Re: [PATCH 1/2] x86: Ignore VM_LOCKED when determining ifhugetlb-backed page tables can be shared or not
From: Mel Gorman
Date: Thu May 28 2009 - 04:55:26 EST
On Thu, May 28, 2009 at 01:18:03AM +0200, Ingo Molnar wrote:
> * Mel Gorman <mel@xxxxxxxxx> 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@xxxxxxxxx>
> > Acked-by: Hugh Dickins <hugh.dickins@xxxxxxxxxxxxx>
> > ---
> > 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
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab
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