racing anon_vma_prepares

[Hugh Dickins]

Just noticed that you rely on mmap_sem to protect anon_vma_prepare:
but it doesn't, since concurrent faults can both down_read(&mmap_sem).
I think the anon_vma_alloc should be done where you have anon_vma_prepare,
but some kind of set_anon_vma under page_table_lock to set or free it.

Hugh

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