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 15:24:41 +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 03:25:38PM +0200, Andi Kleen wrote:
> 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.

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).

 
> > > 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 ...

+static int me_pagecache_clean(struct page *p)
+{
+       if (!isolate_lru_page(p))
+               page_cache_release(p);
+
+       if (page_has_private(p))
+               do_invalidatepage(p, 0);
+       if (page_has_private(p) && !try_to_release_page(p, GFP_NOIO))
+               Dprintk(KERN_ERR "MCE %#lx: failed to release buffers\n",
+                       page_to_pfn(p));
+
+       /*
+        * remove_from_page_cache assumes (mapping && !mapped)
+        */
+       if (page_mapping(p) && !page_mapped(p)) {
                               ^^^^^^^^^^^^^^^

+               remove_from_page_cache(p);
+               page_cache_release(p);
+       }
+
+       return RECOVERED;


> > > 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 it's that bad, then I'll be happy to rewrite it for you.

 
> > > > 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.

So I just don't know why you argue the point that you have lots
of large holes left.
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