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

[Ewan Milne <emilne@redhat.com>]

>> ---
>> 
>> In other words, this commit made the bad behavior go away in 4.8.
>> Need to look at this in more detail, it doesn't appear as if this patch
>> was intended to fix such a problem.
>> 
>> -Ewan
>
>Are you sure it did? I can repropduce copy_to_user() errors with 4.8 as well.
>Using the very same reproducer. On 4.8 it's just harder to trigger and
>doesn't trigger on AHCI as fas as I can telli (maybe I just haven't hit
>it hard enough). I can trigger it on QEMUs SCSI CDROM emulation and hpsa
>though. I cannot however trigger this with a minimal config inside an initrd.

It did for Eyal's supplied test case on my machine, but that was not an
exhaustive test, and I am a little suspicious that the behavior change was
due to a side-effect of the patch rather than actually fixing something.

I think what we need to understand is what caused the regression in the
first place, I probably should have been bisecting the original failure
rather than trying to find where it started working.

I was running against an internal (physical) drive.

-Ewan