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

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Date:   Wed, 1 Mar 2017 11:22:51 +0100
From:   Vlastimil Babka <vbabka.lkml@...il.com>
To:     Nikolay Borisov <nborisov@...e.com>, peterz@...radead.org
Cc:     mingo@...hat.com, linux-kernel@...r.kernel.org,
        Michal Hocko <mhocko@...nel.org>
Subject: Re: [PATCH] lockdep: Teach lockdep about memalloc_noio_save

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.

The function uses "current" though, which __lockdep_trace_alloc()
aliases to "curr" for some reason.

>  		return;
>  
>  	/* We're only interested __GFP_FS allocations for now */
>