Re: [PATCH RFC] rcu/tree: Use GFP_MEMALLOC for alloc memory to free memory pattern

[Joel Fernandes]

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@xxxxxxxxx
> Cc: rcu@xxxxxxxxxxxxxxx
> Cc: willy@xxxxxxxxxxxxx
> Cc: peterz@xxxxxxxxxxxxx
> Cc: neilb@xxxxxxxx
> Cc: vbabka@xxxxxxx
> Cc: mgorman@xxxxxxx
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> ---
> 
> 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@xxxxxxxxxxxxxxxxx/
> 
> 
>  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