Re: [PATCH] mm/hugetlb: fix huge page reserve accounting for private mappings

[Hillf Danton]

> 
> When creating a private mapping of a hugetlbfs file, it is possible to
> unmap pages via ftruncate or fallocate hole punch.  If subsequent faults
> repopulate these mappings, the reserve counts will go negative.  This
> is because the code currently assumes all faults to private mappings will
> consume reserves.  The problem can be recreated as follows:
> - mmap(MAP_PRIVATE) a file in hugetlbfs filesystem
> - write fault in pages in the mapping
> - fallocate(FALLOC_FL_PUNCH_HOLE) some pages in the mapping
> - write fault in pages in the hole
> This will result in negative huge page reserve counts and negative subpool
> usage counts for the hugetlbfs.  Note that this can also be recreated with
> ftruncate, but fallocate is more straight forward.
> 
> This patch modifies the routines vma_needs_reserves and vma_has_reserves
> to examine the reserve map associated with private mappings similar to that
> for shared mappings.  However, the reserve map semantics for private and
> shared mappings are very different.  This results in subtly different code
> that is explained in the comments.
> 
> Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> ---

Acked-by: Hillf Danton <hillf.zj@xxxxxxxxxxxxxxx>

>  mm/hugetlb.c | 42 ++++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 40 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 949d806..0949d0d 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -831,8 +831,27 @@ static bool vma_has_reserves(struct vm_area_struct *vma, long chg)
>  	 * Only the process that called mmap() has reserves for
>  	 * private mappings.
>  	 */
> -	if (is_vma_resv_set(vma, HPAGE_RESV_OWNER))
> -		return true;
> +	if (is_vma_resv_set(vma, HPAGE_RESV_OWNER)) {
> +		/*
> +		 * Like the shared case above, a hole punch or truncate
> +		 * could have been performed on the private mapping.
> +		 * Examine the value of chg to determine if reserves
> +		 * actually exist or were previously consumed.
> +		 * Very Subtle - The value of chg comes from a previous
> +		 * call to vma_needs_reserves().  The reserve map for
> +		 * private mappings has different (opposite) semantics
> +		 * than that of shared mappings.  vma_needs_reserves()
> +		 * has already taken this difference in semantics into
> +		 * account.  Therefore, the meaning of chg is the same
> +		 * as in the shared case above.  Code could easily be
> +		 * combined, but keeping it separate draws attention to
> +		 * subtle differences.
> +		 */
> +		if (chg)
> +			return false;
> +		else
> +			return true;
> +	}
> 
>  	return false;
>  }
> @@ -1815,6 +1834,25 @@ static long __vma_reservation_common(struct hstate *h,
> 
>  	if (vma->vm_flags & VM_MAYSHARE)
>  		return ret;
> +	else if (is_vma_resv_set(vma, HPAGE_RESV_OWNER) && ret >= 0) {
> +		/*
> +		 * In most cases, reserves always exist for private mappings.
> +		 * However, a file associated with mapping could have been
> +		 * hole punched or truncated after reserves were consumed.
> +		 * As subsequent fault on such a range will not use reserves.
> +		 * Subtle - The reserve map for private mappings has the
> +		 * opposite meaning than that of shared mappings.  If NO
> +		 * entry is in the reserve map, it means a reservation exists.
> +		 * If an entry exists in the reserve map, it means the
> +		 * reservation has already been consumed.  As a result, the
> +		 * return value of this routine is the opposite of the
> +		 * value returned from reserve map manipulation routines above.
> +		 */
> +		if (ret)
> +			return 0;
> +		else
> +			return 1;
> +	}
>  	else
>  		return ret < 0 ? ret : 0;
>  }
> --
> 2.4.11