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Message-ID: <83d90b3a-eef0-d4aa-6733-b5c9e5370d4b@oracle.com>
Date: Thu, 14 Dec 2017 12:58:55 -0800
From: Mike Kravetz <mike.kravetz@...cle.com>
To: Michal Hocko <mhocko@...nel.org>
Cc: linux-mm@...ck.org, Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
Andrew Morton <akpm@...ux-foundation.org>,
LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 4/5] mm, hugetlb: get rid of surplus page accounting
tricks
On 12/13/2017 11:50 PM, Michal Hocko wrote:
> On Wed 13-12-17 16:45:55, Mike Kravetz wrote:
>> On 12/04/2017 06:01 AM, Michal Hocko wrote:
>>> From: Michal Hocko <mhocko@...e.com>
>>>
>>> alloc_surplus_huge_page increases the pool size and the number of
>>> surplus pages opportunistically to prevent from races with the pool size
>>> change. See d1c3fb1f8f29 ("hugetlb: introduce nr_overcommit_hugepages
>>> sysctl") for more details.
>>>
>>> The resulting code is unnecessarily hairy, cause code duplication and
>>> doesn't allow to share the allocation paths. Moreover pool size changes
>>> tend to be very seldom so optimizing for them is not really reasonable.
>>> Simplify the code and allow to allocate a fresh surplus page as long as
>>> we are under the overcommit limit and then recheck the condition after
>>> the allocation and drop the new page if the situation has changed. This
>>> should provide a reasonable guarantee that an abrupt allocation requests
>>> will not go way off the limit.
>>>
>>> If we consider races with the pool shrinking and enlarging then we
>>> should be reasonably safe as well. In the first case we are off by one
>>> in the worst case and the second case should work OK because the page is
>>> not yet visible. We can waste CPU cycles for the allocation but that
>>> should be acceptable for a relatively rare condition.
>>>
>>> Signed-off-by: Michal Hocko <mhocko@...e.com>
>>> ---
>>> mm/hugetlb.c | 60 +++++++++++++++++++++---------------------------------------
>>> 1 file changed, 21 insertions(+), 39 deletions(-)
>>>
>>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>>> index a1b8b2888ec9..0c7dc269b6c0 100644
>>> --- a/mm/hugetlb.c
>>> +++ b/mm/hugetlb.c
>>> @@ -1538,62 +1538,44 @@ int dissolve_free_huge_pages(unsigned long start_pfn, unsigned long end_pfn)
>>> static struct page *__alloc_surplus_huge_page(struct hstate *h, gfp_t gfp_mask,
>>> int nid, nodemask_t *nmask)
>>> {
>>> - struct page *page;
>>> - unsigned int r_nid;
>>> + struct page *page = NULL;
>>>
>>> if (hstate_is_gigantic(h))
>>> return NULL;
>>>
>>> - /*
>>> - * Assume we will successfully allocate the surplus page to
>>> - * prevent racing processes from causing the surplus to exceed
>>> - * overcommit
>>> - *
>>> - * This however introduces a different race, where a process B
>>> - * tries to grow the static hugepage pool while alloc_pages() is
>>> - * called by process A. B will only examine the per-node
>>> - * counters in determining if surplus huge pages can be
>>> - * converted to normal huge pages in adjust_pool_surplus(). A
>>> - * won't be able to increment the per-node counter, until the
>>> - * lock is dropped by B, but B doesn't drop hugetlb_lock until
>>> - * no more huge pages can be converted from surplus to normal
>>> - * state (and doesn't try to convert again). Thus, we have a
>>> - * case where a surplus huge page exists, the pool is grown, and
>>> - * the surplus huge page still exists after, even though it
>>> - * should just have been converted to a normal huge page. This
>>> - * does not leak memory, though, as the hugepage will be freed
>>> - * once it is out of use. It also does not allow the counters to
>>> - * go out of whack in adjust_pool_surplus() as we don't modify
>>> - * the node values until we've gotten the hugepage and only the
>>> - * per-node value is checked there.
>>> - */
>>> spin_lock(&hugetlb_lock);
>>> - if (h->surplus_huge_pages >= h->nr_overcommit_huge_pages) {
>>> - spin_unlock(&hugetlb_lock);
>>> - return NULL;
>>> - } else {
>>> - h->nr_huge_pages++;
>>> - h->surplus_huge_pages++;
>>> - }
>>> + if (h->surplus_huge_pages >= h->nr_overcommit_huge_pages)
>>> + goto out_unlock;
>>> spin_unlock(&hugetlb_lock);
>>>
>>> page = __hugetlb_alloc_buddy_huge_page(h, gfp_mask, nid, nmask);
>>> + if (!page)
>>> + goto out_unlock;
>>>
>>> spin_lock(&hugetlb_lock);
>>> - if (page) {
>>> + /*
>>> + * We could have raced with the pool size change.
>>> + * Double check that and simply deallocate the new page
>>> + * if we would end up overcommiting the surpluses. Abuse
>>> + * temporary page to workaround the nasty free_huge_page
>>> + * codeflow
>>> + */
>>> + if (h->surplus_huge_pages >= h->nr_overcommit_huge_pages) {
>>> + SetPageHugeTemporary(page);
>>> + put_page(page);
>>> + page = NULL;
>>> + } else {
>>> + h->surplus_huge_pages_node[page_to_nid(page)]++;
>>> + h->surplus_huge_pages++;
>>> INIT_LIST_HEAD(&page->lru);
>>> r_nid = page_to_nid(page);
>>> set_compound_page_dtor(page, HUGETLB_PAGE_DTOR);
>>> set_hugetlb_cgroup(page, NULL);
>>> - /*
>>> - * We incremented the global counters already
>>> - */
>>> h->nr_huge_pages_node[r_nid]++;
>>> h->surplus_huge_pages_node[r_nid]++;
>>> - } else {
>>> - h->nr_huge_pages--;
>>> - h->surplus_huge_pages--;
>>
>> In the case of a successful surplus allocation, the following counters
>> are incremented:
>>
>> h->surplus_huge_pages_node[page_to_nid(page)]++;
>> h->surplus_huge_pages++;
>> h->nr_huge_pages_node[r_nid]++;
>> h->surplus_huge_pages_node[r_nid]++;
>>
>> Looks like per-node surplus_huge_pages_node is incremented twice, and
>> global nr_huge_pages is not incremented at all.
>>
>> Also, you removed r_nid so I'm guessing this will not compile?
>
> Ups a hickup during the rebase/split up. The following code removes all
> this so I haven't noticed. Thanks for catching that!
> The incremental diff
> ---
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 41d2d9082f0d..3c16cde72ceb 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1565,8 +1565,10 @@ static struct page *__alloc_surplus_huge_page(struct hstate *h, gfp_t gfp_mask,
> put_page(page);
> page = NULL;
> } else {
> - h->surplus_huge_pages_node[page_to_nid(page)]++;
> + int r_nid;
> +
> h->surplus_huge_pages++;
> + h->nr_huge_pages++;
> INIT_LIST_HEAD(&page->lru);
> r_nid = page_to_nid(page);
> set_compound_page_dtor(page, HUGETLB_PAGE_DTOR);
>
With the incremental diff,
Reviewed-by: Mike Kravetz <mike.kravetz@...cle.com>
--
Mike Kravetz
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