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. > This commit also changes drain_all_pages() to not trust reading pcp->count during > drain for page isolation - I believe that could be racy and lead to missing some > cpu's to drain. If others agree, this can be separated and potentially backported. > > [1] https://lore.kernel.org/linux-mm/20200903140032.380431-1-pasha.tatashin@xxxxxxxxxx/ > > Suggested-by: David Hildenbrand <david@xxxxxxxxxx> > Suggested-by: Michal Hocko <mhocko@xxxxxxxx> > Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx> > --- > include/linux/gfp.h | 1 + > mm/internal.h | 4 +++ > mm/memory_hotplug.c | 55 ++++++++++++++++++++++++++++------------- > mm/page_alloc.c | 60 +++++++++++++++++++++++++++++---------------- > mm/page_isolation.c | 45 ++++++++++++++++++++++++++++------ > 5 files changed, 119 insertions(+), 46 deletions(-) This has turned out much larger than I would expect. -- Michal Hocko SUSE Labs
