On Fri, Oct 19, 2018 at 10:53:49AM +0800, Ming Lei wrote:

On Thu, Oct 18, 2018 at 05:22:19PM +0200, Christoph Hellwig wrote:

quoted

On Thu, Oct 18, 2018 at 08:11:23AM -0700, Matthew Wilcox wrote:

quoted

On Thu, Oct 18, 2018 at 04:42:07PM +0200, Christoph Hellwig wrote:

quoted

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;
}

This looks a lot like page_frag_alloc() except I think page_frag_alloc()
may be more efficient.

Oh, nice. Sounds like XFS should just use page_frag_alloc.  I'll give
it a spin.

XFS or other fs can use page_frag_alloc() directly, seems not necessary to
introduce this change in block layer any more given 512-aligned buffer
should be fine everywhere.

The only benefit to make it as block helper is that the offset or size
can be checked with q->dma_alignment.

Dave/Jens, do you think which way is better? Put allocation as block
helper or fs uses page_frag_alloc() directly for allocating 512*N-byte 
buffer(total size is less than PAGE_SIZE)?

Cristoph has already said he's looking at using page_frag_alloc()
directly in XFS....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com