Re: [PATCH] mm: thp: fix pmd_bad() triggering in code paths holding mmap_sem read mode

[Rik van Riel <riel@redhat.com>]

On 03/15/2012 10:44 AM, Andrea Arcangeli wrote:
> In some cases it may happen that pmd_none_or_clear_bad() is called
> with the mmap_sem hold in read mode. In those cases the huge page
> faults can allocate hugepmds under pmd_none_or_clear_bad() and that
> can trigger a false positive from pmd_bad() that will not like to see
> a pmd materializing as trans huge.
>
> It's not khugepaged the problem, khugepaged holds the mmap_sem in
> write mode (and all those sites must hold the mmap_sem in read mode to
> prevent pagetables to go away from under them, during code review it
> seems vm86 mode on 32bit kernels requires that too unless it's
> restricted to 1 thread per process or UP builds). The race is only
> with the huge pagefaults that can convert a pmd_none() into a
> pmd_trans_huge().

Acked-by: Rik van Riel <riel@redhat.com>

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