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Subject: Re: [PATCH 4/5] block: introduce helpers for allocating IO buffers
 from slab
To: Ming Lei <ming.lei@redhat.com>, Christoph Hellwig <hch@lst.de>
Cc: Matthew Wilcox <willy@infradead.org>, linux-block@vger.kernel.org,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Dave Chinner <dchinner@redhat.com>,
        Linux FS Devel <linux-fsdevel@vger.kernel.org>,
        "Darrick J . Wong" <darrick.wong@oracle.com>,
        linux-xfs@vger.kernel.org, Bart Van Assche <bvanassche@acm.org>
References: <20181018131817.11813-1-ming.lei@redhat.com>
 <20181018131817.11813-5-ming.lei@redhat.com> <20181018144207.GD26828@lst.de>
 <20181018151123.GD32429@bombadil.infradead.org>
 <20181018152219.GB28300@lst.de> <20181019025348.GB14531@ming.t460p>
From: Jens Axboe <axboe@kernel.dk>
Message-ID: <368d23d7-e35e-a078-50bc-d8da97552990@kernel.dk>
Date: Thu, 18 Oct 2018 22:06:11 -0600
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On 10/18/18 8:53 PM, Ming Lei wrote:
> On Thu, Oct 18, 2018 at 05:22:19PM +0200, Christoph Hellwig wrote:
>> On Thu, Oct 18, 2018 at 08:11:23AM -0700, Matthew Wilcox wrote:
>>> 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.
>>
>> 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)?

I'd greatly prefer having the FS use that directly, seems kind of
pointless to provide an abstraction for that at that point.

-- 
Jens Axboe