linux-kernel - Re: Recent removal of bsg read/write support

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Date:   Tue, 4 Sep 2018 05:38:21 +0200
From:   Douglas Gilbert <dgilbert@...erlog.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Richard Weinberger <richard.weinberger@...il.com>,
        drorl@...inidat.com, LKML <linux-kernel@...r.kernel.org>,
        linux-block@...r.kernel.org, linux-scsi@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Christoph Hellwig <hch@...radead.org>,
        Jens Axboe <axboe@...nel.dk>
Subject: Re: Recent removal of bsg read/write support

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@...inidat.com> 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