Re: [PATCH v2 10/10] mm, hugetlb: decrement reserve count ifVM_NORESERVE alloc page cache

[Michal Hocko]

On Mon 22-07-13 17:36:31, Joonsoo Kim wrote:
> If a vma with VM_NORESERVE allocate a new page for page cache, we should
> check whether this area is reserved or not. If this address is
> already reserved by other process(in case of chg == 0), we should
> decrement reserve count, because this allocated page will go into page
> cache and currently, there is no way to know that this page comes from
> reserved pool or not when releasing inode. This may introduce
> over-counting problem to reserved count. With following example code,
> you can easily reproduce this situation.
> 
>         size = 20 * MB;
>         flag = MAP_SHARED;
>         p = mmap(NULL, size, PROT_READ|PROT_WRITE, flag, fd, 0);
>         if (p == MAP_FAILED) {
>                 fprintf(stderr, "mmap() failed: %s\n", strerror(errno));
>                 return -1;
>         }
> 
>         flag = MAP_SHARED | MAP_NORESERVE;
>         q = mmap(NULL, size, PROT_READ|PROT_WRITE, flag, fd, 0);
>         if (q == MAP_FAILED) {
>                 fprintf(stderr, "mmap() failed: %s\n", strerror(errno));
>         }
>         q[0] = 'c';
> 
> This patch solve this problem.

Again, please describe _how_ it solves the problem.

> Reviewed-by: Wanpeng Li <liwanp@xxxxxxxxxxxxxxxxxx>
> Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 2ea6afd..6782b41 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -443,10 +443,23 @@ void reset_vma_resv_huge_pages(struct vm_area_struct *vma)
>  }
>  
>  /* Returns true if the VMA has associated reserve pages */
> -static int vma_has_reserves(struct vm_area_struct *vma)
> +static int vma_has_reserves(struct vm_area_struct *vma, long chg)
>  {
> -	if (vma->vm_flags & VM_NORESERVE)
> -		return 0;
> +	if (vma->vm_flags & VM_NORESERVE) {
> +		/*
> +		 * This address is already reserved by other process(chg == 0),
> +		 * so, we should decreament reserved count. Without
> +		 * decreamenting, reserve count is remained after releasing
> +		 * inode, because this allocated page will go into page cache
> +		 * and is regarded as coming from reserved pool in releasing
> +		 * step. Currently, we don't have any other solution to deal
> +		 * with this situation properly, so add work-around here.
> +		 */
> +		if (vma->vm_flags & VM_MAYSHARE && chg == 0)
> +			return 1;
> +		else
> +			return 0;
> +	}
>  
>  	/* Shared mappings always use reserves */
>  	if (vma->vm_flags & VM_MAYSHARE)
> @@ -520,7 +533,8 @@ static struct page *dequeue_huge_page_node(struct hstate *h, int nid)
>  
>  static struct page *dequeue_huge_page_vma(struct hstate *h,
>  				struct vm_area_struct *vma,
> -				unsigned long address, int avoid_reserve)
> +				unsigned long address, int avoid_reserve,
> +				long chg)
>  {
>  	struct page *page = NULL;
>  	struct mempolicy *mpol;
> @@ -535,7 +549,7 @@ static struct page *dequeue_huge_page_vma(struct hstate *h,
>  	 * have no page reserves. This check ensures that reservations are
>  	 * not "stolen". The child may still get SIGKILLed
>  	 */
> -	if (!vma_has_reserves(vma) &&
> +	if (!vma_has_reserves(vma, chg) &&
>  			h->free_huge_pages - h->resv_huge_pages == 0)
>  		return NULL;
>  
> @@ -553,8 +567,12 @@ retry_cpuset:
>  		if (cpuset_zone_allowed_softwall(zone, htlb_alloc_mask)) {
>  			page = dequeue_huge_page_node(h, zone_to_nid(zone));
>  			if (page) {
> -				if (!avoid_reserve && vma_has_reserves(vma))
> -					h->resv_huge_pages--;
> +				if (avoid_reserve)
> +					break;
> +				if (!vma_has_reserves(vma, chg))
> +					break;
> +
> +				h->resv_huge_pages--;
>  				break;
>  			}
>  		}
> @@ -1135,7 +1153,7 @@ static struct page *alloc_huge_page(struct vm_area_struct *vma,
>  		return ERR_PTR(-ENOSPC);
>  	}
>  	spin_lock(&hugetlb_lock);
> -	page = dequeue_huge_page_vma(h, vma, addr, avoid_reserve);
> +	page = dequeue_huge_page_vma(h, vma, addr, avoid_reserve, chg);
>  	if (!page) {
>  		spin_unlock(&hugetlb_lock);
>  		page = alloc_buddy_huge_page(h, NUMA_NO_NODE);
> -- 
> 1.7.9.5
> 

-- 
Michal Hocko
SUSE Labs
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