Re: [PATCH] hugetlb: do not use vma_hugecache_offset for vma_prio_tree_foreach

[Johannes Weiner <hannes@cmpxchg.org>]

On Mon, Aug 13, 2012 at 03:55:41PM +0200, Michal Hocko wrote:
> 0c176d5 (mm: hugetlb: fix pgoff computation when unmapping page
> from vma) fixed pgoff calculation but it has replaced it by
> vma_hugecache_offset which is not approapriate for offsets used for
> vma_prio_tree_foreach because that one expects index in page units
> rather than in huge_page_shift.
> Using vma_hugecache_offset is not incorrect because the pgoff will fit
> into the same vmas but it is confusing so the standard PAGE_SHIFT based
> index calculation is used instead.

I do think it's incorrect.  The resulting index may not be too big,
but it can be too small: assume hpage size of 2M and the address to
unmap to be 0x200000.  This is regular page index 512 and hpage index
1.  If you have a VMA that maps the file only starting at the second
huge page, that VMAs vm_pgoff will be 512 but you ask for offset 1 and
miss it even though it does map the page of interest.  hugetlb_cow()
will try to unmap, miss the vma, and retry the cow until the
allocation succeeds or the skipped vma(s) go away.

Unless I missed something, this should not be deferred as a cleanup.

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