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

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 20 Nov 2015 15:57:21 +0800 "Hillf Danton" <hillf.zj@alibaba-inc.com> 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.
> > 
> > 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,
> > - the allocation should fail but the reserve count is still hold.
> > 
> > Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> > Cc: <stable@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.

It's against 4.3.

Here's the version I have, against current -linus:

--- a/mm/hugetlb.c~mm-hugetlb-fix-hugepage-memory-leak-caused-by-wrong-reserve-count
+++ a/mm/hugetlb.c
@@ -1886,7 +1886,10 @@ struct page *alloc_huge_page(struct vm_a
 		page = __alloc_buddy_huge_page_with_mpol(h, vma, addr);
 		if (!page)
 			goto out_uncharge_cgroup;
-
+		if (!avoid_reserve && vma_has_reserves(vma, gbl_chg)) {
+			SetPagePrivate(page);
+			h->resv_huge_pages--;
+		}
 		spin_lock(&hugetlb_lock);
 		list_move(&page->lru, &h->hugepage_activelist);
 		/* Fall through */

It needs a careful re-review and, preferably, retest please.

Probably when Greg comes to merge this he'll hit problems and we'll
need to provide him with the against-4.3 patch.

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