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

[Joel Fernandes <joel@joelfernandes.org>]

On Tue, Mar 31, 2020 at 05:34:50PM +0200, Michal Hocko wrote:
> On Tue 31-03-20 10:58:06, Joel Fernandes wrote:
> [...]
> > > 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.
> 
> 
> __GFP_ATOMIC | __GFP_HIGH is the way to get an additional access to
> memory reserves regarless of the sleeping status.
> 
> Using __GFP_MEMALLOC is quite dangerous because it can deplete _all_ the
> memory. What does prevent the above code path to do that?

Can you suggest what prevents other users of GFP_MEMALLOC from doing that
also? That's the whole point of having a reserve, in normal usage no one will
use it, but some times you need to use it. Keep in mind this is not a common
case in this code here, this is triggered only if earlier allocation attempts
failed. Only *then* we try with GFP_MEMALLOC with promises to free additional
memory soon.

> If a partial access to reserves is sufficient then why the existing
> modifiers (mentioned above are not sufficient?

The point with using GFP_MEMALLOC is it is useful for situations where you
are about to free memory and needed some memory temporarily, to free that. It
depletes it a bit temporarily to free even more. Is that not the point of
PF_MEMALLOC?
* %__GFP_MEMALLOC allows access to all memory. This should only be used when
 * the caller guarantees the allocation will allow more memory to be freed
 * very shortly e.g. process exiting or swapping. Users either should
 * be the MM or co-ordinating closely with the VM (e.g. swap over NFS).

I was just recommending usage of this flag here because it fits the
requirement of allocating some memory to free some memory. I am also Ok with
GFP_ATOMIC with the GFP_NOWARN removed, if you are Ok with that.

thanks,

 - Joel