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Date:   Tue, 31 Mar 2020 10:58:06 -0400
From:   Joel Fernandes <joel@joelfernandes.org>
To:     linux-kernel@vger.kernel.org
Cc:     linux-mm@kvack.org, rcu@vger.kernel.org, willy@infradead.org,
        peterz@infradead.org, neilb@suse.com, vbabka@suse.cz,
        mgorman@suse.de, Andrew Morton <akpm@linux-foundation.org>,
        Josh Triplett <josh@joshtriplett.org>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>
Subject: Re: [PATCH RFC] rcu/tree: Use GFP_MEMALLOC for alloc memory to free
 memory pattern
Message-ID: <20200331145806.GB236678@google.com>
References: <20200331131628.153118-1-joel@joelfernandes.org>
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On Tue, Mar 31, 2020 at 09:16:28AM -0400, Joel Fernandes (Google) wrote:
> In kfree_rcu() headless implementation (where the caller need not pass
> an rcu_head, but rather directly pass a pointer to an object), we have
> a fall-back where we allocate a rcu_head wrapper for the caller (not the
> common case). This brings the pattern of needing to allocate some memory
> to free some memory.  Currently we use GFP_ATOMIC flag to try harder for
> this allocation, however the GFP_MEMALLOC flag is more tailored to this
> pattern. We need to try harder not only during atomic context, but also
> during non-atomic context anyway. So use the GFP_MEMALLOC flag instead.
> 
> Also remove the __GFP_NOWARN flag simply because although we do have a
> synchronize_rcu() fallback for absolutely worst case, we still would
> like to not enter that path and atleast trigger a warning to the user.
> 
> Cc: linux-mm@kvack.org
> Cc: rcu@vger.kernel.org
> Cc: willy@infradead.org
> Cc: peterz@infradead.org
> Cc: neilb@suse.com
> Cc: vbabka@suse.cz
> Cc: mgorman@suse.de
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> ---
> 
> This patch is based on the (not yet upstream) code in:
> git://git.kernel.org/pub/scm/linux/kernel/git/jfern/linux.git (branch rcu/kfree)
> 
> It is a follow-up to the posted series:
> https://lore.kernel.org/lkml/20200330023248.164994-1-joel@joelfernandes.org/
> 
> 
>  kernel/rcu/tree.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 4be763355c9fb..965deefffdd58 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -3149,7 +3149,7 @@ static inline struct rcu_head *attach_rcu_head_to_object(void *obj)
>  
>  	if (!ptr)
>  		ptr = kmalloc(sizeof(unsigned long *) +
> -				sizeof(struct rcu_head), GFP_ATOMIC | __GFP_NOWARN);
> +				sizeof(struct rcu_head), GFP_MEMALLOC);

Just to add, the main requirements here are:
1. Allocation should be bounded in time.
2. Allocation should try hard (possibly tapping into reserves)
3. Sleeping is Ok but should not affect the time bound.

Considering this, GFP_MEMALLOC|GFP_NOWAIT seems appropriate. Thoughts?

thanks,

 - Joel