Messages in this thread | | | Subject | Re: Recent removal of bsg read/write support | From | Douglas Gilbert <> | Date | Tue, 4 Sep 2018 05:38:21 +0200 |
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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@infinidat.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
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