linux-kernel - Re: [PATCH] arch/sh: use slow path for get_user_pages

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Date:   Mon, 23 Jul 2018 15:46:57 +0200
From:   Thomas Petazzoni <thomas.petazzoni@...tlin.com>
To:     Sergei Shtylyov <sergei.shtylyov@...entembedded.com>
Cc:     Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Rich Felker <dalias@...c.org>, linux-sh@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arch/sh: use slow path for get_user_pages_fast() on SMP

Hello,

On Fri, 15 Jun 2018 19:04:45 +0300, Sergei Shtylyov wrote:
> Hello!
> 
> On 06/15/2018 06:52 PM, Thomas Petazzoni wrote:
> 
> > The fast path of get_user_pages_fast() disables IRQs and then does:
> > 
> >  - gup_pud_range()
> >    - gup_pmd_range()
> >      - gup_pte_range()
> >        - flush_dcache_page()
> > 
> > However, flush_dcache_page() makes a smp_call_function(), and using
> > smp_call_function() when IRQs are disabled is not allowed.
> > 
> > In order to work around this problem, this commit prevents the fast
> > path from get_user_pages_fast() from being used on SMP, and directly  
>        ^^^^ of?
>    Else it sounds a bit tautological...

Yes, agreed.

I can fix that up for a v2, or perhaps such a small typo can be fixed
while applying the patches.

Could someone review and/or merge those patches ?

I sent four patches, which are still pending:

  [PATCH] arch/sh: kernel: use KERN_CONT in print_sh_insn()
  [PATCH] arch/sh: kernel: use KERN_CONT in dump_mem()
  [PATCH] arch/sh: fix SH4 flush_dcache_all() for SMP
  [PATCH] arch/sh: use slow path for get_user_pages_fast() on SMP

Note that this last one is more a workaround than a proper fix. It does
the job, but there is very likely a better solution, and I was hoping
to get some insights/suggestions.

Thanks a lot,

Thomas
-- 
Thomas Petazzoni, CTO, Bootlin (formerly Free Electrons)
Embedded Linux and Kernel engineering
https://bootlin.com