From: Johannes Weiner <hannes@cmpxchg.org>
To: Michal Hocko <mhocko@suse.cz>
Cc: Andrew Morton <akpm@linux-foundation.org>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
Hillf Danton <dhillf@gmail.com>, Mel Gorman <mel@csn.ul.ie>,
KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
Andrea Arcangeli <aarcange@redhat.com>,
David Rientjes <rientjes@google.com>
Subject: Re: [PATCH] hugetlb: do not use vma_hugecache_offset for vma_prio_tree_foreach
Date: Wed, 26 Sep 2012 16:56:17 -0400 [thread overview]
Message-ID: <20120926205617.GA2667@cmpxchg.org> (raw)
In-Reply-To: <1344866141-27906-1-git-send-email-mhocko@suse.cz>
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.
WARNING: multiple messages have this Message-ID (diff)
From: Johannes Weiner <hannes@cmpxchg.org>
To: Michal Hocko <mhocko@suse.cz>
Cc: Andrew Morton <akpm@linux-foundation.org>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
Hillf Danton <dhillf@gmail.com>, Mel Gorman <mel@csn.ul.ie>,
KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
Andrea Arcangeli <aarcange@redhat.com>,
David Rientjes <rientjes@google.com>
Subject: Re: [PATCH] hugetlb: do not use vma_hugecache_offset for vma_prio_tree_foreach
Date: Wed, 26 Sep 2012 16:56:17 -0400 [thread overview]
Message-ID: <20120926205617.GA2667@cmpxchg.org> (raw)
In-Reply-To: <1344866141-27906-1-git-send-email-mhocko@suse.cz>
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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next prev parent reply other threads:[~2012-09-26 20:56 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-08-13 13:55 [PATCH] hugetlb: do not use vma_hugecache_offset for vma_prio_tree_foreach Michal Hocko
2012-08-13 13:55 ` Michal Hocko
2012-08-16 12:45 ` Hillf Danton
2012-08-16 12:45 ` Hillf Danton
2012-08-16 15:11 ` Michal Hocko
2012-08-16 15:11 ` Michal Hocko
2012-09-26 20:56 ` Johannes Weiner [this message]
2012-09-26 20:56 ` Johannes Weiner
2012-10-01 16:22 ` Michal Hocko
2012-10-01 16:22 ` Michal Hocko
2012-10-01 18:29 ` Johannes Weiner
2012-10-01 18:29 ` Johannes Weiner
-- strict thread matches above, loose matches on Subject: below --
2012-08-13 13:09 [patch] hugetlb: correct page offset index for sharing pmd Michal Hocko
2012-08-13 13:24 ` [PATCH] hugetlb: do not use vma_hugecache_offset for vma_prio_tree_foreach Hillf Danton
2012-08-13 13:24 ` Hillf Danton
2012-08-13 13:49 ` Michal Hocko
2012-08-13 13:49 ` Michal Hocko
2012-08-13 13:51 ` Hillf Danton
2012-08-13 13:51 ` Hillf Danton
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