Re: remove exofs and the T10 OSD code V2

[Douglas Gilbert]

On 2018-10-31 4:57 p.m., Boaz Harrosh wrote:
> On 30/10/18 09:45, Christoph Hellwig wrote:
> > On Mon, Oct 29, 2018 at 02:42:12PM -0600, Jens Axboe wrote:
> > > LGTM, for both:
> >
> > I also have this one on top as requested by Martin.  The core block
> > bidi support is unfortunately also used by bsg-lib, although it is
> > not anywhere near as invasive.  But that is another argument for
> > looking into moving bsg-lib away from using block queues..
>
> BUT this patch is very very wrong.
>
> Totally apart from T10-OSD and its use in the field. Support for scsi BIDI
> commands is not exclusive to T10-OSD at all. Even the simple scsi-array
> command-set has BIDI operations defined. for example the write-return-xor
> and so on.
>
> Also some private administrative CDBs of some vendor devices uses SCSI-BIDI.
> So this patch just broke some drivers. (User-mode apps use bsg pass through)
>
> Also you might (try hard and) remove all usage of scsi-bidi as an initiator
> from the Linux Kernel. But what about target mode. As a target we have supported
> on the wire bidi protocols like write-return-xor and others for a long time.
> Are you willing to silently break all these setups in the field on the next update?
> Are you so sure these are never used?
>
> PLEASE, I beg of you guys. Do not remove SCSI-BIDI. It is a cry of generations.
>
> And I think by the rules of Linus, as far as target mode. You are not allowed
> to break users in this way.

Hi,
I'm in the process of rebuilding the sg driver with the following goals:

  - undo 20 years of road wear, some of which is caused by literally
    hundreds of "laparoscopic" patches that gradually ruin a driver,
    at least from the maintainer's viewpoint. Comments that once made
    sense become cryptic or just nonsense; naming schemes that
    obliterate variable names to the extent that a random name
    generator would be easier to follow; and just plain broken code.
    For example, why sort out the existing locking at a particular
    level when you can add a new lock in a completely non-orthogonal
    way? [Yes, I looking at you, google.] Anyway, my first cut at this
    is out there (on the linux-scsi list, see: "[PATCH v3 0/8] sg:
    major cleanup, remove max_queue limit"). Not much new there,
    unless you look very closely

  - the next step is to add to the sg driver async SCSI command
    capability based on the sg_io_v4 structure previously only used
    by the bsg driver and now removed from bsg. The main advantage
    of the sg_io_v4 structure over previous pass-through interface
    attempts is the support of SCSI bidi commands

  - as part of this effort introduce two new ioctls: SG_IOSUBMIT and
    SG_IORECEIVE to replace the write()/read() technique currently
    in use (since Linux 1.0 in 1992). The write()/read() technique
    seems to be responsible for various security folks losing clumps
    of their hair. One advantage of ioctls, as Alan Cox pointed out,
    is the ability to write to and read back from the kernel in a way
    that is naturally synchronized. [Actually, those security folks
    shouldn't look too closely at sg_read() in that respect.]

In discussions with several folks who are on the T10 committee, I
wondered why there was no READ GATHERED command (there has been a
WRITE SCATTERED for 2 years). The answer was lack of interest ***,
plus the difficultly of implementation. You see, READ GATHERED needs
to send a scatter gather list to the device and get the corresponding
data back (as a linear array). And that requires either:
  a) bidi commands (scatter gather list in the data-out, corresponding
     "read" data in the data-in), or
  b) loooong SCSI commands, up to around 256 bytes long in which the
     sgat list is the latter part of that command

And the T10 folks say neither of those options is well supported or
is expensive. I'm guessing they are referring to Linux and Windows.
I haven't argued much beyond that point, but it looks like a bit of
a chicken and egg situation.


Don't know too much about the T10 OSD stuff. But I can see that it
uses both long SCSI commands and a lot of bidi. IMO it seems to be
10 or 20 years before its time. Maybe ibm/redhat need to
(re-)discover it for it to catch on.


Plus there are proprietary SCSI bidi commands out there. People contact
me and ask me how to issue them with sg3_utils package. Easy, I tell them,
just use sg_raw with a bsg device. Typically, in my experience, "no news
is good news" after suggestions like that. When I give bad advice, I
usually hear back relatively quickly. Anyone who wants SCSI bidi _async_
support is currently out of luck.

Enough already
Doug Gilbert


*** example: loading all or most kernel modules in a few READ SCATTERED
    commands might speed up boot time ...