Re: [PATCH 01/10] xfs: split up xfs_buf_allocate_memory

["Darrick J. Wong" <djwong@kernel.org>]

On Thu, May 27, 2021 at 08:47:13AM +1000, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
> 
> Based on a patch from Christoph Hellwig.
> 
> This splits out the heap allocation and page allocation portions of
> the buffer memory allocation into two separate helper functions.
> 
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> ---
>  fs/xfs/xfs_buf.c | 126 ++++++++++++++++++++++++++++-------------------
>  1 file changed, 74 insertions(+), 52 deletions(-)
> 
> diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
> index 592800c8852f..2e35d344a69b 100644
> --- a/fs/xfs/xfs_buf.c
> +++ b/fs/xfs/xfs_buf.c
> @@ -347,65 +347,55 @@ xfs_buf_free(
>  	kmem_cache_free(xfs_buf_zone, bp);
>  }
>  
> -/*
> - * Allocates all the pages for buffer in question and builds it's page list.
> - */
> -STATIC int
> -xfs_buf_allocate_memory(
> -	struct xfs_buf		*bp,
> -	uint			flags)
> +static int
> +xfs_buf_alloc_kmem(
> +	struct xfs_buf	*bp,
> +	size_t		size,
> +	xfs_buf_flags_t	flags)
>  {
> -	size_t			size;
> -	size_t			nbytes, offset;
> -	gfp_t			gfp_mask = xb_to_gfp(flags);
> -	unsigned short		page_count, i;
> -	xfs_off_t		start, end;
> -	int			error;
> -	xfs_km_flags_t		kmflag_mask = 0;
> +	int		align_mask = xfs_buftarg_dma_alignment(bp->b_target);
> +	xfs_km_flags_t	kmflag_mask = KM_NOFS;
>  
> -	/*
> -	 * assure zeroed buffer for non-read cases.
> -	 */
> -	if (!(flags & XBF_READ)) {
> +	/* Assure zeroed buffer for non-read cases. */
> +	if (!(flags & XBF_READ))
>  		kmflag_mask |= KM_ZERO;
> -		gfp_mask |= __GFP_ZERO;
> -	}
>  
> -	/*
> -	 * for buffers that are contained within a single page, just allocate
> -	 * the memory from the heap - there's no need for the complexity of
> -	 * page arrays to keep allocation down to order 0.
> -	 */
> -	size = BBTOB(bp->b_length);
> -	if (size < PAGE_SIZE) {
> -		int align_mask = xfs_buftarg_dma_alignment(bp->b_target);
> -		bp->b_addr = kmem_alloc_io(size, align_mask,
> -					   KM_NOFS | kmflag_mask);
> -		if (!bp->b_addr) {
> -			/* low memory - use alloc_page loop instead */
> -			goto use_alloc_page;
> -		}
> +	bp->b_addr = kmem_alloc_io(size, align_mask, kmflag_mask);
> +	if (!bp->b_addr)
> +		return -ENOMEM;
>  
> -		if (((unsigned long)(bp->b_addr + size - 1) & PAGE_MASK) !=
> -		    ((unsigned long)bp->b_addr & PAGE_MASK)) {
> -			/* b_addr spans two pages - use alloc_page instead */
> -			kmem_free(bp->b_addr);
> -			bp->b_addr = NULL;
> -			goto use_alloc_page;
> -		}
> -		bp->b_offset = offset_in_page(bp->b_addr);
> -		bp->b_pages = bp->b_page_array;
> -		bp->b_pages[0] = kmem_to_page(bp->b_addr);
> -		bp->b_page_count = 1;
> -		bp->b_flags |= _XBF_KMEM;
> -		return 0;
> +	if (((unsigned long)(bp->b_addr + size - 1) & PAGE_MASK) !=
> +	    ((unsigned long)bp->b_addr & PAGE_MASK)) {
> +		/* b_addr spans two pages - use alloc_page instead */
> +		kmem_free(bp->b_addr);
> +		bp->b_addr = NULL;
> +		return -ENOMEM;
>  	}
> +	bp->b_offset = offset_in_page(bp->b_addr);
> +	bp->b_pages = bp->b_page_array;
> +	bp->b_pages[0] = kmem_to_page(bp->b_addr);
> +	bp->b_page_count = 1;
> +	bp->b_flags |= _XBF_KMEM;
> +	return 0;
> +}
> +
> +static int
> +xfs_buf_alloc_pages(
> +	struct xfs_buf	*bp,
> +	uint		page_count,
> +	xfs_buf_flags_t	flags)
> +{
> +	gfp_t		gfp_mask = xb_to_gfp(flags);
> +	size_t		size;
> +	size_t		offset;
> +	size_t		nbytes;
> +	int		i;
> +	int		error;
> +
> +	/* Assure zeroed buffer for non-read cases. */
> +	if (!(flags & XBF_READ))
> +		gfp_mask |= __GFP_ZERO;
>  
> -use_alloc_page:
> -	start = BBTOB(bp->b_maps[0].bm_bn) >> PAGE_SHIFT;
> -	end = (BBTOB(bp->b_maps[0].bm_bn + bp->b_length) + PAGE_SIZE - 1)
> -								>> PAGE_SHIFT;
> -	page_count = end - start;
>  	error = _xfs_buf_get_pages(bp, page_count);
>  	if (unlikely(error))
>  		return error;
> @@ -458,6 +448,38 @@ xfs_buf_allocate_memory(
>  	return error;
>  }
>  
> +
> +/*
> + * Allocates all the pages for buffer in question and builds it's page list.
> + */
> +static int
> +xfs_buf_allocate_memory(
> +	struct xfs_buf		*bp,
> +	uint			flags)
> +{
> +	size_t			size;
> +	xfs_off_t		start, end;
> +	int			error;
> +
> +	/*
> +	 * For buffers that fit entirely within a single page, first attempt to
> +	 * allocate the memory from the heap to minimise memory usage. If we
> +	 * can't get heap memory for these small buffers, we fall back to using
> +	 * the page allocator.
> +	 */
> +	size = BBTOB(bp->b_length);
> +	if (size < PAGE_SIZE) {
> +		error = xfs_buf_alloc_kmem(bp, size, flags);
> +		if (!error)
> +			return 0;
> +	}
> +
> +	start = BBTOB(bp->b_maps[0].bm_bn) >> PAGE_SHIFT;
> +	end = (BBTOB(bp->b_maps[0].bm_bn + bp->b_length) + PAGE_SIZE - 1)
> +								>> PAGE_SHIFT;

round_down and round_up?

As a straight translation this seems fine, but you might as well take
the opportunity to declutter some of this. :)

Reviewed-by: Darrick J. Wong <djwong@kernel.org>

--D

> +	return xfs_buf_alloc_pages(bp, end - start, flags);
> +}
> +
>  /*
>   *	Map buffer into kernel address-space if necessary.
>   */
> -- 
> 2.31.1
>