Re: [PATCH] SCSI Core patches

[James Bottomley <James.Bottomley@steeleye.com>]

I got around to looking at the patch.

I think the idea of slab allocation for Scsi_Cmnds is generally worth the 
effort.  There are several issues with the current patch, though:

1) The command for the device has to be allocated in space that respects the 
dma_mask for the device (on a non MMIO bus).  Pretty much for scsi, this 
translates to using GFP_DMA as the allocation flag if unchecked_isa_dma is 
set.  GFP_DMA has to be passed in at kmem_cache_create time.  We therefore 
probably need the capacity to use a different kmem_cache on a per Scsi_Host 
basis and the ability to set up a GFP_DMA kmem_cache if a host with 
unchecked_isa_dma appears.

2) scsi_get_command takes the host lock when obtaining a command from it's 
free_list.  Unfortunately, scsi_get_command looks to be called from places 
(like the prep_fn) where the lock is already held => deadlock.

3) The free_list employs all the list machinery for potentially storing 
multiple commands, but in practice it looks like it only ever stores one of 
them.  Is there a plan to make this free_list size tuneable (probably tuneable 
per device)?

James