linux-kernel - Re: [PATCH] mm/memory_hotplug: drain per-cpu pages again during memory offline

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Date:   Thu, 3 Sep 2020 08:38:06 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Pavel Tatashin <pasha.tatashin@...een.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH] mm/memory_hotplug: drain per-cpu pages again during
 memory offline

On Wed 02-09-20 19:51:45, Vlastimil Babka wrote:
> On 9/2/20 5:13 PM, Michal Hocko wrote:
> > On Wed 02-09-20 16:55:05, Vlastimil Babka wrote:
> >> On 9/2/20 4:26 PM, Pavel Tatashin wrote:
> >> > On Wed, Sep 2, 2020 at 10:08 AM Michal Hocko <mhocko@...e.com> 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.
> > 
> > So the page block is marked MIGRATE_ISOLATE but the allocation itself
> > could be used for non migrateable objects. Or does anything prevent that
> > from happening?
> 
> In a movable zone, the allocation should not be used for non migrateable
> objects. E.g. if the zone was not ZONE_MOVABLE, the offlining could fail
> regardless of this race (analogically for migrating away from CMA pageblocks).
> 
> > We really do depend on isolation to not allow reuse when offlining.
> 
> This is not really different than if the page on pcplist was allocated just a
> moment before the offlining, thus isolation started. We ultimately rely on being
> able to migrate any allocated pages away during the isolation. This "freeing to
> pcplists" race doesn't fundamentally change anything in this regard. We just
> have to guarantee that pages on pcplists will be eventually flushed, to make
> forward progress, and there was a bug in this aspect.

You are right. I managed to confuse myself yesterday. The race is
impossible for !ZONE_MOVABLE because we do PageBuddy check there. And on
the movable zone we are not losing the migrateability property.

Pavel I think this will be a useful information to add to the changelog.
We should also document this in the code to prevent from further
confusion. I would suggest something like the following:

diff --git a/mm/page_isolation.c b/mm/page_isolation.c
index 242c03121d73..56d4892bceb8 100644
--- a/mm/page_isolation.c
+++ b/mm/page_isolation.c
@@ -170,6 +170,14 @@ __first_valid_page(unsigned long pfn, unsigned long nr_pages)
  * pageblocks we may have modified and return -EBUSY to caller. This
  * prevents two threads from simultaneously working on overlapping ranges.
  *
+ * Please note that there is no strong synchronization with the page allocator
+ * either. Pages might be freed while their page blocks are marked ISOLATED.
+ * In some cases pages might still end up on pcp lists and that would allow
+ * for their allocation even when they are in fact isolated already. Depending on
+ * how strong of a guarantee the caller needs drain_all_pages might be needed
+ * (e.g. __offline_pages will need to call it after check for isolated range for
+ * a next retry).
+ *
  * Return: the number of isolated pageblocks on success and -EBUSY if any part
  * of range cannot be isolated.
  */
-- 
Michal Hocko
SUSE Labs