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Date:   Tue, 11 Aug 2020 12:26:49 +0200
From:   Michal Hocko <mhocko@suse.com>
To:     Uladzislau Rezki <urezki@gmail.com>
Cc:     LKML <linux-kernel@vger.kernel.org>, RCU <rcu@vger.kernel.org>,
        linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        "Paul E . McKenney" <paulmck@kernel.org>,
        Matthew Wilcox <willy@infradead.org>,
        "Theodore Y . Ts'o" <tytso@mit.edu>,
        Joel Fernandes <joel@joelfernandes.org>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Oleksiy Avramchenko <oleksiy.avramchenko@sonymobile.com>
Subject: Re: [RFC-PATCH 1/2] mm: Add __GFP_NO_LOCKS flag
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On Tue 11-08-20 11:37:13, Uladzislau Rezki wrote:
> On Tue, Aug 11, 2020 at 10:19:17AM +0200, Michal Hocko wrote:
> > On Mon 10-08-20 21:25:26, Michal Hocko wrote:
> > > On Mon 10-08-20 18:07:39, Uladzislau Rezki wrote:
> > [...]
> > > > The problem that i see is we can not use the page allocator from atomic
> > > > contexts, what is our case:
> > > > 
> > > > <snip>
> > > >     local_irq_save(flags) or preempt_disable() or raw_spinlock();
> > > >     __get_free_page(GFP_ATOMIC);
> > > > <snip>
> > > > 
> > > > So if we can convert the page allocator to raw_* lock it will be appreciated,
> > > > at least from our side, IMHO, not from RT one. But as i stated above we need
> > > > to sort raised questions out if converting is done.
> > > > 
> > > > What is your view?
> > > 
> > > To me it would make more sense to support atomic allocations also for
> > > the RT tree. Having both GFP_NOWAIT and GFP_ATOMIC which do not really
> > > work for atomic context in RT sounds subtle and wrong.
> > 
> > I was thinking about this some more. I still think the above would be a
> > reasonable goal we should try to achieve. If for not other then for
> > future maintainability (especially after the RT patchset is merged).
> > I have tried to search for any known problems/attempts to make
> > zone->lock raw but couldn't find anything. Maybe somebody more involved
> > in RT world have something to say about that.
> > 
> I tried yesterday to convert zone->lock. See below files i had to modify:
> <snip>
>         modified:   include/linux/mmzone.h
>         modified:   mm/compaction.c
>         modified:   mm/memory_hotplug.c
>         modified:   mm/page_alloc.c
>         modified:   mm/page_isolation.c
>         modified:   mm/page_reporting.c
>         modified:   mm/shuffle.c
>         modified:   mm/vmscan.c
>         modified:   mm/vmstat.c
> <snip>
> 
> There is one more lock, that is zone->lru_lock one. Both zone->lock and this
> one intersect between each other. If the lru_lock can be nested under zone->lock
> it should be converted as well. But i need to analyze it farther. There are
> two wrapper functions which are used as common interface to lock/unlock both
> locks. See compact_lock_irqsave()/compact_unlock_should_abort_lru() in the 
> mm/compaction.c.
> 
> Any thoughts here?

I am not an expert on compaction. Vlastimil would know better. My
thinking was that zone->lock is a tail lock but compaction/page
isolation might be doing something I am not aware of right now.

> Anyway i tried to convert only zone->lock and use page allocator passing there
> gfp_mask=0 as argument. So it works. CONFIG_PROVE_RAW_LOCK_NESTING does not
> complain about any "bad" lock nesting.
> 
> > Anyway, if the zone->lock is not a good fit for raw_spin_lock then the
> > only way I can see forward is to detect real (RT) atomic contexts and
> > bail out early before taking the lock in the allocator for NOWAIT/ATOMIC
> > requests.
> >
> For RT kernel we can detect it for sure. preemtable() works just fine there,
> i.e. we can identify the context we are currently in.

In previous email I didn't mention why I prefer full NOWAIT semantic
over rt specific bailouts. There are users making NOWAIT allocation
attempts as an opportunistic allocation request which is OK to fail
as they have a fallback to go through. This would imply they would
prefer to know this ASAP rather then get blocked and sleep. A lack of
reports for PREEMPT_RT would suggest that nobody has noticed as this
though.
-- 
Michal Hocko
SUSE Labs