Re: [PATCH] mm/hugetlbfs Fix bugs in fallocate hole punch of areas with holes

[Naoya Horiguchi]

On Fri, Oct 30, 2015 at 04:32:12PM -0700, Mike Kravetz wrote:
> Hugh Dickins pointed out problems with the new hugetlbfs fallocate
> hole punch code.  These problems are in the routine remove_inode_hugepages
> and mostly occur in the case where there are holes in the range of
> pages to be removed.  These holes could be the result of a previous hole
> punch or simply sparse allocation.
> 
> remove_inode_hugepages handles both hole punch and truncate operations.
> Page index handling was fixed/cleaned up so that holes are properly
> handled.  In addition, code was changed to ensure multiple passes of the
> address range only happens in the truncate case.  More comments were added
> to explain the different actions in each case.  A cond_resched() was added
> after removing up to PAGEVEC_SIZE pages.
> 
> Some totally unnecessary code in hugetlbfs_fallocate() that remained from
> early development was also removed.
> 
> Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> ---
>  fs/hugetlbfs/inode.c | 44 +++++++++++++++++++++++++++++---------------
>  1 file changed, 29 insertions(+), 15 deletions(-)
> 
> diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
> index 316adb9..30cf534 100644
> --- a/fs/hugetlbfs/inode.c
> +++ b/fs/hugetlbfs/inode.c
> @@ -368,10 +368,25 @@ static void remove_inode_hugepages(struct inode *inode, loff_t lstart,
>  			lookup_nr = end - next;
>  
>  		/*
> -		 * This pagevec_lookup() may return pages past 'end',
> -		 * so we must check for page->index > end.
> +		 * When no more pages are found, take different action for
> +		 * hole punch and truncate.
> +		 *
> +		 * For hole punch, this indicates we have removed each page
> +		 * within the range and are done.  Note that pages may have
> +		 * been faulted in after being removed in the hole punch case.
> +		 * This is OK as long as each page in the range was removed
> +		 * once.
> +		 *
> +		 * For truncate, we need to make sure all pages within the
> +		 * range are removed when exiting this routine.  We could
> +		 * have raced with a fault that brought in a page after it
> +		 * was first removed.  Check the range again until no pages
> +		 * are found.
>  		 */
>  		if (!pagevec_lookup(&pvec, mapping, next, lookup_nr)) {
> +			if (!truncate_op)
> +				break;
> +
>  			if (next == start)
>  				break;
>  			next = start;
> @@ -382,19 +397,23 @@ static void remove_inode_hugepages(struct inode *inode, loff_t lstart,
>  			struct page *page = pvec.pages[i];
>  			u32 hash;
>  
> +			/*
> +			 * The page (index) could be beyond end.  This is
> +			 * only possible in the punch hole case as end is
> +			 * LLONG_MAX for truncate.
> +			 */
> +			if (page->index >= end) {
> +				next = end;	/* we are done */
> +				break;
> +			}
> +			next = page->index;
> +
>  			hash = hugetlb_fault_mutex_hash(h, current->mm,
>  							&pseudo_vma,
>  							mapping, next, 0);
>  			mutex_lock(&hugetlb_fault_mutex_table[hash]);
>  
>  			lock_page(page);
> -			if (page->index >= end) {
> -				unlock_page(page);
> -				mutex_unlock(&hugetlb_fault_mutex_table[hash]);
> -				next = end;	/* we are done */
> -				break;
> -			}
> -
>  			/*
>  			 * If page is mapped, it was faulted in after being
>  			 * unmapped.  Do nothing in this race case.  In the
> @@ -423,15 +442,13 @@ static void remove_inode_hugepages(struct inode *inode, loff_t lstart,
>  				}
>  			}
>  
> -			if (page->index > next)
> -				next = page->index;
> -
>  			++next;

You set next = page->index above, so this increment takes effect only in
the final iteration. Can we put this outside (just after) this for-loop?

Thanks,
Naoya Horiguchi

>  			unlock_page(page);
>  
>  			mutex_unlock(&hugetlb_fault_mutex_table[hash]);
>  		}
>  		huge_pagevec_release(&pvec);
> +		cond_resched();
>  	}
>  
>  	if (truncate_op)
> @@ -647,9 +664,6 @@ static long hugetlbfs_fallocate(struct file *file, int mode, loff_t offset,
>  	if (!(mode & FALLOC_FL_KEEP_SIZE) && offset + len > inode->i_size)
>  		i_size_write(inode, offset + len);
>  	inode->i_ctime = CURRENT_TIME;
> -	spin_lock(&inode->i_lock);
> -	inode->i_private = NULL;
> -	spin_unlock(&inode->i_lock);
>  out:
>  	mutex_unlock(&inode->i_mutex);
>  	return error;
> -- 
> 2.4.3
> --
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