RE: mm: fix BUG in __split_huge_page_pmd

[Kirill A. Shutemov]

Hugh Dickins wrote:
> Occasionally we hit the BUG_ON(pmd_trans_huge(*pmd)) at the end of
> __split_huge_page_pmd(): seen when doing madvise(,,MADV_DONTNEED).
> 
> It's invalid: we don't always have down_write of mmap_sem there:
> a racing do_huge_pmd_wp_page() might have copied-on-write to another
> huge page before our split_huge_page() got the anon_vma lock.
> 
> Forget the BUG_ON, just go back and try again if this happens.
>     
> Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx

Looks reasonable to me.

Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>

madvise(MADV_DONTNEED) was aproblematic with THP before. Is a big win having
mmap_sem taken on read rather than on write for it?

-- 
 Kirill A. Shutemov
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