Re: [PATCH v1] mm: hugetlb: fix hugepage memory leak caused by wrong reserve count

[Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>]

On Fri, Nov 20, 2015 at 01:56:18PM -0800, Mike Kravetz wrote:
> On 11/19/2015 11:57 PM, Hillf Danton wrote:
> >>
> >> When dequeue_huge_page_vma() in alloc_huge_page() fails, we fall back to
> >> alloc_buddy_huge_page() to directly create a hugepage from the buddy allocator.
> >> In that case, however, if alloc_buddy_huge_page() succeeds we don't decrement
> >> h->resv_huge_pages, which means that successful hugetlb_fault() returns without
> >> releasing the reserve count. As a result, subsequent hugetlb_fault() might fail
> >> despite that there are still free hugepages.
> >>
> >> This patch simply adds decrementing code on that code path.
> 
> In general, I agree with the patch.  If we allocate a huge page via the
> buddy allocator and that page will be used to satisfy a reservation, then
> we need to decrement the reservation count.
> 
> As Hillf mentions, this code is not exactly the same in linux-next.
> Specifically, there is the new call to take the memory policy of the
> vma into account when calling the buddy allocator.  I do not think,
> this impacts your proposed change but you may want to test with that
> in place.
> 
> >>
> >> I reproduced this problem when testing v4.3 kernel in the following situation:
> >> - the test machine/VM is a NUMA system,
> >> - hugepage overcommiting is enabled,
> >> - most of hugepages are allocated and there's only one free hugepage
> >>   which is on node 0 (for example),
> >> - another program, which calls set_mempolicy(MPOL_BIND) to bind itself to
> >>   node 1, tries to allocate a hugepage,
> 
> I am curious about this scenario.  When this second program attempts to
> allocate the page, I assume it creates a reservation first.  Is this
> reservation before or after setting mempolicy?  If the mempolicy was set
> first, I would have expected the reservation to allocate a page on
> node 1 to satisfy the reservation.

My testing called set_mempolicy() at first then called mmap(), but things
didn't change if I reordered them, because currently hugetlb reservation is
not NUMA-aware.

Thanks,
Naoya Horiguchi
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