Re: [PATCH 05/12] xfs: refactor backing memory allocations for buffers

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

On Wed, Feb 26, 2025 at 07:51:33AM -0800, Christoph Hellwig wrote:
> Lift handling of shmem and slab backed buffers into xfs_buf_alloc_pages
> and rename the result to xfs_buf_alloc_backing_mem.  This shares more
> code and ensures uncached buffers can also use slab, which slightly
> reduces the memory usage of growfs on 512 byte sector size file systems,
> but more importantly means the allocation invariants are the same for
> cached and uncached buffers.  Document these new invariants with a big
> fat comment mostly stolen from a patch by Dave Chinner.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>

That's a nice refactoring.  I'd wondered about get_buf_uncached not
going for slab memory when I was writing the xmbuf code, but didn't want
to overcomplicate that patchset and then forgot about it :/

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

--D

> ---
>  fs/xfs/xfs_buf.c | 55 +++++++++++++++++++++++++++++++-----------------
>  1 file changed, 36 insertions(+), 19 deletions(-)
> 
> diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
> index af1389ebdd69..e8783ee23623 100644
> --- a/fs/xfs/xfs_buf.c
> +++ b/fs/xfs/xfs_buf.c
> @@ -329,19 +329,49 @@ xfs_buf_alloc_kmem(
>  	return 0;
>  }
>  
> +/*
> + * Allocate backing memory for a buffer.
> + *
> + * For tmpfs-backed buffers used by in-memory btrees this directly maps the
> + * tmpfs page cache folios.
> + *
> + * For real file system buffers there are two different kinds backing memory:
> + *
> + * The first type backs the buffer by a kmalloc allocation.  This is done for
> + * less than PAGE_SIZE allocations to avoid wasting memory.
> + *
> + * The second type of buffer is the multi-page buffer. These are always made
> + * up of single pages so that they can be fed to vmap_ram() to return a
> + * contiguous memory region we can access the data through, or mark it as
> + * XBF_UNMAPPED and access the data directly through individual page_address()
> + * calls.
> + */
>  static int
> -xfs_buf_alloc_pages(
> +xfs_buf_alloc_backing_mem(
>  	struct xfs_buf	*bp,
>  	xfs_buf_flags_t	flags)
>  {
> +	size_t		size = BBTOB(bp->b_length);
>  	gfp_t		gfp_mask = GFP_KERNEL | __GFP_NOLOCKDEP | __GFP_NOWARN;
>  	long		filled = 0;
>  
> +	if (xfs_buftarg_is_mem(bp->b_target))
> +		return xmbuf_map_page(bp);
> +
> +	/*
> +	 * 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.
> +	 */
> +	if (size < PAGE_SIZE && xfs_buf_alloc_kmem(new_bp, flags) == 0)
> +		return 0;
> +
>  	if (flags & XBF_READ_AHEAD)
>  		gfp_mask |= __GFP_NORETRY;
>  
>  	/* Make sure that we have a page list */
> -	bp->b_page_count = DIV_ROUND_UP(BBTOB(bp->b_length), PAGE_SIZE);
> +	bp->b_page_count = DIV_ROUND_UP(size, PAGE_SIZE);
>  	if (bp->b_page_count <= XB_PAGES) {
>  		bp->b_pages = bp->b_page_array;
>  	} else {
> @@ -622,18 +652,7 @@ xfs_buf_find_insert(
>  	if (error)
>  		goto out_drop_pag;
>  
> -	if (xfs_buftarg_is_mem(new_bp->b_target)) {
> -		error = xmbuf_map_page(new_bp);
> -	} else if (BBTOB(new_bp->b_length) >= PAGE_SIZE ||
> -		   xfs_buf_alloc_kmem(new_bp, flags) < 0) {
> -		/*
> -		 * 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.
> -		 */
> -		error = xfs_buf_alloc_pages(new_bp, flags);
> -	}
> +	error = xfs_buf_alloc_backing_mem(new_bp, flags);
>  	if (error)
>  		goto out_free_buf;
>  
> @@ -995,14 +1014,12 @@ xfs_buf_get_uncached(
>  	if (error)
>  		return error;
>  
> -	if (xfs_buftarg_is_mem(bp->b_target))
> -		error = xmbuf_map_page(bp);
> -	else
> -		error = xfs_buf_alloc_pages(bp, flags);
> +	error = xfs_buf_alloc_backing_mem(bp, flags);
>  	if (error)
>  		goto fail_free_buf;
>  
> -	error = _xfs_buf_map_pages(bp, 0);
> +	if (!bp->b_addr)
> +		error = _xfs_buf_map_pages(bp, 0);
>  	if (unlikely(error)) {
>  		xfs_warn(target->bt_mount,
>  			"%s: failed to map pages", __func__);
> -- 
> 2.45.2
> 
>