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

[Ingo Molnar]

* 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?

	Ingo
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