Re: command emulation fix

[Christoph Hellwig <hch@lst.de>]

On Thu, Jan 19, 2017 at 03:34:58PM -0500, Tejun Heo wrote:
> So, unless we allocate one mempool per port, we're gonna have to
> synchronize around its use anyway.  mempool can't guarantee allocation
> to multiple users at the same time.  If this is something which
> affects scalability, I'm completley fine with making it per-port (or
> device).  Each ata_port carries 512 byte buffer anyway.  Maybe we can
> reuse that?

I'll play around with these things a bit more.  For the slow
path ops I think I can get away without any dynamic allocation
at all - just use small on-stack buffers.

For DSM / Write Same we rewrite into the buffer the SCSI layer
provided us.  This is a bit of an issue anyway as this might
modify user data that is not expected to be rewritten or even
mapped read-only.  Maybe we need to kill off that emulation
entirely and just have ATA DSM using the ATA_16 CDB as another
provisioning option in sd.c.  While that is a bit of a layering
violation it would solve a lot of problems with the way TRIM
is currently implemented.