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

Coly Li
2020-07-15T16:00:17Z

On 2020/7/15 23:36, Hannes Reinecke wrote:
> 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>

This is from existing bcache code, let me think how to improve in future.

Thanks.

Coly Li