On 09.09.20 13:55, Vlastimil Babka wrote: > On 9/9/20 1:36 PM, 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. > > I hoped so too, but we can't know the moment when all processes that were in the > critical part of freeing pages to pcplists have moved on (they might have been > rescheduled). > We could change free_unref_page() to disable IRQs sooner, before > free_unref_page_prepare(), or at least the get_pfnblock_migratetype() part. Then > after the single drain, we should be safe, AFAICS? At least moving it before getting the migratetype should not be that severe? > RT guys might not be happy though, but it's much simpler than this patch. I > still like some of the cleanups in 1-4 though tbh :) It would certainly make this patch much simpler. Do you have a prototype lying around? -- Thanks, David / dhildenb
