linux-kernel - Re: [PATCH v3 2/2] btrfs: allocate page arrays using bulk page allocator

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Date:   Thu, 31 Mar 2022 19:35:25 +0200
From:   David Sterba <dsterba@...e.cz>
To:     Sweet Tea Dorminy <sweettea-kernel@...miny.me>
Cc:     Chris Mason <clm@...com>, Josef Bacik <josef@...icpanda.com>,
        David Sterba <dsterba@...e.com>,
        Nick Terrell <terrelln@...com>, linux-kernel@...r.kernel.org,
        linux-btrfs@...r.kernel.org, kernel-team@...com,
        Nikolay Borisov <nborisov@...e.com>
Subject: Re: [PATCH v3 2/2] btrfs: allocate page arrays using bulk page
 allocator

On Wed, Mar 30, 2022 at 04:11:23PM -0400, Sweet Tea Dorminy wrote:
> While calling alloc_page() in a loop is an effective way to populate an
> array of pages, the kernel provides a method to allocate pages in bulk.
> alloc_pages_bulk_array() populates the NULL slots in a page array, trying to
> grab more than one page at a time.
> 
> Unfortunately, it doesn't guarantee allocating all slots in the array,
> but it's easy to call it in a loop and return an error if no progress
> occurs. Similar code can be found in xfs/xfs_buf.c:xfs_buf_alloc_pages().
> 
> Signed-off-by: Sweet Tea Dorminy <sweettea-kernel@...miny.me>
> Reviewed-by: Nikolay Borisov <nborisov@...e.com>
> ---
> Changes in v3:
>  - Added a newline after variable declaration
> Changes in v2:
>  - Moved from ctree.c to extent_io.c
> ---
>  fs/btrfs/extent_io.c | 24 +++++++++++++++---------
>  1 file changed, 15 insertions(+), 9 deletions(-)
> 
> diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
> index ab4c1c4d1b59..b268e47aa2b7 100644
> --- a/fs/btrfs/extent_io.c
> +++ b/fs/btrfs/extent_io.c
> @@ -3144,19 +3144,25 @@ static void end_bio_extent_readpage(struct bio *bio)
>   */
>  int btrfs_alloc_page_array(unsigned long nr_pages, struct page **page_array)
>  {
> -	int i;
> +	long allocated = 0;
> +
> +	for (;;) {
> +		long last = allocated;
>  
> -	for (i = 0; i < nr_pages; i++) {
> -		struct page *page;
> +		allocated = alloc_pages_bulk_array(GFP_NOFS, nr_pages,
> +						   page_array);
> +		if (allocated == nr_pages)
> +			return 0;
>  
> -		if (page_array[i])
> +		if (allocated != last)
>  			continue;
> -		page = alloc_page(GFP_NOFS);
> -		if (!page)
> -			return -ENOMEM;
> -		page_array[i] = page;
> +		/*
> +		 * During this iteration, no page could be allocated, even
> +		 * though alloc_pages_bulk_array() falls back to alloc_page()
> +		 * if  it could not bulk-allocate. So we must be out of memory.
> +		 */
> +		return -ENOMEM;
>  	}

I find the way the loop is structured a bit cumbersome so I'd suggest to
rewrite it as:

int btrfs_alloc_page_array(unsigned int nr_pages, struct page **page_array)
{
        unsigned int allocated;

        for (allocated = 0; allocated < nr_pages;) {
                unsigned int last = allocated;

                allocated = alloc_pages_bulk_array(GFP_NOFS, nr_pages, page_array);

                /*
                 * During this iteration, no page could be allocated, even
                 * though alloc_pages_bulk_array() falls back to alloc_page()
                 * if  it could not bulk-allocate. So we must be out of memory.
                 */
                if (allocated == last)
                        return -ENOMEM;
        }
        return 0;
}

Also in the xfs code there's memalloc_retry_wait() which is supposed to be
called when repeated memory allocation is retried. What was the reason
you removed it?