Re: Huge pages: Memory leak on mmap failure

[Mel Gorman <mel@csn.ul.ie>]

On Thu, May 17, 2012 at 03:50:17PM -0700, Dave Hansen wrote:
> On 05/17/2012 02:07 PM, Christoph Lameter wrote:
> > 
> > On 2.6.32 and 3.4-rc6 mmap failure of a huge page causes a memory
> > leak. The 32 byte kmalloc cache grows by 10 mio entries if running
> > the following code:
> 
> When called for anonymous (non-shared) mappings, hugetlb_reserve_pages()
> does a resv_map_alloc().  It depends on code in hugetlbfs's
> vm_ops->close() to release that allocation.
> 
> However, in the mmap() failure path, we do a plain unmap_region()
> without the remove_vma() which actually calls vm_ops->close().
> 
> As the code stands today, I think we can fix this by just making sure we
> release the resv_map after hugetlb_acct_memory() fails. 

This appears to be the most practical solution.

> But, this seems
> like a bit of a superficial fix and if we end up with another path or
> two that can return -ESOMETHING, this might get reintroduced.  The
> assumption that vm_ops->close() will get called on all VMAs passed in to
> hugetlbfs_file_mmap() seems like something that needs to get corrected.
> 

It does not look practical to move the allocation to somewhere like
hugetlb_vm_op_open() as minimally that operation is never expected to
fail. That leaves no sane way to communicate that a kmalloc() failed
for example.  ->close() will get called once hugetlb_reserve_pages()
returns successfully so right now, I'm not seeing a better fix than the
superficial fix.

-- 
Mel Gorman
SUSE Labs

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