Re: [PATCH V4 2/3] scsi: core: avoid to pre-allocate big chunk for protection meta data

[Bart Van Assche <bvanassche@acm.org>]

On Sun, 2019-04-28 at 15:39 +0800, Ming Lei wrote:
> Now scsi_mq_setup_tags() pre-allocates a big buffer for protection
> sg entries, and the buffer size is scsi_mq_sgl_size().
> 
> This way isn't correct, scsi_mq_sgl_size() is used to pre-allocate
> sg entries for IO data. And the protection data buffer is much less,
> for example, one 512byte sector needs 8byte protection data, and
> the max sector number for one request is 2560(BLK_DEF_MAX_SECTORS),
> so the max protection data size is just 20k.
> 
> The usual case is that one bio builds one single bip segment. Attribute
> to bio split, bio merge is seldom done for big IO, and it is only done
> in case of small bios. And protection data segment number is usually
> same with bio count in the request, so the number won't be very big,
> and allocating from slab is fast enough.
> 
> Reduce to pre-allocate one sg entry for protection data, and switch
> to runtime allocation in case that the protection data segment number
> is bigger than 1. Then we can save huge pre-alocation, for example,
> 500

Reviewed-by: Bart Van Assche <bvanassche@acm.org>