Re: [PATCH] Do not account for the address space used by hugetlbfs using VM_ACCOUNT V2 (Was Linus 2.6.29-rc4)

[Andy Whitcroft <apw@canonical.com>]

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