On Fri, Feb 28, 2020 at 09:50:06AM -0400, Jason Gunthorpe wrote:
> On Tue, Feb 11, 2020 at 04:52:52PM -0400, Jason Gunthorpe wrote:
> > Many users of the mmu_notifier invalidate_range callbacks maintain
> > locking/counters/etc on a paired basis and have long expected that
> > invalidate_range_start/end() are always paired.
> >
> > For instance kvm_mmu_notifier_invalidate_range_end() undoes
> > kvm->mmu_notifier_count which was incremented during start().
> >
> > The recent change to add non-blocking notifiers breaks this assumption
> > when multiple notifiers are present in the list. When EAGAIN is returned
> > from an invalidate_range_start() then no invalidate_range_ends() are
> > called, even if the subscription's start had previously been called.
> >
> > Unfortunately, due to the RCU list traversal we can't reliably generate a
> > subset of the linked list representing the notifiers already called to
> > generate an invalidate_range_end() pairing.
> >
> > One case works correctly, if only one subscription requires
> > invalidate_range_end() and it is the last entry in the hlist. In this
> > case, when invalidate_range_start() returns -EAGAIN there will be nothing
> > to unwind.
> >
> > Keep the notifier hlist sorted so that notifiers that require
> > invalidate_range_end() are always last, and if two are added then disable
> > non-blocking invalidation for the mm.
> >
> > A warning is printed for this case, if in future we determine this never
> > happens then we can simply fail during registration when there are
> > unsupported combinations of notifiers.
> >
> > Fixes: 93065ac753e4 ("mm, oom: distinguish blockable mode for mmu notifiers")
> > Cc: Michal Hocko <mhocko@xxxxxxxx>
> > Cc: "Jérôme Glisse" <jglisse@xxxxxxxxxx>
> > Cc: Christoph Hellwig <hch@xxxxxx>
> > Signed-off-by: Jason Gunthorpe <jgg@xxxxxxxxxxxx>
> > mm/mmu_notifier.c | 53 ++++++++++++++++++++++++++++++++++++++++++++---
> > 1 file changed, 50 insertions(+), 3 deletions(-)
> >
> > v1: https://lore.kernel.org/linux-mm/20190724152858.GB28493@xxxxxxxx/
> > v2: https://lore.kernel.org/linux-mm/20190807191627.GA3008@xxxxxxxx/
> > * Abandon attempting to fix it by calling invalidate_range_end() during an
> > EAGAIN start
> > * Just trivially ban multiple subscriptions
> > v3:
> > * Be more sophisticated, ban only multiple subscriptions if the result is
> > a failure. Allows multiple subscriptions without invalidate_range_end
> > * Include a printk when this condition is hit (Michal)
> >
> > At this point the rework Christoph requested during the first posting
> > is completed and there are now only 3 drivers using
> > invalidate_range_end():
> >
> > drivers/misc/mic/scif/scif_dma.c: .invalidate_range_end = scif_mmu_notifier_invalidate_range_end};
> > drivers/misc/sgi-gru/grutlbpurge.c: .invalidate_range_end = gru_invalidate_range_end,
> > virt/kvm/kvm_main.c: .invalidate_range_end = kvm_mmu_notifier_invalidate_range_end,
> >
> > While I think it is unlikely that any of these drivers will be used in
> > combination with each other, display a printk in hopes to check.
> >
> > Someday I expect to just fail the registration on this condition.
> >
> > I think this also addresses Michal's concern about a 'big hammer' as
> > it probably won't ever trigger now.
>
> I'm going to put this in linux-next to see if there are any reports of
> the pr_warn failing.
>
> Michal, are you happy with this solution now?
It's been a month in linux-next now, with no complaints. If there are
no comments I will go ahead to send it in the hmm PR.
Thanks,
Jason
