Re: [PATCH 1/2] x86: Ignore VM_LOCKED when determining if hugetlb-backed page tables can be shared or not

[Eric B Munson <ebmunson@us.ibm.com>]

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On Wed, 27 May 2009, Mel Gorman wrote:

> On x86 and x86-64, it is possible that page tables are shared beween shared
> mappings backed by hugetlbfs. As part of this, page_table_shareable() checks
> a pair of vma->vm_flags and they must match if they are to be shared. All
> VMA flags are taken into account, including VM_LOCKED.
> 
> The problem is that VM_LOCKED is cleared on fork(). When a process with a
> shared memory segment forks() to exec() a helper, there will be shared VMAs
> with different flags. The impact is that the shared segment is sometimes
> considered shareable and other times not, depending on what process is
> checking.
> 
> What happens is that the segment page tables are being shared but the count is
> inaccurate depending on the ordering of events. As the page tables are freed
> with put_page(), bad pmd's are found when some of the children exit. The
> hugepage counters also get corrupted and the Total and Free count will
> no longer match even when all the hugepage-backed regions are freed. This
> requires a reboot of the machine to "fix".
> 
> This patch addresses the problem by comparing all flags except VM_LOCKED when
> deciding if pagetables should be shared or not for hugetlbfs-backed mapping.
> 
> Signed-off-by: Mel Gorman <mel@csn.ul.ie>
> Acked-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>

I tested this patch using 2.6.30-rc7 and the libhugetlbfs test suite on x86_64.
Everything looks good to me.

Acked-by: Eric B Munson <ebmunson@us.ibm.com>
Tested-by: Eric B Munson <ebmunson@us.ibm.com>

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