Re: [PATCH 4/5] block: introduce helpers for allocating IO buffers from slab

[Christoph Hellwig <hch@lst.de>]

This all seems quite complicated.

I think the interface we'd want is more one that has a little
cache of a single page in the queue, and a little bitmap which
sub-page size blocks of it are used.

Something like (pseudo code minus locking):

void *blk_alloc_sector_buffer(struct block_device *bdev, gfp_t gfp)
{
	unsigned block_size = block_size(bdev);

	if (blocksize >= PAGE_SIZE)
		return (void *)__get_free_pages(gfp, get_order(blocksize));

	if (bdev->fragment_cache_page) {
		[ <find fragment in bdev->fragment_cache_page using
		  e.g. bitmap and return if found]
	}

	bdev->fragment_cache_page = (void *)__get_free_page(gfp);
	goto find_again;
}

note that the locking also is important, preferably we'd be able to do
something lockless.