On 09.09.20 13:36, Michal Hocko wrote: > On Wed 09-09-20 12:48:54, Vlastimil Babka wrote: >> Here's a version that will apply on top of next-20200908. The first 4 patches need no change. >> >> ----8<---- >> >From 8febc17272b8e8b378e2e5ea5e76b2616f029c5b Mon Sep 17 00:00:00 2001 >> From: Vlastimil Babka <vbabka@xxxxxxx> >> Date: Mon, 7 Sep 2020 17:20:39 +0200 >> Subject: [PATCH] mm, page_alloc: disable pcplists during page isolation >> >> Page isolation can race with process freeing pages to pcplists in a way that >> a page from isolated pageblock can end up on pcplist. This can be fixed by >> repeated draining of pcplists, as done by patch "mm/memory_hotplug: drain >> per-cpu pages again during memory offline" in [1]. >> >> David and Michal would prefer that this race was closed in a way that callers >> of page isolation don't need to care about drain. David suggested disabling >> pcplists usage completely during page isolation, instead of repeatedly draining >> them. >> >> To achieve this without adding special cases in alloc/free fastpath, we can use >> the same 'trick' as boot pagesets - when pcp->high is 0, any pcplist addition >> will be immediately flushed. >> >> The race can thus be closed by setting pcp->high to 0 and draining pcplists >> once in start_isolate_page_range(). The draining will serialize after processes >> that already disabled interrupts and read the old value of pcp->high in >> free_unref_page_commit(), and processes that have not yet disabled interrupts, >> will observe pcp->high == 0 when they are rescheduled, and skip pcplists. >> This guarantees no stray pages on pcplists in zones where isolation happens. >> >> We can use the variable zone->nr_isolate_pageblock (protected by zone->lock) >> to detect transitions from 0 to 1 (to change pcp->high to 0 and issue drain) >> and from 1 to 0 (to restore original pcp->high and batch values cached in >> struct zone). We have to avoid external updates to high and batch by taking >> pcp_batch_high_lock. To allow multiple isolations in parallel, change this >> lock from mutex to rwsem. >> >> For callers that pair start_isolate_page_range() with >> undo_isolated_page_range() properly, this is transparent. Currently that's >> alloc_contig_range(). __offline_pages() doesn't call undo_isolated_page_range() >> in the succes case, so it has to be carful to handle restoring pcp->high and batch >> and unlocking pcp_batch_high_lock. > > I was hoping that it would be possible to have this completely hidden > inside start_isolate_page_range code path. If we need some sort of > disable_pcp_free/enable_pcp_free then it seems like a better fit to have > an explicit API for that (the naming would be obviously different > because we do not want to call out pcp free lists). I strongly suspect > that only the memory hotplug really cares for this hard guanrantee. > alloc_contig_range simply goes with EBUSY. There will be different alloc_contig_range() demands in the future: try fast (e.g., loads of small CMA allocations) vs. try hard (e.g., virtio-mem). We can add ways to specify that. -- Thanks, David / dhildenb
