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

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Message-ID: <20200117152631.GJ19428@dhcp22.suse.cz>
Date:   Fri, 17 Jan 2020 16:26:31 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     Qian Cai <cai@....pw>, akpm@...ux-foundation.org,
        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 v4] mm/hotplug: silence a lockdep splat with
 printk()

On Fri 17-01-20 15:43:58, David Hildenbrand wrote:
> On 17.01.20 15:42, Michal Hocko wrote:
> > On Fri 17-01-20 07:40:15, Qian Cai wrote:
> >>
> >>
> >>> On Jan 17, 2020, at 3:51 AM, David Hildenbrand <david@...hat.com> wrote:
> >>>
> >>> -> you are accessing the pageblock without the zone lock. It could
> >>> change to "isolate" again in the meantime if I am not wrong!
> >>
> >> Since we are just dumping the state for debugging, it should be fine
> >> to accept a bit inaccuracy here due to racing. I could put a bit
> >> comments over there.
> > 
> > Sorry, I could have been more specific. The race I was talking about is
> > not about accuracy. The current code is racy in that sense already
> > because you are looking at a struct page you do not own so its state can
> > change at any time. Please note that the zone->lock doesn't really
> 
> The pageblock state cannot change with the zone->lock. That's what I was
> referring to here. (this specific check)

CMA pages tend to have a stable pageblock state. More importantly this
is not the most important information from my experience. It is usually
reference count and page flags that are of interest. Or details about
what kind of filesystem is owning the page and it doesn't want to
release it.
-- 
Michal Hocko
SUSE Labs