, for example, a raised error
> > count).
> 
> It's a failure because the page was still used and not successfully
> isolated.

But you're predicating success on page_count, so there can be
other users anyway. You do check page_count later and emit
a different message in this case, but even that isn't enough
to tell you if it has no more users.

I wouldn't have thought it's worth the complication, but
there is nothing preventing you using my truncate function
and also keeping this error check to test afterwards.
 

> > +        * remove_from_page_cache assumes (mapping && !mapped)
> > +        */
> > +       if (page_mapping(p) && !page_mapped(p)) {
> 
> Ok you're right. That one is not needed. I will remove it.
> 
> > > 
> > > 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.
> > 
> > So I just don't know why you argue the point that you have lots
> > of large holes left.
> 
> I didn't argue that. My point was just that I currently don't have
> data what holes are the worst on given workloads. If I figure out at
> some point that writeback pages are a significant part of some important
> workload I would be interested in tackling them.
> That said I think that's unlikely, but I'm not ruling it out.

Well, it sounds like maybe there is a sane way to do them with your
IO interception... but anyway let's not worry about this right
now ;)
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