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

[Jason Gunthorpe]

On Thu, Aug 15, 2019 at 10:49:31PM +0200, Daniel Vetter wrote:
> On Thu, Aug 15, 2019 at 10:27 PM Jason Gunthorpe <jgg@xxxxxxxx> wrote:
> > On Thu, Aug 15, 2019 at 10:16:43PM +0200, Daniel Vetter wrote:
> > > So if someone can explain to me how that works with lockdep I can of
> > > course implement it. But afaics that doesn't exist (I tried to explain
> > > that somewhere else already), and I'm no really looking forward to
> > > hacking also on lockdep for this little series.
> >
> > Hmm, kind of looks like it is done by calling preempt_disable()
> 
> Yup. That was v1, then came the suggestion that disabling preemption
> is maybe not the best thing (the oom reaper could still run for a long
> time comparatively, if it's cleaning out gigabytes of process memory
> or what not, hence this dedicated debug infrastructure).

Oh, I'm coming in late, sorry

Anyhow, I was thinking since we agreed this can trigger on some
CONFIG_DEBUG flag, something like

    /* This is a sleepable region, but use preempt_disable to get debugging
     * for calls that are not allowed to block for OOM [.. insert
     * Michal's explanation.. ] */
    if (IS_ENABLED(CONFIG_DEBUG_ATOMIC_SLEEP) && !mmu_notifier_range_blockable(range))
	preempt_disable();
    ops->invalidate_range_start();

And I have also been idly mulling doing something like

   if (IS_ENABLED(CONFIG_DEBUG_NOTIFIERS) && 
       rand &&
       mmu_notifier_range_blockable(range)) {
     range->flags = 0
     if (!ops->invalidate_range_start(range))
	continue

     // Failed, try again as blockable
     range->flags = MMU_NOTIFIER_RANGE_BLOCKABLE
   }
   ops->invalidate_range_start(range);

Which would give coverage for this corner case without forcing OOM.

Jason