Re: [PATCH v3 3/3] mm/hugetlb: handle races in alloc_huge_page and hugetlb_reserve_pages

[Mike Kravetz]

On 05/27/2015 10:56 AM, Mike Kravetz wrote:
> alloc_huge_page and hugetlb_reserve_pages use region_chg to
> calculate the number of pages which will be added to the reserve
> map.  Subpool and global reserve counts are adjusted based on
> the output of region_chg.  Before the pages are actually added
> to the reserve map, these routines could race and add fewer
> pages than expected.  If this happens, the subpool and global
> reserve counts are not correct.
>
> Compare the number of pages actually added (region_add) to those
> expected to added (region_chg).  If fewer pages are actually added,
> this indicates a race and adjust counters accordingly.
>
> Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> ---
>   mm/hugetlb.c | 34 ++++++++++++++++++++++++++++++----
>   1 file changed, 30 insertions(+), 4 deletions(-)
>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index b3d3d59..038c84e 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1544,7 +1544,7 @@ static struct page *alloc_huge_page(struct vm_area_struct *vma,
>   	struct hugepage_subpool *spool = subpool_vma(vma);
>   	struct hstate *h = hstate_vma(vma);
>   	struct page *page;
> -	long chg;
> +	long chg, commit;
>   	int ret, idx;
>   	struct hugetlb_cgroup *h_cg;
>
> @@ -1585,7 +1585,20 @@ static struct page *alloc_huge_page(struct vm_area_struct *vma,
>
>   	set_page_private(page, (unsigned long)spool);
>
> -	vma_commit_reservation(h, vma, addr);
> +	commit = vma_commit_reservation(h, vma, addr);
> +	if (unlikely(chg > commit)) {
> +		/*
> +		 * The page was added to the reservation map between
> +		 * vma_needs_reservation and vma_commit_reservation.
> +		 * This indicates a race with hugetlb_reserve_pages.
> +		 * Adjust for the subpool count incremented above AND
> +		 * in hugetlb_reserve_pages for the same page.  Also,
> +		 * the reservation count added in hugetlb_reserve_pages
> +		 * no longer applies.
> +		 */
> +		hugepage_subpool_put_pages(spool, 1);
> +		hugetlb_acct_memory(h, -1);
> +	}
>   	return page;

Well, this is embarrassing.  The code to fix up counts is incomplete
and incorrect.  It does not take into account the hugetlbfs min_size
option to reserve a set of pages for a filesystem.  The code should
look like:

		long rsv_adjust;

		rsv_adjust = hugepage_subpool_put_pages(spool, 1);
		hugetlb_acct_memory(h, -rsv_adjust);

A similar change is required in hugetlb_reserve_pages().

I have verified that the global reserve count is not properly
adjusted with the original patch, and verified that it is properly
adjusted with the above change.

Should I create a new patch set, or create a patch on top of the
existing set?

I think this oversight points out the need to encapsulate the
subpool and global reserve count adjustments.  There are too
many counters/adjustments in the code and it is pretty easy not
to take them all into account.  I'll start work on some
encapsulation routines.

--
Mike Kravetz

>   out_uncharge_cgroup:
> @@ -3699,8 +3712,21 @@ int hugetlb_reserve_pages(struct inode *inode,
>   	 * consumed reservations are stored in the map. Hence, nothing
>   	 * else has to be done for private mappings here
>   	 */
> -	if (!vma || vma->vm_flags & VM_MAYSHARE)
> -		region_add(resv_map, from, to);
> +	if (!vma || vma->vm_flags & VM_MAYSHARE) {
> +		long add = region_add(resv_map, from, to);
> +
> +		if (unlikely(chg > add)) {
> +			/*
> +			 * pages in this range were added to the reserve
> +			 * map between region_chg and region_add.  This
> +			 * indicates a race with alloc_huge_page.  Adjust
> +			 * the subpool and reserve counts modified above
> +			 * based on the difference.
> +			 */
> +			hugepage_subpool_put_pages(spool, chg - add);
> +			hugetlb_acct_memory(h, -(chg - ret));
> +		}
> +	}
>   	return 0;
>   out_err:
>   	if (vma && is_vma_resv_set(vma, HPAGE_RESV_OWNER))
--
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