linux-kernel - Re: Hugepage regression

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Message-ID: <20061011011816.GA21235@localhost.localdomain>
Date:	Wed, 11 Oct 2006 11:18:16 +1000
From:	'David Gibson' <david@...son.dropbear.id.au>
To:	"Chen, Kenneth W" <kenneth.w.chen@...el.com>
Cc:	'Hugh Dickins' <hugh@...itas.com>, Andrew Morton <akpm@...l.org>,
	linux-kernel@...r.kernel.org
Subject: Re: Hugepage regression

On Tue, Oct 10, 2006 at 04:34:40PM -0700, Chen, Kenneth W wrote:
> Hugh Dickins wrote on Tuesday, October 10, 2006 12:18 PM
> > Yes, I'd expect your i_mmap_lock to solve the problem: and since
> > you're headed in that direction anyway, it makes most sense to use
> > that solution rather than get into defining arrays, or sacrificing
> > the lazy flush, or risking page_count races.
> > 
> > So please extract the __unmap_hugepage_range mods from your shared
> > pagetable patch, and use that to fix the bug.
> 
> 
> OK, here is a bug fix patch fixing earlier "bug fix" patch :-(
> 
> 
> [patch] hugetlb: fix linked list corruption in unmap_hugepage_range
> 
> commit fe1668ae5bf0145014c71797febd9ad5670d5d05 causes kernel to oops with
> libhugetlbfs test suite.  The problem is that hugetlb pages can be shared
> by multiple mappings. Multiple threads can fight over page->lru in the unmap
> path and bad things happen.  We now serialize __unmap_hugepage_range to void
> concurrent linked list manipulation.  Such serialization is also needed for
> shared page table page on hugetlb area. This patch will fixed the bug and
> also serve as a prepatch for shared page table.

Can I suggest that you put a big comment on the linked list
declaration itself saying that you're relying on serialization here.
Otherwise I'm worried someone will try to de-serialize it again, and
break it without realizing.  Given the number of people who failed to
spot the problem with the patch the first time around..

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson
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