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

[Douglas Gilbert <dgilbert@interlog.com>]

On 2016-05-30 03:09 PM, Christoph Hellwig wrote:
> 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.

Christoph,
Interesting idea but a time waster due to:
    s/sg_copy_from_buffer/sg_pcopy_from_buffer/

Otherwise the start of the sgl is overwritten by each subsequent
call. The "p" variant has a trailing 'off_t skip' argument and
currently there are no helper functions in scsi_cmnd.h for the
"pcopy" variants. Also handling the resid is fiddly, not helped by
initializing it to zero rather than scsi_bufflen().

Doug Gilbert