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

[Mel Gorman <mel@csn.ul.ie>]

On Thu, May 28, 2009 at 01:18:03AM +0200, Ingo Molnar wrote:
> 
> * Mel Gorman <mel@csn.ul.ie> 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>
> > ---
> >  arch/x86/mm/hugetlbpage.c |    6 +++++-
> >  1 files changed, 5 insertions(+), 1 deletions(-)
> 
> i suspect it would be best to do this due -mm, due to the (larger) 
> mm/hugetlb.c cross section, right?
> 

I'm happy with that approach. Almost all hugetlbfs-related patches have
gone through -mm to date AFAIK even when they have been arch specific
like this.

Thanks

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab

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