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.
Adding Andrew to Cc as I accidentally left him off originally, and he
may have
sparked additional comments.
--
Mike Kravetz
> Cc: stable@xxxxxxxxxxxxxxx
> Reported-by: Jan Stancek <jstancek@xxxxxxxxxx>
> Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> ---
> mm/hugetlb.c | 66 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 66 insertions(+)
>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index ec49d9e..418bf01 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1826,11 +1826,17 @@ static void return_unused_surplus_pages(struct hstate *h,
> * is not the case is if a reserve map was changed between calls. It
> * is the responsibility of the caller to notice the difference and
> * take appropriate action.
> + *
> + * vma_add_reservation is used in error paths where a reservation must
> + * be restored when a newly allocated huge page must be freed. It is
> + * to be called after calling vma_needs_reservation to determine if a
> + * reservation exists.
> */
> enum vma_resv_mode {
> VMA_NEEDS_RESV,
> VMA_COMMIT_RESV,
> VMA_END_RESV,
> + VMA_ADD_RESV,
> };
> static long __vma_reservation_common(struct hstate *h,
> struct vm_area_struct *vma, unsigned long addr,
> @@ -1856,6 +1862,14 @@ static long __vma_reservation_common(struct hstate *h,
> region_abort(resv, idx, idx + 1);
> ret = 0;
> break;
> + case VMA_ADD_RESV:
> + if (vma->vm_flags & VM_MAYSHARE)
> + ret = region_add(resv, idx, idx + 1);
> + else {
> + region_abort(resv, idx, idx + 1);
> + ret = region_del(resv, idx, idx + 1);
> + }
> + break;
> default:
> BUG();
> }
> @@ -1903,6 +1917,56 @@ static void vma_end_reservation(struct hstate *h,
> (void)__vma_reservation_common(h, vma, addr, VMA_END_RESV);
> }
>
> +static long vma_add_reservation(struct hstate *h,
> + struct vm_area_struct *vma, unsigned long addr)
> +{
> + return __vma_reservation_common(h, vma, addr, VMA_ADD_RESV);
> +}
> +
> +/*
> + * This routine is called to restore a reservation on error paths. In the
> + * specific error paths, a huge page was allocated (via alloc_huge_page)
> + * and is about to be freed. If a reservation for the page existed,
> + * alloc_huge_page would have consumed the reservation and set PagePrivate
> + * in the newly allocated page. When the page is freed via free_huge_page,
> + * the global reservation count will be incremented if PagePrivate is set.
> + * However, free_huge_page can not adjust the reserve map. Adjust the
> + * reserve map here to be consistent with global reserve count adjustments
> + * to be made by free_huge_page.
> + */
> +static void restore_reserve_on_error(struct hstate *h,
> + struct vm_area_struct *vma, unsigned long address,
> + struct page *page)
> +{
> + if (unlikely(PagePrivate(page))) {
> + long rc = vma_needs_reservation(h, vma, address);
> +
> + if (unlikely(rc < 0)) {
> + /*
> + * Rare out of memory condition in reserve map
> + * manipulation. Clear PagePrivate so that
> + * global reserve count will not be incremented
> + * by free_huge_page. This will make it appear
> + * as though the reservation for this page was
> + * consumed. This may prevent the task from
> + * faulting in the page at a later time. This
> + * is better than inconsistent global huge page
> + * accounting of reserve counts.
> + */
> + ClearPagePrivate(page);
> + } else if (rc) {
> + rc = vma_add_reservation(h, vma, address);
> + if (unlikely(rc < 0))
> + /*
> + * See above comment about rare out of
> + * memory condition.
> + */
> + ClearPagePrivate(page);
> + } else
> + vma_end_reservation(h, vma, address);
> + }
> +}
> +
> struct page *alloc_huge_page(struct vm_area_struct *vma,
> unsigned long addr, int avoid_reserve)
> {
> @@ -3498,6 +3562,7 @@ retry_avoidcopy:
> spin_unlock(ptl);
> mmu_notifier_invalidate_range_end(mm, mmun_start, mmun_end);
> out_release_all:
> + restore_reserve_on_error(h, vma, address, new_page);
> put_page(new_page);
> out_release_old:
> put_page(old_page);
> @@ -3680,6 +3745,7 @@ backout:
> spin_unlock(ptl);
> backout_unlocked:
> unlock_page(page);
> + restore_reserve_on_error(h, vma, address, page);
> put_page(page);
> goto out;
> }
>
--
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