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@xxxxxx>
> >
> > 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
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