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SubjectRe: [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> 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?

Ingo


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