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

[Paul E. McKenney]

On Wed, Apr 01, 2020 at 05:28:05PM +0200, Michal Hocko wrote:
> On Wed 01-04-20 15:22:58, Uladzislau Rezki wrote:
> > > > We call it from atomic context, so we can not sleep, also we do not have
> > > > any existing context coming from the caller. I see that GFP_ATOMIC is high-level
> > > > flag and is differ from __GFP_ATOMIC. It is defined as:
> > > > 
> > > > #define GFP_ATOMIC (__GFP_HIGH|__GFP_ATOMIC|__GFP_KSWAPD_RECLAIM)
> > > > 
> > > > so basically we would like to have __GFP_KSWAPD_RECLAIM that is included in it,
> > > > because it will also help in case of high memory pressure and wake-up kswapd to
> > > > reclaim memory.
> > > > 
> > > > We also can extract:
> > > > 
> > > > __GFP_ATOMIC | __GFP_HIGH | __GFP_RETRY_MAYFAIL | __GFP_KSWAPD_RECLAIM
> > > > 
> > > > but that is longer then
> > > > 
> > > > GFP_ATMOC |  __GFP_RETRY_MAYFAIL
> > > 
> > > OK, if you are always in the atomic context then GFP_ATOMIC is
> > > sufficient. __GFP_RETRY_MAYFAIL will make no difference for allocations
> > > which do not reclaim (and thus not retry). Sorry this was not clear to
> > > me from the previous description.
> > > 
> > Ahh. OK. Then adding __GFP_RETRY_MAYFAIL to GFP_ATOMIC will not make any effect.
> > 
> > Thank you for your explanation!
> 
> Welcome. I wish all those gfp flags were really clear but I fully
> understand that people who are not working with MM regurarly might find
> it confusing. Btw. have __GFP_RETRY_MAYFAIL is documented in gfp.h and
> it is documented as the reclaim modifier which should imply that it has
> no effect when the reclaim is not allowed which is the case for any non
> sleeping allocation. If that relation was not immediately obvious then I
> think we need to make it explicit. Would you find it useful?
> 
> E.g.

One nit below, but either way:

Acked-by: Paul E. McKenney <paulmck@xxxxxxxxxx>

> diff --git a/include/linux/gfp.h b/include/linux/gfp.h
> index e3ab1c0d9140..8f09cefdfa7b 100644
> --- a/include/linux/gfp.h
> +++ b/include/linux/gfp.h
> @@ -127,6 +127,8 @@ struct vm_area_struct;
>   *
>   * Reclaim modifiers
>   * ~~~~~~~~~~~~~~~~~
> + * Please note that all the folloging flags are only applicable to sleepable

s/folloging/following/

> + * allocations (e.g. %GFP_NOWAIT and %GFP_ATOMIC will ignore them).
>   *
>   * %__GFP_IO can start physical IO.
>   *
> -- 
> Michal Hocko
> SUSE Labs