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Date:   Wed, 14 Aug 2019 13:45:58 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Daniel Vetter <daniel.vetter@ffwll.ch>
Cc:     LKML <linux-kernel@vger.kernel.org>, linux-mm@kvack.org,
        DRI Development <dri-devel@lists.freedesktop.org>,
        Intel Graphics Development <intel-gfx@lists.freedesktop.org>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>, Michal Hocko <mhocko@suse.com>,
        David Rientjes <rientjes@google.com>,
        Christian =?ISO-8859-1?Q?K=F6nig?= <christian.koenig@amd.com>,
        =?ISO-8859-1?Q?J=E9r?= =?ISO-8859-1?Q?=F4me?= Glisse 
        <jglisse@redhat.com>,
        Masahiro Yamada <yamada.masahiro@socionext.com>,
        Wei Wang <wvw@google.com>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Jann Horn <jannh@google.com>, Feng Tang <feng.tang@intel.com>,
        Kees Cook <keescook@chromium.org>,
        Randy Dunlap <rdunlap@infradead.org>,
        Daniel Vetter <daniel.vetter@intel.com>
Subject: Re: [PATCH 2/5] kernel.h: Add non_block_start/end()
Message-Id: <20190814134558.fe659b1a9a169c0150c3e57c@linux-foundation.org>
In-Reply-To: <20190814202027.18735-3-daniel.vetter@ffwll.ch>
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        <20190814202027.18735-3-daniel.vetter@ffwll.ch>
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On Wed, 14 Aug 2019 22:20:24 +0200 Daniel Vetter <daniel.vetter@ffwll.ch> wrote:

> In some special cases we must not block, but there's not a
> spinlock, preempt-off, irqs-off or similar critical section already
> that arms the might_sleep() debug checks. Add a non_block_start/end()
> pair to annotate these.
> 
> This will be used in the oom paths of mmu-notifiers, where blocking is
> not allowed to make sure there's forward progress. Quoting Michal:
> 
> "The notifier is called from quite a restricted context - oom_reaper -
> which shouldn't depend on any locks or sleepable conditionals. The code
> should be swift as well but we mostly do care about it to make a forward
> progress. Checking for sleepable context is the best thing we could come
> up with that would describe these demands at least partially."
> 
> Peter also asked whether we want to catch spinlocks on top, but Michal
> said those are less of a problem because spinlocks can't have an
> indirect dependency upon the page allocator and hence close the loop
> with the oom reaper.

I continue to struggle with this.  It introduces a new kernel state
"running preemptibly but must not call schedule()".  How does this make
any sense?

Perhaps a much, much more detailed description of the oom_reaper
situation would help out.