On Tue 26-10-21 16:25:07, Uladzislau Rezki wrote: > On Tue, Oct 26, 2021 at 12:24 PM NeilBrown <neilb@xxxxxxx> wrote: > > > > On Tue, 26 Oct 2021, Michal Hocko wrote: > > > On Tue 26-10-21 10:50:21, Neil Brown wrote: > > > > On Mon, 25 Oct 2021, Uladzislau Rezki wrote: > > > > > On Fri, Oct 22, 2021 at 09:49:08AM +1100, NeilBrown wrote: > > > > > > However I'm not 100% certain, and the behaviour might change in the > > > > > > future. So having one place (the definition of memalloc_retry_wait()) > > > > > > where we can change the sleeping behaviour if the alloc_page behavour > > > > > > changes, would be ideal. Maybe memalloc_retry_wait() could take a > > > > > > gfpflags arg. > > > > > > > > > > > At sleeping is required for __get_vm_area_node() because in case of lack > > > > > of vmap space it will end up in tight loop without sleeping what is > > > > > really bad. > > > > > > > > > So vmalloc() has two failure modes. alloc_page() failure and > > > > __alloc_vmap_area() failure. The caller cannot tell which... > > > > > > > > Actually, they can. If we pass __GFP_NOFAIL to vmalloc(), and it fails, > > > > then it must have been __alloc_vmap_area() which failed. > > > > What do we do in that case? > > > > Can we add a waitq which gets a wakeup when __purge_vmap_area_lazy() > > > > finishes? > > > > If we use the spinlock from that waitq in place of free_vmap_area_lock, > > > > then the wakeup would be nearly free if no-one was waiting, and worth > > > > while if someone was waiting. > > > > > > Is this really required to be part of the initial support? > > > > No.... I was just thinking out-loud. > > > alloc_vmap_area() has an retry path, basically if it fails the code > will try to "purge" > areas and repeat it one more time. So we do not need to purge outside some where > else. I think that Neil was not concerned about the need for purging something but rather a waiting event the retry loop could hook into. So that the sleep wouldn't have to be a random timeout but something that is actually actionable - like somebody freeing an area. -- Michal Hocko SUSE Labs
