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.
--
Mike Kravetz
>> - the allocation should fail but the reserve count is still hold.
>>
>> Signed-off-by: Naoya Horiguchi <n-horigu...@ah.jp.nec.com>
>> Cc: <sta...@vger.kernel.org> [3.16+]
>> ---
>> - the reason why I set stable target to "3.16+" is that this patch can be
>> applied easily/automatically on these versions. But this bug seems to be
>> old one, so if you are interested in backporting to older kernels,
>> please let me know.
>> ---
>> mm/hugetlb.c | 5 ++++-
>> 1 files changed, 4 insertions(+), 1 deletions(-)
>>
>> diff --git v4.3/mm/hugetlb.c v4.3_patched/mm/hugetlb.c
>> index 9cc7734..77c518c 100644
>> --- v4.3/mm/hugetlb.c
>> +++ v4.3_patched/mm/hugetlb.c
>> @@ -1790,7 +1790,10 @@ struct page *alloc_huge_page(struct vm_area_struct
>> *vma,
>> page = alloc_buddy_huge_page(h, NUMA_NO_NODE);
>> if (!page)
>> goto out_uncharge_cgroup;
>> -
>> + if (!avoid_reserve && vma_has_reserves(vma, gbl_chg)) {
>> + SetPagePrivate(page);
>> + h->resv_huge_pages--;
>> + }
>
> I am wondering if this patch was prepared against the next tree.
>
>> spin_lock(&hugetlb_lock);
>> list_move(&page->lru, &h->hugepage_activelist);
>> /* Fall through */
>> --
>> 1.7.1
>
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