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Date: Thu, 18 Oct 2018 08:11:23 -0700
From: Matthew Wilcox <willy@infradead.org>
To: Christoph Hellwig <hch@lst.de>
Cc: Ming Lei <ming.lei@redhat.com>, Jens Axboe <axboe@kernel.dk>,
        linux-block@vger.kernel.org,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
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        "Darrick J . Wong" <darrick.wong@oracle.com>, xfs@vger.kernel.org,
        Bart Van Assche <bvanassche@acm.org>
Subject: Re: [PATCH 4/5] block: introduce helpers for allocating IO buffers
 from slab
Message-ID: <20181018151123.GD32429@bombadil.infradead.org>
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 <20181018144207.GD26828@lst.de>
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On Thu, Oct 18, 2018 at 04:42:07PM +0200, Christoph Hellwig wrote:
> 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.