Re: [RFC PATCH 4/5] mm, hugetlb: get rid of surplus page accounting tricks

[Michal Hocko]

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@xxxxxxxx>
> > 
> > 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@xxxxxxxx>
> > ---
> >  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);
-- 
Michal Hocko
SUSE Labs