Re: [PATCH 1/1] mm/hugetlb: fix huge page reservation leak in private mapping error paths

[Hillf Danton]

On Wednesday, November 02, 2016 12:37 AM Mike Kravetz wrote:
> On 10/19/2016 08:11 PM, Mike Kravetz wrote:
> > Error paths in hugetlb_cow() and hugetlb_no_page() may free a newly
> > allocated huge page.  If a reservation was associated with the huge
> > page, alloc_huge_page() consumed the reservation while allocating.
> > When the newly allocated page is freed in free_huge_page(), it will
> > increment the global reservation count.  However, the reservation entry
> > in the reserve map will remain.  This is not an issue for shared
> > mappings as the entry in the reserve map indicates a reservation exists.
> > But, an entry in a private mapping reserve map indicates the reservation
> > was consumed and no longer exists.  This results in an inconsistency
> > between the reserve map and the global reservation count.  This 'leaks'
> > a reserved huge page.
> >
> > Create a new routine restore_reserve_on_error() to restore the reserve
> > entry in these specific error paths.  This routine makes use of a new
> > function vma_add_reservation() which will add a reserve entry for a
> > specific address/page.
> >
> > In general, these error paths were rarely (if ever) taken on most
> > architectures.  However, powerpc contained arch specific code that
> > that resulted in an extra fault and execution of these error paths
> > on all private mappings.
> >
> > Fixes: 67961f9db8c4 ("mm/hugetlb: fix huge page reserve accounting for private mappings)
> >
> 
> Any additional comments on this?
> 
> It does address a regression with private mappings that appears to only be
> visible on powerpc.  Aneesh submitted a patch to workaround the issue on
> powerpc that is in mmotm/linux-next (71271479df7e/955f9aa468e0).  Aneesh's
> patch makes the symptoms go away.  This patch addresses root cause.
> 
Both works are needed, thanks.

Acked-by: Hillf Danton <hillf.zj@xxxxxxxxxxxxxxx>