Re: [PATCH] scsi_debug: fix sleep in invalid context

[Christoph Hellwig <hch@infradead.org>]

On Mon, May 30, 2016 at 12:02:45PM -0700, James Bottomley wrote:
> This might fix the immediate warning, but won't it demand huge
> contiguous memory chunks in high lun configurations and thus fail
> randomly?  Report luns is important to us because if that fails the
> target won't attach.
> 
> What about vmalloc'ing enough space at configuration time, when you do
> have process context, and simply reusing the already allocated buffer
> in this routine?  If you want to be clever, you could do a single
> vmalloc for the biggest LUN size you have and reuse that buffer for
> every report lun command with suitable locking ... we tend to fire off
> report luns sequentially at start of day, so it's not like they have
> huge performance or concurrency requirements.

There is no need for the allocation at all.  Instead of the big
array and a single call to fill_from_dev_buffer we can simply
have a single scsi_lun structure on stack that gets reused for
every lun and individual calls to sg_copy_from_buffer for each
one instead of a single big one.