Mel, Ingo, I want to raise again a question I raised (in offline mail with Mel) a couple of weeks ago.
I see only a page_mapcount check in migrate_misplaced_transhuge_page, and don't understand how migration can be safe against the possibility of an earlier call to get_user_pages or get_user_pages_fast (intended to pin a part of the THP) without a page_count check. (I'm also still somewhat worried about unidentified attempts to pin the page concurrently; but since I don't have an example to give, and concurrent get_user_pages or get_user_pages_fast wouldn't get past the pmd_numa, let's not worry too much about my unidentified anxiety ;) migrate_page_move_mapping and migrate_huge_page_move_mapping check page_count, but migrate_misplaced_transhuge_page doesn't use those. __collapse_huge_page_isolate and khugepaged_scan_pmd (over in huge_memory.c) take commented care to check page_count lest GUP. I can see that page_count might often be raised by concurrent faults on the same pmd_numa, waiting on the lock_page in do_huge_pmd_numa_page. That's unfortunate, and maybe you can find a clever way to discount those. But safety must come first: don't we need to check page_count? Hugh -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
