Re: [PATCH] xfs: use alloc_pages_bulk_array() for buffers

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

On Wed, May 19, 2021 at 11:07:33AM +1000, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
> 
> Because it's more efficient than allocating pages one at a time in a
> loop.
> 
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> ---
>  fs/xfs/xfs_buf.c | 91 +++++++++++++++++++++---------------------------
>  1 file changed, 39 insertions(+), 52 deletions(-)
> 
> diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
> index 592800c8852f..a6cf607bbc4a 100644
> --- a/fs/xfs/xfs_buf.c
> +++ b/fs/xfs/xfs_buf.c
> @@ -276,8 +276,8 @@ _xfs_buf_alloc(
>   *	Allocate a page array capable of holding a specified number
>   *	of pages, and point the page buf at it.
>   */
> -STATIC int
> -_xfs_buf_get_pages(
> +static int
> +xfs_buf_get_pages(
>  	struct xfs_buf		*bp,
>  	int			page_count)
>  {
> @@ -292,8 +292,8 @@ _xfs_buf_get_pages(
>  			if (bp->b_pages == NULL)
>  				return -ENOMEM;
>  		}
> -		memset(bp->b_pages, 0, sizeof(struct page *) * page_count);
>  	}
> +	memset(bp->b_pages, 0, sizeof(struct page *) * bp->b_page_count);

Could this kmem_alloc be converted to kmem_zalloc?

And isn't the xfs_buf allocated with zalloc, which means we don't need
to zero b_page_array itself?

Confused about why this is needed.

>  	return 0;
>  }
>  
> @@ -356,10 +356,10 @@ xfs_buf_allocate_memory(
>  	uint			flags)
>  {
>  	size_t			size;
> -	size_t			nbytes, offset;
> +	size_t			offset;
>  	gfp_t			gfp_mask = xb_to_gfp(flags);
> -	unsigned short		page_count, i;
>  	xfs_off_t		start, end;
> +	long			filled = 0;
>  	int			error;
>  	xfs_km_flags_t		kmflag_mask = 0;
>  
> @@ -405,55 +405,44 @@ xfs_buf_allocate_memory(
>  	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);
> +	error = xfs_buf_get_pages(bp, end - start);
>  	if (unlikely(error))
>  		return error;
>  
>  	offset = bp->b_offset;
>  	bp->b_flags |= _XBF_PAGES;
>  
> -	for (i = 0; i < bp->b_page_count; i++) {
> -		struct page	*page;
> -		uint		retries = 0;
> -retry:
> -		page = alloc_page(gfp_mask);
> -		if (unlikely(page == NULL)) {
> -			if (flags & XBF_READ_AHEAD) {
> -				bp->b_page_count = i;
> -				error = -ENOMEM;
> -				goto out_free_pages;
> -			}
> +	/*
> +	 * Bulk filling of pages can take multiple calls. Not filling the entire
> +	 * array is not an allocation failure, so don't back off if we get at
> +	 * least one extra page.
> +	 */
> +	for (;;) {
> +		int	last = filled;

Any reason why last is int but filled is long?

Oh, heh, nr_pages is int.

Nice how Dr. Demento came on as soon as I started reading
__alloc_pages_bulk.

>  
> -			/*
> -			 * This could deadlock.
> -			 *
> -			 * But until all the XFS lowlevel code is revamped to
> -			 * handle buffer allocation failures we can't do much.
> -			 */
> -			if (!(++retries % 100))
> -				xfs_err(NULL,
> -		"%s(%u) possible memory allocation deadlock in %s (mode:0x%x)",
> -					current->comm, current->pid,
> -					__func__, gfp_mask);
> -
> -			XFS_STATS_INC(bp->b_mount, xb_page_retries);
> -			congestion_wait(BLK_RW_ASYNC, HZ/50);
> -			goto retry;
> +		filled = alloc_pages_bulk_array(gfp_mask, bp->b_page_count,
> +						bp->b_pages);
> +		if (filled == bp->b_page_count) {
> +			XFS_STATS_INC(bp->b_mount, xb_page_found);
> +			break;
>  		}
>  
> -		XFS_STATS_INC(bp->b_mount, xb_page_found);
> +		if (filled != last)
> +			continue;
>  
> -		nbytes = min_t(size_t, size, PAGE_SIZE - offset);
> -		size -= nbytes;
> -		bp->b_pages[i] = page;
> -		offset = 0;
> +		if (flags & XBF_READ_AHEAD) {
> +			error = -ENOMEM;
> +			goto out_free_pages;
> +		}
> +
> +		XFS_STATS_INC(bp->b_mount, xb_page_retries);
> +		congestion_wait(BLK_RW_ASYNC, HZ/50);

Silly nit: spaces around HZ / 50.

Otherwise this mostly looks ok to me.  The bulk allocation is a big
improvement.

--D

>  	}
>  	return 0;
>  
>  out_free_pages:
> -	for (i = 0; i < bp->b_page_count; i++)
> -		__free_page(bp->b_pages[i]);
> +	while (--filled >= 0)
> +		__free_page(bp->b_pages[filled]);
>  	bp->b_flags &= ~_XBF_PAGES;
>  	return error;
>  }
> @@ -950,8 +939,8 @@ xfs_buf_get_uncached(
>  	int			flags,
>  	struct xfs_buf		**bpp)
>  {
> -	unsigned long		page_count;
> -	int			error, i;
> +	unsigned long		filled;
> +	int			error;
>  	struct xfs_buf		*bp;
>  	DEFINE_SINGLE_BUF_MAP(map, XFS_BUF_DADDR_NULL, numblks);
>  
> @@ -962,17 +951,15 @@ xfs_buf_get_uncached(
>  	if (error)
>  		goto fail;
>  
> -	page_count = PAGE_ALIGN(numblks << BBSHIFT) >> PAGE_SHIFT;
> -	error = _xfs_buf_get_pages(bp, page_count);
> +	error = xfs_buf_get_pages(bp, PAGE_ALIGN(BBTOB(numblks)) >> PAGE_SHIFT);
>  	if (error)
>  		goto fail_free_buf;
>  
> -	for (i = 0; i < page_count; i++) {
> -		bp->b_pages[i] = alloc_page(xb_to_gfp(flags));
> -		if (!bp->b_pages[i]) {
> -			error = -ENOMEM;
> -			goto fail_free_mem;
> -		}
> +	filled = alloc_pages_bulk_array(xb_to_gfp(flags), bp->b_page_count,
> +					bp->b_pages);
> +	if (filled != bp->b_page_count) {
> +		error = -ENOMEM;
> +		goto fail_free_mem;
>  	}
>  	bp->b_flags |= _XBF_PAGES;
>  
> @@ -988,8 +975,8 @@ xfs_buf_get_uncached(
>  	return 0;
>  
>   fail_free_mem:
> -	while (--i >= 0)
> -		__free_page(bp->b_pages[i]);
> +	while (--filled >= 0)
> +		__free_page(bp->b_pages[filled]);
>  	_xfs_buf_free_pages(bp);
>   fail_free_buf:
>  	xfs_buf_free_maps(bp);
> -- 
> 2.31.1
>