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

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Date:   Tue, 14 Jan 2020 15:53:39 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Qian Cai <cai@....pw>
Cc:     Michal Hocko <mhocko@...nel.org>,
        David Hildenbrand <david@...hat.com>,
        sergey.senozhatsky.work@...il.com, pmladek@...e.com,
        rostedt@...dmis.org, peterz@...radead.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH -next] mm/hotplug: silence a lockdep splat with printk()

> On Jan 14, 2020, at 4:02 PM, Michal Hocko <mhocko@...nel.org> 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.