linux-kernel - Re: [PATCH v2] lockdep: Teach lockdep about memalloc_noio

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Message-ID: <20170301100304.GI6500@twins.programming.kicks-ass.net>
Date:   Wed, 1 Mar 2017 11:03:04 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Nikolay Borisov <nborisov@...e.com>
Cc:     mingo@...hat.com, linux-kernel@...r.kernel.org,
        ming.lei@...onical.com, minchan@...nel.org, mel@....ul.ie,
        junxiao.bi@...cle.com
Subject: Re: [PATCH v2] lockdep: Teach lockdep about memalloc_noio_save

On Wed, Mar 01, 2017 at 11:57:13AM +0200, Nikolay Borisov wrote:
> 
> 
> On  1.03.2017 11:46, Peter Zijlstra wrote:
> > On Wed, Mar 01, 2017 at 09:59:00AM +0200, Nikolay Borisov wrote:
> >> Commit 21caf2fc1931 ("mm: teach mm by current context info to not do I/O
> >> during memory allocation") added the memalloc_noio_(save|restore) functions
> >> to enable people to modify the MM behavior by disbaling I/O during memory
> >> allocation. This prevents allocation paths recursing back into the filesystem
> >> without explicitly changing the flags for every allocation site. Yet, lockdep
> >> not being aware of that is prone to showing false positives. Fix this
> >> by teaching it that the presence of PF_MEMALLOC_NOIO flag mean we are not
> >> going to issue any I/O
> > 
> > I'm not up to date on the specific, but GFP_IO is separate from GFP_FS.
> > 
> > And MEMALLOC_NOIO only clears GFP_IO but leaves GFP_FS set.
> 
> static inline gfp_t memalloc_noio_flags(gfp_t flags)                            
> {                                                                               
>     if (unlikely(current->flags & PF_MEMALLOC_NOIO))                            
>         flags &= ~(__GFP_IO | __GFP_FS);                                        
>     return flags;                                                               
> }
> 

Ah, so in the initial patch you referenced there was:

+static inline gfp_t memalloc_noio_flags(gfp_t flags)
+{
+       if (unlikely(current->flags & PF_MEMALLOC_NOIO))
+               flags &= ~__GFP_IO;
+       return flags;
+}

OK, so then this commit needs something like:

Fixes: 934f3072c17c ("mm: clear __GFP_FS when PF_MEMALLOC_NOIO is set")