On Thu 13-04-23 13:11:39, Pavel Tatashin wrote: > On Thu, Apr 13, 2023 at 11:25 AM Michal Hocko <mhocko@xxxxxxxx> wrote: > > > > On Thu 13-04-23 11:05:20, Pavel Tatashin wrote: [...] > > > This is a theoretical concern. Freeing a 1G page requires 16M of free > > > memory. A machine might need to be reconfigured from one task to > > > another, and release a large number of 1G pages back to the system if > > > allocating 16M fails, the release won't work. > > > > This is really an important "detail" changelog should mention. While I > > am not really against that change I would much rather see that as a > > result of a real world fix rather than a theoretical concern. Mostly > > because a real life scenario would allow us to test the > > __GFP_RETRY_MAYFAIL effectivness. As that request might fail as well we > > just end up with a theoretical fix for a theoretical problem. Something > > that is easy to introduce but much harder to get rid of should we ever > > need to change __GFP_RETRY_MAYFAIL implementation for example. > > I will add this to changelog in v3. If __GFP_RETRY_MAYFAIL is > ineffective we will receive feedback once someone hits this problem. I do not remember anybody hitting this with the current __GFP_NORETRY. So arguably there is nothing to be fixed ATM. > Otherwise, we will never hear about it. I think overall it is safer to > keep this code with __GFP_RETRY_MAYFAIL flag. > > > > > > In an ideal scenario we should guarantee that this never fails: that > > > we always can free HugeTLB pages back to the system. At the very least > > > we could steal the memory for vmemmap from the page that is being > > > released. > > > > Yes, this really bothered me when the concept was introduced initially. > > I am always concerned when you need to allocate in order to free memory. > > Practically speaking we haven't heard about bug reports so maybe this is > > not such a big deal as I thought. > > I suspect this is because at the moment it is not that frequent when a > machine is reconfigured from having a lot of HugeTLB based workload to > non-HugeTLB workload. Yes, hugetlb workloads tend to be pretty static from my experience. -- Michal Hocko SUSE Labs
