On Fri, Dec 18, 2020 at 8:14 AM Michal Hocko <mhocko@xxxxxxxx> wrote: > > On Fri 18-12-20 07:43:15, Pavel Tatashin wrote: > > On Fri, Dec 18, 2020 at 5:46 AM Michal Hocko <mhocko@xxxxxxxx> wrote: > > > > > > On Thu 17-12-20 13:52:41, Pavel Tatashin wrote: > > > [...] > > > > +#define PINNABLE_MIGRATE_MAX 10 > > > > +#define PINNABLE_ISOLATE_MAX 100 > > > > > > Why would we need to limit the isolation retries. Those should always be > > > temporary failure unless I am missing something. > > > > Actually, during development, I was retrying isolate errors > > infinitely, but during testing found a hung where when FOLL_TOUCH > > without FOLL_WRITE is passed (fault in kernel without write flag), the > > zero page is faulted. The isolation of the zero page was failing every > > time, therefore the process was hanging. > > Why would you migrate zero page in the first place? Simply instantiate > it. This is exactly the idea behind FOLL_WRITE; it causes zero pages to be created in the right zone right away, and no migration is necessary. > > > Since then, I fixed this problem by adding FOLL_WRITE unconditionally > > to FOLL_LONGTERM, but I was worried about other possible bugs that > > would cause hangs, so decided to limit isolation errors. If you think > > it its not necessary, I can unlimit isolate retires. > > It should have a really good reason to exist. Worries about some corner > cases is definitely not a reason to put some awkward retry mechanism. > My historical experience is that these things are extremely hard to get > rid of later. > > > > I am not sure about the > > > PINNABLE_MIGRATE_MAX either. Why do we want to limit that? migrate_pages > > > already implements its retry logic why do you want to count retries on > > > top of that? I do agree that the existing logic is suboptimal because > > > > True, but again, just recently, I worked on a race bug where pages can > > end up in per-cpu list after lru_add_drain_all() but before isolation, > > so I think retry is necessary. > > There are ways to make sure pages are not ending on pcp list. Have a > look at how hotplug does that. Sounds good to me, I will remove PINNABLE_MIGRATE_MAX, and leave only PINNABLE_ISOLATE_MAX for transient isolation errors. > > > > the migration failure might be ephemeral or permanent but that should be > > > IMHO addressed at migrate_pages (resp. unmap_and_move) and simply report > > > failures that are permanent - e.g. any potential pre-existing long term > > > pin - if that is possible at all. If not what would cause permanent > > > migration failure? OOM? > > > > Yes, OOM is the main cause for migration failures. > > Then you can treat ENOMEM as a permanent failure. Sounds good.
