Re: [PATCH -next v4] mm/hotplug: silence a lockdep splat with printk()

[Qian Cai <cai@lca.pw>]

> On Jan 17, 2020, at 10:46 AM, Michal Hocko <mhocko@kernel.org> wrote:
> 
> On Fri 17-01-20 10:05:12, Qian Cai wrote:
>> 
>> 
>>> On Jan 17, 2020, at 9:39 AM, Michal Hocko <mhocko@kernel.org> wrote:
>>> 
>>> Thanks a lot. Having it in a separate patch would be great.
>> 
>> I was thinking about removing that WARN together in this v5 patch,
>> so there is less churn to touch the same function again. However, I
>> am fine either way, so just shout out if you feel strongly towards a
>> separate patch.
> 
> I hope you meant moving rather than removing ;). The warning is useful
> because we shouldn't see unmovable pages in the movable zone. And a
> separate patch makes more sense because the justification is slightly
> different. We do not want to have a way for userspace to trigger the
> warning from userspace - even though it shouldn't be possible, but
> still. Only the offlining path should complain.

Something like this?

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 621716a25639..32c854851e1f 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -8307,7 +8307,6 @@ struct page *has_unmovable_pages(struct zone *zone, struct page *page,
        }
        return NULL;
 unmovable:
-       WARN_ON_ONCE(zone_idx(zone) == ZONE_MOVABLE);
        return pfn_to_page(pfn + iter);
 }
 
diff --git a/mm/page_isolation.c b/mm/page_isolation.c
index e70586523ca3..08571b515d9f 100644
--- a/mm/page_isolation.c
+++ b/mm/page_isolation.c
@@ -54,9 +54,11 @@ static int set_migratetype_isolate(struct page *page, int migratetype, int isol_
 
 out:
        spin_unlock_irqrestore(&zone->lock, flags);
+
        if (!ret)
                drain_all_pages(zone);
        else if ((isol_flags & REPORT_FAILURE) && unmovable)
+               WARN_ON_ONCE(zone_idx(zone) == ZONE_MOVABLE);
                /*
                 * printk() with zone->lock held will guarantee to trigger a
                 * lockdep splat, so defer it here.