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SubjectRe: [PATCH] Do not account for the address space used by hugetlbfs using VM_ACCOUNT V2 (Was Linus 2.6.29-rc4)
Acked-by: Andy Whitcroft <apw@canonical.com>

> Subject: [PATCH] Do not account for hugetlbfs quota at mmap() time if mapping [SHM|MAP]_NORESERVE
>
> Commit 5a6fe125950676015f5108fb71b2a67441755003 brought hugetlbfs more in line
> with the core VM by obeying VM_NORESERVE and not reserving hugepages for both
> shared and private mappings when [SHM|MAP]_NORESERVE are specified. However,
> it is still taking filesystem quota unconditionally.
>
> At fault time, if there are no reserves and attempt is made to allocate
> the page and account for filesystem quota. If either fail, the fault
> fails. The impact is that quota is getting accounted for twice. This patch
> partially reverts 5a6fe125950676015f5108fb71b2a67441755003. To help
> prevent this mistake happening again, it improves the documentation of
> hugetlb_reserve_pages()
>
> Reported-by: Andy Whitcroft <apw@canonical.com>
> Signed-off-by: Mel Gorman <mel@csn.ul.ie>
>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 2074642..107da3d 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -2272,10 +2272,18 @@ int hugetlb_reserve_pages(struct inode *inode,
> struct vm_area_struct *vma,
> int acctflag)
> {
> - long ret = 0, chg;
> + long ret, chg;
> struct hstate *h = hstate_inode(inode);
>
> /*
> + * Only apply hugepage reservation if asked. At fault time, an
> + * attempt will be made for VM_NORESERVE to allocate a page
> + * and filesystem quota without using reserves
> + */
> + if (acctflag & VM_NORESERVE)
> + return 0;
> +
> + /*
> * Shared mappings base their reservation on the number of pages that
> * are already allocated on behalf of the file. Private mappings need
> * to reserve the full area even if read-only as mprotect() may be
> @@ -2283,42 +2291,47 @@ int hugetlb_reserve_pages(struct inode *inode,
> */
> if (!vma || vma->vm_flags & VM_SHARED)
> chg = region_chg(&inode->i_mapping->private_list, from, to);
> - else
> + else {
> + struct resv_map *resv_map = resv_map_alloc();
> + if (!resv_map)
> + return -ENOMEM;
> +
> chg = to - from;
>
> + set_vma_resv_map(vma, resv_map);
> + set_vma_resv_flags(vma, HPAGE_RESV_OWNER);
> + }
> +
> if (chg < 0)
> return chg;
>
> + /* There must be enough filesystem quota for the mapping */
> if (hugetlb_get_quota(inode->i_mapping, chg))
> return -ENOSPC;
>
> /*
> - * Only apply hugepage reservation if asked. We still have to
> - * take the filesystem quota because it is an upper limit
> - * defined for the mount and not necessarily memory as a whole
> + * Check enough hugepages are available for the reservation.
> + * Hand back the quota if there are not
> */
> - if (acctflag & VM_NORESERVE) {
> - reset_vma_resv_huge_pages(vma);
> - return 0;
> - }
> -
> ret = hugetlb_acct_memory(h, chg);
> if (ret < 0) {
> hugetlb_put_quota(inode->i_mapping, chg);
> return ret;
> }
> +
> + /*
> + * Account for the reservations made. Shared mappings record regions
> + * that have reservations as they are shared by multiple VMAs.
> + * When the last VMA disappears, the region map says how much
> + * the reservation was and the page cache tells how much of
> + * the reservation was consumed. Private mappings are per-VMA and
> + * only the consumed reservations are tracked. When the VMA
> + * disappears, the original reservation is the VMA size and the
> + * consumed reservations are stored in the map. Hence, nothing
> + * else has to be done for private mappings here
> + */
> if (!vma || vma->vm_flags & VM_SHARED)
> region_add(&inode->i_mapping->private_list, from, to);
> - else {
> - struct resv_map *resv_map = resv_map_alloc();
> -
> - if (!resv_map)
> - return -ENOMEM;
> -
> - set_vma_resv_map(vma, resv_map);
> - set_vma_resv_flags(vma, HPAGE_RESV_OWNER);
> - }
> -
> return 0;
> }

-apw


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