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SubjectRe: [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@dorminy.me>
> Reviewed-by: Nikolay Borisov <nborisov@suse.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?

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