Re: [PATCH] mm/hmm: replace hmm_update with mmu_notifier_range

[Michal Hocko <mhocko@kernel.org>]

On Wed 24-07-19 12:28:58, Jason Gunthorpe wrote:
> On Wed, Jul 24, 2019 at 09:05:53AM +0200, Christoph Hellwig wrote:
> > Looks good:
> > 
> > Reviewed-by: Christoph Hellwig <hch@lst.de>
> > 
> > One comment on a related cleanup:
> > 
> > >  	list_for_each_entry(mirror, &hmm->mirrors, list) {
> > >  		int rc;
> > >  
> > > -		rc = mirror->ops->sync_cpu_device_pagetables(mirror, &update);
> > > +		rc = mirror->ops->sync_cpu_device_pagetables(mirror, nrange);
> > >  		if (rc) {
> > > -			if (WARN_ON(update.blockable || rc != -EAGAIN))
> > > +			if (WARN_ON(mmu_notifier_range_blockable(nrange) ||
> > > +			    rc != -EAGAIN))
> > >  				continue;
> > >  			ret = -EAGAIN;
> > >  			break;
> > 
> > This magic handling of error seems odd.  I think we should merge rc and
> > ret into one variable and just break out if any error happens instead
> > or claiming in the comments -EAGAIN is the only valid error and then
> > ignoring all others here.
> 
> The WARN_ON is enforcing the rules already commented near
> mmuu_notifier_ops.invalidate_start - we could break or continue, it
> doesn't much matter how to recover from a broken driver, but since we
> did the WARN_ON this should sanitize the ret to EAGAIN or 0
> 
> Humm. Actually having looked this some more, I wonder if this is a
> problem:
> 
> I see in __oom_reap_task_mm():
> 
> 			if (mmu_notifier_invalidate_range_start_nonblock(&range)) {
> 				tlb_finish_mmu(&tlb, range.start, range.end);
> 				ret = false;
> 				continue;
> 			}
> 			unmap_page_range(&tlb, vma, range.start, range.end, NULL);
> 			mmu_notifier_invalidate_range_end(&range);
> 
> Which looks like it creates an unbalanced start/end pairing if any
> start returns EAGAIN?
> 
> This does not seem OK.. Many users require start/end to be paired to
> keep track of their internal locking. Ie for instance hmm breaks
> because the hmm->notifiers counter becomes unable to get to 0.
> 
> Below is the best idea I've had so far..
> 
> Michal, what do you think?

IIRC we have discussed this with Jerome back then when I've introduced
this code and unless I misremember he said the current code was OK.
Maybe new users have started relying on a new semantic in the meantime,
back then, none of the notifier has even started any action in blocking
mode on a EAGAIN bailout. Most of them simply did trylock early in the
process and bailed out so there was nothing to do for the range_end
callback.

Has this changed?
-- 
Michal Hocko
SUSE Labs