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
