Re: [PATCH 1/2] Revert "page cache: fix page_cache_next/prev_miss off by one"

[Sidhartha Kumar <sidhartha.kumar@oracle.com>]

On 6/21/23 2:24 PM, Mike Kravetz wrote:
> This reverts commit 9425c591e06a9ab27a145ba655fb50532cf0bcc9
> 
> The reverted commit fixed up routines primarily used by readahead code
> such that they could also be used by hugetlb.  Unfortunately, this
> caused a performance regression as pointed out by the Closes: tag.
> 
> The hugetlb code which uses page_cache_next_miss will be addressed in
> a subsequent patch.
> 
> Reported-by: kernel test robot <oliver.sang@intel.com>
> Closes: https://lore.kernel.org/oe-lkp/202306211346.1e9ff03e-oliver.sang@intel.com
> Fixes: 9425c591e06a ("page cache: fix page_cache_next/prev_miss off by one")
> Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
> ---
>   mm/filemap.c | 26 ++++++++++----------------
>   1 file changed, 10 insertions(+), 16 deletions(-)
> 
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 3b73101f9f86..9e44a49bbd74 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -1728,9 +1728,7 @@ bool __folio_lock_or_retry(struct folio *folio, struct mm_struct *mm,
>    *
>    * Return: The index of the gap if found, otherwise an index outside the
>    * range specified (in which case 'return - index >= max_scan' will be true).
> - * In the rare case of index wrap-around, 0 will be returned.  0 will also
> - * be returned if index == 0 and there is a gap at the index.  We can not
> - * wrap-around if passed index == 0.
> + * In the rare case of index wrap-around, 0 will be returned.
>    */
>   pgoff_t page_cache_next_miss(struct address_space *mapping,
>   			     pgoff_t index, unsigned long max_scan)
> @@ -1740,13 +1738,12 @@ pgoff_t page_cache_next_miss(struct address_space *mapping,
>   	while (max_scan--) {
>   		void *entry = xas_next(&xas);
>   		if (!entry || xa_is_value(entry))
> -			return xas.xa_index;
> -		if (xas.xa_index == 0 && index != 0)
> -			return xas.xa_index;
> +			break;
> +		if (xas.xa_index == 0)
> +			break;
>   	}
>   
> -	/* No gaps in range and no wrap-around, return index beyond range */
> -	return xas.xa_index + 1;
> +	return xas.xa_index;
>   }
>   EXPORT_SYMBOL(page_cache_next_miss);
>   
> @@ -1767,9 +1764,7 @@ EXPORT_SYMBOL(page_cache_next_miss);
>    *
>    * Return: The index of the gap if found, otherwise an index outside the
>    * range specified (in which case 'index - return >= max_scan' will be true).
> - * In the rare case of wrap-around, ULONG_MAX will be returned.  ULONG_MAX
> - * will also be returned if index == ULONG_MAX and there is a gap at the
> - * index.  We can not wrap-around if passed index == ULONG_MAX.
> + * In the rare case of wrap-around, ULONG_MAX will be returned.
>    */
>   pgoff_t page_cache_prev_miss(struct address_space *mapping,
>   			     pgoff_t index, unsigned long max_scan)
> @@ -1779,13 +1774,12 @@ pgoff_t page_cache_prev_miss(struct address_space *mapping,
>   	while (max_scan--) {
>   		void *entry = xas_prev(&xas);
>   		if (!entry || xa_is_value(entry))
> -			return xas.xa_index;
> -		if (xas.xa_index == ULONG_MAX && index != ULONG_MAX)
> -			return xas.xa_index;
> +			break;
> +		if (xas.xa_index == ULONG_MAX)
> +			break;
>   	}
>   
> -	/* No gaps in range and no wrap-around, return index beyond range */
> -	return xas.xa_index - 1;
> +	return xas.xa_index;
>   }
>   EXPORT_SYMBOL(page_cache_prev_miss);
>   
Reviewed-by: Sidhartha Kumar <sidhartha.kumar@oracle.com>