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SubjectRe: [PATCH -next] mm/hotplug: silence a lockdep splat with printk()
> On Jan 14, 2020, at 4:02 PM, Michal Hocko <> wrote:
>> Yeah, that was a long discussion with a lot of lockdep false positives.
>> I believe I have made it clear that the console code shouldn't depend on
>> memory allocation because that is just too fragile. If that is not
>> possible for some reason then it has to be mentioned in the changelog.
>> I really do not want us to add kludges to the MM code just because of
>> printk deficiencies unless that is absolutely inevitable.
> I don't know how to convince you, but both random number generator and
> printk() maintainers agreed to get ride of printk() with zone->lock
> held as you can see in the approved commit mentioned in this patch
> description because it is a whac-a-mole to fix other places. In other
> word, the patch alone fixes quite a few false positives and potential
> real deadlocks. Maybe Andrew please has a look at this directly?

Well, a few things.

The changelog is quite poor. It doesn't describe the problem (console
drivers allocating memory) not does it describe the solution
(deferring the dump_page() until after release of zone->lock).

So I changed it to this:

: Some console drivers can perform memory allocation at inappropriate times,
: which can result in lockdep warnings (and presumably deadlocks) if printk
: is called with zone->lock held.
: By far the best fix is to reeducate those console drivers to not perform
: these allocations, but this is proving difficult.
: Another but poorer approach is to call printk_deferred() when holding
: zone->lock, but memory offline will call dump_page() which needs to defer
: after the lock.
: So change has_unmovable_pages() so that it no longer calls dump_page()
: itself - instead it passes the page's descripton (as a string) back to the
: caller so that in the case of a has_unmovable_pages() failure, the caller
: can call dump_page() after releasing zone->lock.
: While at it, remove a similar but unnecessary debug printk() as well.

But I see a couple of other issues.

> @@ -8290,8 +8290,10 @@ bool has_unmovable_pages(struct zone *zo
> return false;
> unmovable:
> WARN_ON_ONCE(zone_idx(zone) == ZONE_MOVABLE);
> - if (flags & REPORT_FAILURE)
> - dump_page(pfn_to_page(pfn + iter), reason);
> + if (flags & REPORT_FAILURE) {
> + page = pfn_to_page(pfn + iter);

This statement appears to be unnecessary.

> + strscpy(dump, reason, 64);
> + }

Also, that whole `reason' thing in has_unmovable_pages() is just there
to tell us whether it was an "unmovable page" or a "CMA page". This
doesn't seem terribly useful to me. Also, I expect that the
dump_page() output will permit the user to determine that it was a CMA
page anyway. If not, we can change dump_page() to add that info.

So how about we remove that whole `reason' thing and possibly enhance
dump_page()? The patch then becomes much simpler.

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