On 03.12.20 12:47, Michal Hocko wrote: > On Thu 03-12-20 10:47:02, David Hildenbrand wrote: >> On 03.12.20 09:28, Michal Hocko wrote: > [...] >>> I think we should aim at easy and very highlevel behavior: >>> - GFP_NOWAIT - unsupported currently IIRC but something that something >>> that should be possible to implement. Isolation is non blocking, >>> migration could be skipped >>> - GFP_KERNEL - default behavior whatever that means >>> - GFP_NORETRY - opportunistic allocation as lightweight as we can get. >>> Failures to be expected also for transient reasons. >>> - GFP_RETRY_MAYFAIL - try hard but not as hard as to trigger disruption >>> (e.g. via oom killer). >> >> I think we currently see demand for 3 modes for alloc_contig_range() >> >> a) normal >> >> As is. Try, but don't try too hard. E.g., drain LRU, drain PCP, retry a >> couple of times. Failures in some cases (short-term pinning, PCP races) >> are still possible and acceptable. >> >> GFP_RETRY_MAYFAIL ? > > normal shouldn't really require anybody to think about gfp flags hard. > That to most people really means GFP_KERNEL. > >> E.g., "Allocations with this flag may fail, but only when there is >> genuinely little unused memory." - current description does not match at >> all. When allocating ranges things behave completely different. >> >> >> b) fast >> >> Try, but fail fast. Leave optimizations that can improve the result to >> the caller. E.g., don't drain LRU, don't drain PCP, don't retry. >> Frequent failures are expected and acceptable. >> >> __GFP_NORETRY ? >> >> E.g., "The VM implementation will try only very lightweight memory >> direct reclaim to get some memory under memory pressure" - again, I >> think current description does not really match. > > Agreed. As mentioned above this would be an opportunistic allocation > mode. > > >> c) hard >> >> Try hard, E.g., temporarily disabling the PCP. Certainly not >> __GFP_NOFAIL, that would be highly dangerous. So no flags / GFP_KERNEL? > > NOFAIL semantic is out of question. Should we have a mode to try harder > than the default? I dunno. Do we have users? I think RETRY_MAYFAIL is a > middle ground between the default and NORETRY which is just too easy to > fail. This is the case for the allocator as well. And from what I have > seen people are already using MAYFAIL in order to prevent oom killer so > this is a generally recognized pattern. virtio-mem might be one user. It might first try in normal mode to get as much memory out as possible, but switch to hard mode when it might make sense. > >>> - __GFP_THIS_NODE - stick to a node without fallback >>> - we can support zone modifiers although there is no existing user. >>> - __GFP_NOWARN - obvious >>> >>> And that is it. Or maybe I am seeing that oversimplified. >>> >> >> Again, I think most flags make sense for the migration target allocation >> path and mainly deal with OOM situations and reclaim. For the migration >> path - which is specific to the alloc_contig_range() allocater - they >> don't really apply and create more confusion than they actually help - IMHO. > > Migration is really an implementation detail of this interface. You > shouldn't be even thinking that there is a migration underneath not even > mention to actually trying to control it. CMA? I tend to agree. alloc_contig_range? I disagree. -- Thanks, David / dhildenb
