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
