On 15 Apr 2021, at 2:45, Huang, Ying wrote: > "Zi Yan" <z...@nvidia.com> writes: > >> On 13 Apr 2021, at 23:00, Huang, Ying wrote: >> >>> Yang Shi <shy828...@gmail.com> writes: >>> >>>> The generic migration path will check refcount, so no need check refcount >>>> here. >>>> But the old code actually prevents from migrating shared THP (mapped by >>>> multiple >>>> processes), so bail out early if mapcount is > 1 to keep the behavior. >>> >>> What prevents us from migrating shared THP? If no, why not just remove >>> the old refcount checking? >> >> If two or more processes are in different NUMA nodes, a THP shared by them >> can be >> migrated back and forth between NUMA nodes, which is quite costly. Unless we >> have >> a better way of figuring out a good location for such pages to reduce the >> number >> of migration, it might be better not to move them, right? >> > > Some mechanism has been provided in should_numa_migrate_memory() to > identify the shared pages from the private pages. Do you find it > doesn't work well in some situations? > > The multiple threads in one process which run on different NUMA nodes > may share pages too. So it isn't a good solution to exclude pages > shared by multiple processes.
After recheck the patch, it seems that no shared THP migration here is a side effect of the original page_count check, which might not be intended and be worth fixing. But Yang just want to solve one problem, simplifying THP NUMA migration, at a time. Maybe a separate patch would be better for both discussing and fixing this problem. — Best Regards, Yan Zi
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