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

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Date:   Wed, 1 Mar 2017 10:46:57 +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
Subject: Re: [PATCH v2] lockdep: Teach lockdep about memalloc_noio_save

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.

Therefore I think your change is wrong, but I might have overlooked a
detail. Added original authors to Cc to clarify.


> Signed-off-by: Nikolay Borisov <nborisov@...e.com>
> ---
>  kernel/locking/lockdep.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> 
>  Turned out there was another place where RELCAIM_FS was being set, fix it 
>  by using the memalloc_noio_flags when setting ->lockdep_reclaim_gfp flags.
> 
> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> index 9812e5dd409e..87cf9910e66f 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)
>  		return;
>  
>  	/* We're only interested __GFP_FS allocations for now */
> @@ -3852,7 +3853,7 @@ EXPORT_SYMBOL_GPL(lock_unpin_lock);
>  
>  void lockdep_set_current_reclaim_state(gfp_t gfp_mask)
>  {
> -	current->lockdep_reclaim_gfp = gfp_mask;
> +	current->lockdep_reclaim_gfp = memalloc_noio_flags(gfp_mask);
>  }
>  
>  void lockdep_clear_current_reclaim_state(void)
> -- 
> 2.7.4
>