Re: [PATCH v3 9/9] mm, hugetlb: decrement reserve count if VM_NORESERVE alloc page cache

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On Mon, Jul 29, 2013 at 1:28 PM, Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> 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.
>
> Assume 2MB, nr_hugepages = 100
>
>         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';
>
> After finish the program, run 'cat /proc/meminfo'.
> You can see below result.
>
> HugePages_Free:      100
> HugePages_Rsvd:        1
>
> To fix this, we should check our mapping type and tracked region.
> If our mapping is VM_NORESERVE, VM_MAYSHARE and chg is 0, this imply
> that current allocated page will go into page cache which is already
> reserved region when mapping is created. In this case, we should decrease
> reserve count. As implementing above, this patch solve 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>
>
Acked-by: Hillf Danton <dhillf@xxxxxxxxx>

> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 4b1b043..b3b8252 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)
>                 goto err;
>
> @@ -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;
>                         }
>                 }
> @@ -1138,7 +1156,7 @@ 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
>

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