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

[Greg KH]

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@xxxxxxxx>
> Suggested-by: Michal Hocko <mhocko@xxxxxxxxxx>
> Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> ---
>  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>