Re: [PATCH 04/11] xfs: cleanup _xfs_buf_get_pages

[Dave Chinner <david@fromorbit.com>]

On Wed, May 19, 2021 at 09:08:53PM +0200, Christoph Hellwig wrote:
> Remove the check for an existing b_pages array as this function is always
> called right after allocating a buffer, so this can't happen.  Also
> use kmem_zalloc to allocate the page array instead of doing a manual
> memset gіven that the inline array is already pre-zeroed as part of the
> freshly allocated buffer anyway.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>  fs/xfs/xfs_buf.c | 23 +++++++++++------------
>  1 file changed, 11 insertions(+), 12 deletions(-)
> 
> diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
> index 392b85d059bff5..9c64c374411081 100644
> --- a/fs/xfs/xfs_buf.c
> +++ b/fs/xfs/xfs_buf.c
> @@ -281,19 +281,18 @@ _xfs_buf_get_pages(
>  	struct xfs_buf		*bp,
>  	int			page_count)
>  {
> -	/* Make sure that we have a page list */
> -	if (bp->b_pages == NULL) {
> -		bp->b_page_count = page_count;
> -		if (page_count <= XB_PAGES) {
> -			bp->b_pages = bp->b_page_array;
> -		} else {
> -			bp->b_pages = kmem_alloc(sizeof(struct page *) *
> -						 page_count, KM_NOFS);
> -			if (bp->b_pages == NULL)
> -				return -ENOMEM;
> -		}
> -		memset(bp->b_pages, 0, sizeof(struct page *) * page_count);
> +	ASSERT(bp->b_pages == NULL);
> +
> +	bp->b_page_count = page_count;
> +	if (page_count > XB_PAGES) {
> +		bp->b_pages = kmem_zalloc(sizeof(struct page *) * page_count,
> +					  KM_NOFS);
> +		if (!bp->b_pages)
> +			return -ENOMEM;
> +	} else {
> +		bp->b_pages = bp->b_page_array;
>  	}
> +
>  	return 0;
>  }

This will not apply (and break) the bulk alloc patch I sent out - we
have to ensure that the b_pages array is always zeroed before we
call the bulk alloc function, hence I moved the memset() in this
function to be unconditional. I almost cleaned up this function in
that patchset....

Just doing this:

	bp->b_page_count = page_count;
	if (page_count > XB_PAGES) {
		bp->b_pages = kmem_alloc(sizeof(struct page *) * page_count,
					 KM_NOFS);
		if (!bp->b_pages)
			return -ENOMEM;
	} else {
		bp->b_pages = bp->b_page_array;
	}
	memset(bp->b_pages, 0, sizeof(struct page *) * page_count);

	return 0;

will make it work fine with bulk alloc.

Cheers,

Dave.

-- 
Dave Chinner
david@fromorbit.com