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

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Date:   Wed, 1 Mar 2017 11:31:32 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Vlastimil Babka <vbabka.lkml@...il.com>
Cc:     Nikolay Borisov <nborisov@...e.com>, peterz@...radead.org,
        mingo@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] lockdep: Teach lockdep about memalloc_noio_save

On Wed 01-03-17 11:22:51, Vlastimil Babka wrote:
> On 03/01/2017 08:48 AM, 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
> > 
> > Signed-off-by: Nikolay Borisov <nborisov@...e.com>
> > ---
> >  kernel/locking/lockdep.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> > index 9812e5dd409e..5715fdcede28 100644
> > --- a/kernel/locking/lockdep.c
> > +++ b/kernel/locking/lockdep.c
> > @@ -2866,7 +2866,8 @@ static void __lockdep_trace_alloc(gfp_t gfp_mask, unsigned long flags)
> >  		return;
> >  
> >  	/* this guy won't enter reclaim */
> > -	if ((curr->flags & PF_MEMALLOC) && !(gfp_mask & __GFP_NOMEMALLOC))
> > +	if (((curr->flags & PF_MEMALLOC) && !(gfp_mask & __GFP_NOMEMALLOC)) ||
> > +			curr->flags & PF_MEMALLOC_NOIO)
> 
> It would be slightly better to use memalloc_noio_flags() here. Michal is
> planning to convert it to take also a new PF_MEMALLOC_NOFS flag into
> account, and there would be less chance of forgetting to update this place.

Yes, you are right. The following should do the trick. I am really
surprised we haven't noticed this before. I thought we were shaving the
gfp_mask before the allocator goes the lockdep_trace_alloc way. But it
is not and what is worse SLAB tracks this as well so we cannot rely on
the proper gfp mask. The positive thing is that the recursion avoidance
works because we always clear GFP_IO and GFP_FS when doing reclaim.

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 7c38f8f3d97b..0c70b26849ce 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -2861,6 +2861,8 @@ static void __lockdep_trace_alloc(gfp_t gfp_mask, unsigned long flags)
 	if (unlikely(!debug_locks))
 		return;
 
+	gfp_mask = memalloc_noio_flags(gfp_mask);
+
 	/* no reclaim without waiting on it */
 	if (!(gfp_mask & __GFP_DIRECT_RECLAIM))
 		return;
-- 
Michal Hocko
SUSE Labs