linux-kernel - Re: [PATCH] mm/hugetlb.c: fix reservation race when freeing surplus pages

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Date:   Thu, 19 Jan 2017 19:16:51 +0100
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Paul Cassella <cassella@...y.com>,
        Michal Hocko <mhocko@...nel.org>,
        Masayoshi Mizuma <m.mizuma@...fujitsu.com>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        Aneesh Kumar <aneesh.kumar@...ux.vnet.ibm.com>,
        Hillf Danton <hillf.zj@...baba-inc.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        stable@...r.kernel.org
Subject: Re: [PATCH] mm/hugetlb.c: fix reservation race when freeing surplus
 pages

On Mon, Jan 09, 2017 at 11:56:07AM -0800, Mike Kravetz wrote:
> The routine return_unused_surplus_pages decrements the global
> reservation count, and frees any unused surplus pages that were
> backing the reservation.  Commit 7848a4bf51b3 ("mm/hugetlb.c:
> add cond_resched_lock() in return_unused_surplus_pages()") added
> a call to cond_resched_lock in the loop freeing the pages.  As
> a result, the hugetlb_lock could be dropped, and someone else
> could use the pages that will be freed in subsequent iterations
> of the loop.  This could result in inconsistent global hugetlb
> page state, application api failures (such as mmap) failures or
> application crashes.
> 
> When dropping the lock in return_unused_surplus_pages, make sure
> that the global reservation count (resv_huge_pages) remains
> sufficiently large to prevent someone else from claiming pages
> about to be freed.
> 
> Fixes: 7848a4bf51b3 ("mm/hugetlb.c: add cond_resched_lock() in return_unused_surplus_pages()")
> Reported-and-analyzed-by: Paul Cassella <cassella@...y.com>
> Suggested-by: Michal Hocko <mhocko@...nel.org>
> Signed-off-by: Mike Kravetz <mike.kravetz@...cle.com>
> ---
>  mm/hugetlb.c | 37 ++++++++++++++++++++++++++++---------
>  1 file changed, 28 insertions(+), 9 deletions(-)

<formletter>

This is not the correct way to submit patches for inclusion in the
stable kernel tree.  Please read Documentation/stable_kernel_rules.txt
for how to do this properly.

</formletter>