Re: [PATCH v3 08/16] bcache: introduce meta_bucket_pages() related helper routines

[Hannes Reinecke <hare@suse.de>]

On 7/15/20 4:30 PM, colyli@suse.de wrote:
> From: Coly Li <colyli@suse.de>
> 
> Currently the in-memory meta data like c->uuids or c->disk_buckets
> are allocated by alloc_bucket_pages(). The macro alloc_bucket_pages()
> calls __get_free_pages() to allocated continuous pages with order
> indicated by ilog2(bucket_pages(c)),
>   #define alloc_bucket_pages(gfp, c)                      \
>       ((void *) __get_free_pages(__GFP_ZERO|gfp, ilog2(bucket_pages(c))))
> 
> The maximum order is defined as MAX_ORDER, the default value is 11 (and
> can be overwritten by CONFIG_FORCE_MAX_ZONEORDER). In bcache code the
> maximum bucket size width is 16bits, this is restricted both by KEY_SIZE
> size and bucket_size size from struct cache_sb_disk. The maximum 16bits
> width and power-of-2 value is (1<<15) in unit of sector (512byte). It
> means the maximum value of bucket size in bytes is (1<<24) bytes a.k.a
> 4096 pages.
> 
> When the bucket size is set to maximum permitted value, ilog2(4096) is
> 12, which exceeds the default maximum order __get_free_pages() can
> accepted, the failed pages allocation will fail cache set registration
> procedure and print a kernel oops message for the exceeded pages order.
> 
> This patch introduces meta_bucket_pages(), meta_bucket_bytes(), and
> alloc_bucket_pages() helper routines. meta_bucket_pages() indicates the
> maximum pages can be allocated to meta data bucket, meta_bucket_bytes()
> indicates the according maximum bytes, and alloc_bucket_pages() does
> the pages allocation for meta bucket. Because meta_bucket_pages()
> chooses the smaller value among the bucket size and MAX_ORDER_NR_PAGES,
> it still works when MAX_ORDER overwritten by CONFIG_FORCE_MAX_ZONEORDER.
> 
> Following patches will use these helper routines to decide maximum pages
> can be allocated for different meta data buckets. If the bucket size is
> larger than meta_bucket_bytes(), the bcache registration can continue to
> success, just the space more than meta_bucket_bytes() inside the bucket
> is wasted. Comparing bcache failed for large bucket size, wasting some
> space for meta data buckets is acceptable at this moment.
> 
> Signed-off-by: Coly Li <colyli@suse.de>
> ---
>   drivers/md/bcache/bcache.h | 20 ++++++++++++++++++++
>   drivers/md/bcache/super.c  |  3 +++
>   2 files changed, 23 insertions(+)
> 
> diff --git a/drivers/md/bcache/bcache.h b/drivers/md/bcache/bcache.h
> index 80e3c4813fb0..972f1aff0f70 100644
> --- a/drivers/md/bcache/bcache.h
> +++ b/drivers/md/bcache/bcache.h
> @@ -762,6 +762,26 @@ struct bbio {
>   #define bucket_bytes(c)		((c)->sb.bucket_size << 9)
>   #define block_bytes(c)		((c)->sb.block_size << 9)
>   
> +static inline unsigned int meta_bucket_pages(struct cache_sb *sb)
> +{
> +	unsigned int n, max_pages;
> +
> +	max_pages = min_t(unsigned int,
> +			  __rounddown_pow_of_two(USHRT_MAX) / PAGE_SECTORS,
> +			  MAX_ORDER_NR_PAGES);
> +
> +	n = sb->bucket_size / PAGE_SECTORS;
> +	if (n > max_pages)
> +		n = max_pages;
> +
> +	return n;
> +}
> +
> +static inline unsigned int meta_bucket_bytes(struct cache_sb *sb)
> +{
> +	return meta_bucket_pages(sb) << PAGE_SHIFT;
> +}
> +
>   #define prios_per_bucket(c)				\
>   	((bucket_bytes(c) - sizeof(struct prio_set)) /	\
>   	 sizeof(struct bucket_disk))
I'm not particular happy with the division followed by a shift; might be 
an idea to replace the division by a shift.

But that's minor, so:

Reviewed-by: Hannes Reinecke <hare@suse.de>

Cheers,

Hannes
-- 
Dr. Hannes Reinecke            Teamlead Storage & Networking
hare@suse.de                               +49 911 74053 688
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