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

[Andrew Morton]

On Wed, 14 Aug 2019 22:20:24 +0200 Daniel Vetter <daniel.vetter at 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.