Re: [PATCH 3/4] kernel.h: Add non_block_start/end()

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Aug 20, 2019 at 10:24:40PM +0200, Daniel Vetter wrote:
> On Tue, Aug 20, 2019 at 10:19:01AM +0200, Daniel Vetter 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.
> > 
> > Suggested by Michal Hocko.
> > 
> > v2:
> > - Improve commit message (Michal)
> > - Also check in schedule, not just might_sleep (Peter)
> > 
> > v3: It works better when I actually squash in the fixup I had lying
> > around :-/
> > 
> > v4: Pick the suggestion from Andrew Morton to give non_block_start/end
> > some good kerneldoc comments. I added that other blocking calls like
> > wait_event pose similar issues, since that's the other example we
> > discussed.
> > 
> > Cc: Jason Gunthorpe <jgg@ziepe.ca>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Ingo Molnar <mingo@redhat.com>
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: Michal Hocko <mhocko@suse.com>
> > Cc: David Rientjes <rientjes@google.com>
> > Cc: "Christian König" <christian.koenig@amd.com>
> > Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
> > Cc: "Jérôme Glisse" <jglisse@redhat.com>
> > Cc: linux-mm@kvack.org
> > Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
> > Cc: Wei Wang <wvw@google.com>
> > Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: Jann Horn <jannh@google.com>
> > Cc: Feng Tang <feng.tang@intel.com>
> > Cc: Kees Cook <keescook@chromium.org>
> > Cc: Randy Dunlap <rdunlap@infradead.org>
> > Cc: linux-kernel@vger.kernel.org
> > Acked-by: Christian König <christian.koenig@amd.com> (v1)
> > Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> 
> Hi Peter,
> 
> Iirc you've been involved at least somewhat in discussing this. -mm folks
> are a bit undecided whether these new non_block semantics are a good idea.
> Michal Hocko still is in support, but Andrew Morton and Jason Gunthorpe
> are less enthusiastic. Jason said he's ok with merging the hmm side of
> this if scheduler folks ack. If not, then I'll respin with the
> preempt_disable/enable instead like in v1.
> 
> So ack/nack for this from the scheduler side?

Right, I had memories of seeing this before, and I just found a fairly
long discussion on this elsewhere in the vacation inbox (*groan*).

Yeah, this is something I can live with,

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>