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

[Davidlohr Bueso]

On Wed, 2015-05-27 at 10:56 -0700, 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>

Reviewed-by: Davidlohr Bueso <dave@xxxxxxxxxxxx>

With one nit below.

> ---
>  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);

Should these fixups be encapsulated in a helper? The comment is the same
for both alloc_huge_page and hugetlb_reserve_pages.

Thanks,
Davidlohr

> +	}
>  	return page;
>  
>  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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