Re: SG does not ignore dxferp (direct io + mmap)

[Laurence Oberman <loberman@redhat.com>]

----- Original Message -----
> From: "Eyal Ben David" <bdeyal@gmail.com>
> To: linux-scsi@vger.kernel.org
> Sent: Sunday, November 20, 2016 11:02:49 AM
> Subject: SG does not ignore dxferp (direct io + mmap)
> 
> Hi all,
> 
> We have some IO utility that perform the IOs using sg and direct io with
> mmap.
> Our current systems are Ubuntu 14.04, RHEL 6,7
> The IO utility always set dxferp to either the address or mmap of
> other allocation (valloc)
> Setting dxferp was harmless since SG is supposed to ignore the address
> if mmap IO is selected.
> When porting to Ubuntu 16.04, we had a corruption problem - first byte
> of a read task is always 0.
> When setting dxferp as NULL the corruption does not occur any more.
> This is a regression and not according to SCSI generic documentation.
> 
> I wrote a small program that shows the change:
> 
> Read indirect (no mmap), lba=0:
> =======================
> $ ./sg_mmap_read -d /dev/sg0 -l 0
> 0000000 eb 63 90 10 8e d0 bc 00 b0 b8 00 00 8e d8 8e c0
> 
> Read with mmap, lba=0, dxferp=NULL:
> ============================
> $ ./sg_mmap_read -d /dev/sg0 -l 0 -m
> 0000000 eb 63 90 10 8e d0 bc 00 b0 b8 00 00 8e d8 8e c0
> 
> Read with mmap, lba=0, dxferp=address from mmap
> ======================================
> $ ./sg_mmap_read -d /dev/sg0 -l 0 -m -b
> 0000000 00 63 90 10 8e d0 bc 00 b0 b8 00 00 8e d8 8e c0
> 
> On the older systems all results are the same.
> 
> Thanks for any answer!
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Hello

Given that we cannot see what your utility (sg_mmap_read) is doing, can we get the source for that or an strace of your test here.
I am sure Doug will then be able to help you.

Thanks
Laurence