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

Hillf Danton
2013-07-29T09:43:28Z

On Mon, Jul 29, 2013 at 1:28 PM, Joonsoo Kim <iamjoonsoo.kim@lge.com> 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@linux.vnet.ibm.com>
> Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
>
Acked-by: Hillf Danton <dhillf@gmail.com>

> 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
>