linux-kernel - Re: [RESEND][PATCH v5 3/3] hugetlbfs: add swap entry check in follow_hugetlb

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Message-ID: <CAHGf_=o6+RxyutMxai-82eHQDrx5KfiJ0-MCjqAO-nkbp1eiCA@mail.gmail.com>
Date:	Wed, 10 Apr 2013 12:31:39 -0400
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>, Hugh Dickins <hughd@...gle.com>,
	Rik van Riel <riel@...hat.com>,
	Konstantin Khlebnikov <khlebnikov@...nvz.org>,
	Michal Hocko <mhocko@...e.cz>,
	HATAYAMA Daisuke <d.hatayama@...fujitsu.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RESEND][PATCH v5 3/3] hugetlbfs: add swap entry check in follow_hugetlb_page()

On Wed, Apr 10, 2013 at 12:17 PM, Naoya Horiguchi
<n-horiguchi@...jp.nec.com> wrote:
> # I suspended Reviewed and Acked given for the previous version, because
> # it has a non-minor change. If you want to restore it, please let me know.
> -----
> With applying the previous patch "hugetlbfs: stop setting VM_DONTDUMP in
> initializing vma(VM_HUGETLB)" to reenable hugepage coredump, if a memory
> error happens on a hugepage and the affected processes try to access
> the error hugepage, we hit VM_BUG_ON(atomic_read(&page->_count) <= 0)
> in get_page().
>
> The reason for this bug is that coredump-related code doesn't recognise
> "hugepage hwpoison entry" with which a pmd entry is replaced when a memory
> error occurs on a hugepage.
> In other words, physical address information is stored in different bit layout
> between hugepage hwpoison entry and pmd entry, so follow_hugetlb_page()
> which is called in get_dump_page() returns a wrong page from a given address.
>
> The expected behavior is like this:
>
>   absent   is_swap_pte   FOLL_DUMP   Expected behavior
>   -------------------------------------------------------------------
>    true     false         false       hugetlb_fault
>    false    true          false       hugetlb_fault
>    false    false         false       return page
>    true     false         true        skip page (to avoid allocation)
>    false    true          true        hugetlb_fault
>    false    false         true        return page
>
> With this patch, we can call hugetlb_fault() and take proper actions
> (we wait for migration entries, fail with VM_FAULT_HWPOISON_LARGE for
> hwpoisoned entries,) and as the result we can dump all hugepages except
> for hwpoisoned ones.
>
> ChangeLog v5:
>  - improve comment and description.
>
> ChangeLog v4:
>  - move is_swap_page() to right place.
>
> ChangeLog v3:
>  - add comment about using is_swap_pte()
>
> Signed-off-by: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
> Cc: stable@...r.kernel.org

Acked-by: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
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