netdev - Re: [4.15-rc9] fs_reclaim lockdep trace

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Date:   Mon, 29 Jan 2018 14:55:47 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc:     torvalds@...ux-foundation.org, davej@...emonkey.org.uk,
        npiggin@...il.com, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, netdev@...r.kernel.org, mhocko@...nel.org
Subject: Re: [4.15-rc9] fs_reclaim lockdep trace

On Mon, Jan 29, 2018 at 08:47:20PM +0900, Tetsuo Handa wrote:
> Peter Zijlstra wrote:
> > On Sun, Jan 28, 2018 at 02:55:28PM +0900, Tetsuo Handa wrote:
> > > This warning seems to be caused by commit d92a8cfcb37ecd13
> > > ("locking/lockdep: Rework FS_RECLAIM annotation") which moved the
> > > location of
> > > 
> > >   /* this guy won't enter reclaim */
> > >   if ((current->flags & PF_MEMALLOC) && !(gfp_mask & __GFP_NOMEMALLOC))
> > >           return false;
> > > 
> > > check added by commit cf40bd16fdad42c0 ("lockdep: annotate reclaim context
> > > (__GFP_NOFS)").
> > 
> > I'm not entirly sure I get what you mean here. How did I move it? It was
> > part of lockdep_trace_alloc(), if __GFP_NOMEMALLOC was set, it would not
> > mark the lock as held.
> 
> d92a8cfcb37ecd13 replaced lockdep_set_current_reclaim_state() with
> fs_reclaim_acquire(), and removed current->lockdep_recursion handling.
> 
> ----------
> # git show d92a8cfcb37ecd13 | grep recursion
> -# define INIT_LOCKDEP                          .lockdep_recursion = 0, .lockdep_reclaim_gfp = 0,
> +# define INIT_LOCKDEP                          .lockdep_recursion = 0,
>         unsigned int                    lockdep_recursion;
> -       if (unlikely(current->lockdep_recursion))
> -       current->lockdep_recursion = 1;
> -       current->lockdep_recursion = 0;
> -        * context checking code. This tests GFP_FS recursion (a lock taken
> ----------

That should not matter at all. The only case that would matter for is if
lockdep itself would ever call into lockdep again. Not something that
happens here.

> > The new code has it in fs_reclaim_acquire/release to the same effect, if
> > __GFP_NOMEMALLOC, we'll not acquire/release the lock.
> 
> Excuse me, but I can't catch.
> We currently acquire/release __fs_reclaim_map if __GFP_NOMEMALLOC.

Right, got the case inverted, same difference though. Before we'd do
mark_held_lock(), now we do acquire/release under the same conditions.

> > > Since __kmalloc_reserve() from __alloc_skb() adds
> > > __GFP_NOMEMALLOC | __GFP_NOWARN to gfp_mask, __need_fs_reclaim() is
> > > failing to return false despite PF_MEMALLOC context (and resulted in
> > > lockdep warning).
> > 
> > But that's correct right, __GFP_NOMEMALLOC should negate PF_MEMALLOC.
> > That's what the name says.
> 
> __GFP_NOMEMALLOC negates PF_MEMALLOC regarding what watermark that allocation
> request should use.

Right.

> But at the same time, PF_MEMALLOC negates __GFP_DIRECT_RECLAIM.

Ah indeed.

> Then, how can fs_reclaim contribute to deadlock?

Not sure it can. But if we're going to allow this, it needs to come with
a clear description on why. Not a few clues to a puzzle.

Now, even if its not strictly a deadlock, there is something to be said
for flagging GFP_FS allocs that lead to nested GFP_FS allocs, do we ever
want to allow that?