On 9/2/20 4:26 PM, Pavel Tatashin wrote: > On Wed, Sep 2, 2020 at 10:08 AM Michal Hocko <mhocko@xxxxxxxx> wrote: >> >> > >> > Thread#1 - continue >> > free_unref_page_commit >> > migratetype = get_pcppage_migratetype(page); >> > // get old migration type >> > list_add(&page->lru, &pcp->lists[migratetype]); >> > // add new page to already drained pcp list >> > >> > Thread#2 >> > Never drains pcp again, and therefore gets stuck in the loop. >> > >> > The fix is to try to drain per-cpu lists again after >> > check_pages_isolated_cb() fails. >> >> But this means that the page is not isolated and so it could be reused >> for something else. No? > > The page is in a movable zone, has zero references, and the section is > isolated (i.e. set_pageblock_migratetype(page, MIGRATE_ISOLATE);) is > set. The page should be offlinable, but it is lost in a pcp list as > that list is never drained again after the first failure to migrate > all pages in the range. Yeah. To answer Michal's "it could be reused for something else" - yes, somebody could allocate it from the pcplist before we do the extra drain. But then it becomes "visible again" and the loop in __offline_pages() should catch it by scan_movable_pages() - do_migrate_range(). And this time the pageblock is already marked as isolated, so the page (freed by migration) won't end up on the pcplist again.
