> Am 18.12.2020 um 13:43 schrieb Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>: > > 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. > > 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. > >> 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. > >> 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. 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), > hardware page poisoning is another rare example. > How is concurrent migration handled? Like memory offlining, compaction, alloc_contig_range() while trying to pin? >> -- >> Michal Hocko >> SUSE Labs >
