linux-kernel - Re: [PATCH] hugetlb: do not use vma_hugecache_offset for vma_prio_tree

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Message-ID: <20121001162226.GA24860@dhcp22.suse.cz>
Date:	Mon, 1 Oct 2012 18:22:26 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Hillf Danton <dhillf@...il.com>,
	Mel Gorman <mel@....ul.ie>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	David Rientjes <rientjes@...gle.com>
Subject: Re: [PATCH] hugetlb: do not use vma_hugecache_offset for
 vma_prio_tree_foreach

On Wed 26-09-12 16:56:17, Johannes Weiner wrote:
> 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.

You are right and I have totally missed this because I focused on the
other boundary too much :/ This vma_hugecache_offset is really
confusing.
Andrew has already updated the changelog so we will not get even more
confusion into the Linus tree.
Thanks for spotting this Johannes!
-- 
Michal Hocko
SUSE Labs
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