Re: Recent removal of bsg read/write support

[Douglas Gilbert]

On 2018-09-03 02:28 PM, Michal Hocko wrote:
> On Sun 02-09-18 21:16:10, Douglas Gilbert wrote:
> > On 2018-09-02 01:44 PM, Richard Weinberger wrote:
> > > CC'ing relevant people. Otherwise your mail might get lost.
> > >
> > > On Sun, Sep 2, 2018 at 1:37 PM Dror Levin <drorl@xxxxxxxxxxxxx> wrote:
> > > > Note: I am not subscribed to LKML so please CC replies to this email.
> > > >
> > > > Hi,
> > > >
> > > > We have an internal tool that uses the bsg read/write interface to
> > > > issue SCSI commands as part of a test suite for a storage device.
> > > >
> > > > After recently reading on LWN that this interface is to be removed we
> > > > tried porting our code to use sg instead. However, that raises new
> > > > issues - mainly getting ENOMEM over iSCSI for unknown reasons.
> > > >
> > > > Because of this we would like to continue using the bsg interface,
> > > > even if some changes are required to meet security concerns.
> > > >
> > > > Is there any chance for this removal to be reverted? I saw it was
> > > > already included in 4.19-rc1.
> >
> > Hi,
> > Both bsg and sg are relatively thin shims over the same block layer
> > pass-through calls. And neither driver will themselves generate ENOMEM
> > unless the CPU is running low of memory.
> >
> > In my experience, the main reason for unexpected ENOMEMs *** is from
> > blk_rq_map_user_iov() in block/blk_map.c called from both drivers.
> > That is a particular resource shortage rather than memory in general.
> > I do notice the blk_rq_map_user_iov() is/was called with GFP_KERNEL
> > in bsg and GFP_ATOMIC by sg. That suggests when you call write() on
> > a sg device and get ENOMEM, then wait a little (depends on your app)
> > and try again.
>
> Well, what is the reason to use GFP_ATOMIC in the first place? I am not
> familiar with the code so I might be easily wrong but sg_start_req which
> calls blk_rq_map_user_iov resp. blk_rq_map_user with GFP_ATOMIC uses
> mutex. It is a conditional usage so the sleeping context might depend
> on the caller. But I guess it would be better to double check. It looks
> suspicious to me.

Of the hundreds of 'hacks' on the sg driver over the years, the most
common is an expert arguing that GFP_ATOMIC should be changed to GFP_KERNEL.
They usually get their way. That is followed around 6 to 9 months later by
a sg user complaining about an unexpected broken app. So back it goes to
GFP_ATOMIC.

Welcome to the merry-go-round.

Doug Gilbert