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. > 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. > > 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. > And also a few > cases described in movable zone comment, where it is possible during > boot some pages can be allocated by memblock in movable zone due to > lack of memory resources (even if those resources were added later), Do you have any examples? I find it hard to follow that somebody would be pinning early boot allocations. > hardware page poisoning is another rare example. Could you elaborate please? -- Michal Hocko SUSE Labs
