Date: Tue, 9 Jun 2009 15:28:47 +0200
From: Nick Piggin <npiggin@suse.de>
To: Wu Fengguang <fengguang.wu@intel.com>
Cc: Andi Kleen <andi@firstfloor.org>,
       "akpm@linux-foundation.org" <akpm@linux-foundation.org>,
       "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
       "linux-mm@kvack.org" <linux-mm@kvack.org>
Subject: Re: [PATCH] [11/16] HWPOISON: check and isolate corrupted free pages v2
Message-ID: <20090609132847.GC15219@wotan.suse.de>
References: <20090603846.816684333@firstfloor.org> <20090603184645.68FA21D0286@basil.firstfloor.org> <20090609100229.GE14820@wotan.suse.de> <20090609130304.GF5589@localhost>
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On Tue, Jun 09, 2009 at 09:03:04PM +0800, Wu Fengguang wrote:
> On Tue, Jun 09, 2009 at 06:02:29PM +0800, Nick Piggin wrote:
> > On Wed, Jun 03, 2009 at 08:46:45PM +0200, Andi Kleen wrote:
> > > 
> > > From: Wu Fengguang <fengguang.wu@intel.com>
> > > 
> > > If memory corruption hits the free buddy pages, we can safely ignore them.
> > > No one will access them until page allocation time, then prep_new_page()
> > > will automatically check and isolate PG_hwpoison page for us (for 0-order
> > > allocation).
> > 
> > It would be kinda nice if this could be done in the handler
> > directly (ie. take the page directly out of the allocator
> > or pcp list). Completely avoiding fastpaths would be a
> > wonderful goal.
> 
> In fact Andi have code to do that. We prefer this one because
> - it's simple
> - it's good sanity check for possible software BUGs
> - it mainly adds overhead to high order pages, which is acceptable

Yeah it's not bad. But we don't have much other non-debug options
that check for random memory corruption like this. Given that the
struct page is a very tiny proportion of memory, then I'm not
totally convinced that all this checking in the page allocator is
worthwhile for everyone. It's a much bigger cost if checks and
branches have to be there just for hwpoison.

And I don't think removing a free page from the page allocator is
too much more complex than removing a live page from the pagecache ;)


> 
> Thanks,
> Fengguang
> 
> > >  
> > > +	/* Don't complain about poisoned pages */
> > > +	if (PageHWPoison(page)) {
> > > +		__ClearPageBuddy(page);
> > > +		return;
> > > +	}
> > > +
> 
> I do think the above chunk is not absolutely necessary, though.
> 
> Thanks,
> Fengguang
> 
> 
> > >  	/*
> > >  	 * Allow a burst of 60 reports, then keep quiet for that minute;
> > >  	 * or allow a steady drip of one report per second.
> > > @@ -650,7 +656,7 @@
> > >  /*
> > >   * This page is about to be returned from the page allocator
> > >   */
> > > -static int prep_new_page(struct page *page, int order, gfp_t gfp_flags)
> > > +static inline int check_new_page(struct page *page)
> > >  {
> > >  	if (unlikely(page_mapcount(page) |
> > >  		(page->mapping != NULL)  |
> > > @@ -659,6 +665,18 @@
> > >  		bad_page(page);
> > >  		return 1;
> > >  	}
> > > +	return 0;
> > > +}
> > > +
> > > +static int prep_new_page(struct page *page, int order, gfp_t gfp_flags)
> > > +{
> > > +	int i;
> > > +
> > > +	for (i = 0; i < (1 << order); i++) {
> > > +		struct page *p = page + i;
> > > +		if (unlikely(check_new_page(p)))
> > > +			return 1;
> > > +	}
> > >  
> > >  	set_page_private(page, 0);
> > >  	set_page_refcounted(page);
--
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