Re: [PATCH 2/5] kernel.h: Add non_block_start/end()

[Michal Hocko]

On Wed 14-08-19 13:45:58, Andrew Morton wrote:
> On Wed, 14 Aug 2019 22:20:24 +0200 Daniel Vetter <daniel.vetter@xxxxxxxx> 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.
 
The primary point here is that there is a demand of non blockable mmu
notifiers to be called when the oom reaper tears down the address space.
As the oom reaper is the primary guarantee of the oom handling forward
progress it cannot be blocked on anything that might depend on blockable
memory allocations. These are not really easy to track because they
might be indirect - e.g. notifier blocks on a lock which other context
holds while allocating memory or waiting for a flusher that needs memory
to perform its work. If such a blocking state happens that we can end up
in a silent hang with an unusable machine.
Now we hope for reasonable implementations of mmu notifiers (strong
words I know ;) and this should be relatively simple and effective catch
all tool to detect something suspicious is going on.

Does that make the situation more clear?

-- 
Michal Hocko
SUSE Labs