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

[Uladzislau Rezki <urezki@gmail.com>]

On Wed, Apr 01, 2020 at 02:55:03PM +0200, Michal Hocko wrote:
> On Wed 01-04-20 14:32:30, Uladzislau Rezki wrote:
> > On Wed, Apr 01, 2020 at 09:09:58AM +0200, Michal Hocko wrote:
> > > On Tue 31-03-20 18:12:15, Uladzislau Rezki wrote:
> > > > > 
> > > > > __GFP_ATOMIC | __GFP_HIGH is the way to get an additional access to
> > > > > memory reserves regarless of the sleeping status.
> > > > > 
> > > > Michal, just one question here regarding proposed flags. Can we also
> > > > tight it with __GFP_RETRY_MAYFAIL flag? Means it also can repeat a few
> > > > times in order to increase the chance of being success.
> > > 
> > > yes, __GFP_RETRY_MAYFAIL is perfectly valid with __GFP_ATOMIC. Please
> > > note that __GFP_ATOMIC, despite its name, doesn't imply an atomic
> > > allocation which cannot sleep. Quite confusing, I know. A much better
> > > name would be __GFP_RESERVES or something like that.
> > > 
> > OK. Then we can use GFP_ATOMIC | __GFP_RETRY_MAYFAIL to try in more harder
> > way.
> 
> Please note the difference between __GFP_ATOMIC and GFP_ATOMIC. The
> later is a highlevel flag to use for atomic contexts. The former is an
> explicit way to give an access to memory reserves. I am not familiar
> with your code but if you have an existing gfp context coming from the
> caller then just do (gfp | __GFP_ATOMIC | __GFP_HIGH | __GFP_RETRY_MAYFAIL).
> If you do not have any gfp then decide based on whether the current
> context is allowed to sleep
> 	gfp = GFP_KERNEL | __GFP_ATOMIC | __GFP_HIGH | __GFP_RETRY_MAYFAIL;
> 	if (!sleepable)
> 		gfp &= ~__GFP_DIRECT_RECLAIM;

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

Am i missing something?

Thank you, Michal!

--
Vlad Rezki