On Wed 01-04-20 15:08:16, Uladzislau Rezki wrote: > 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 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. -- Michal Hocko SUSE Labs
