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From: Michal Hocko <mhocko@suse.com>
To: Jason Gunthorpe <jgg@mellanox.com>
Cc: "linux-mm@kvack.org" <linux-mm@kvack.org>,
	"Jérôme Glisse" <jglisse@redhat.com>,
	"Andrea Arcangeli" <aarcange@redhat.com>,
	"Christoph Hellwig" <hch@lst.de>
Subject: Re: [PATCH] mm/mmn: prevent unpaired invalidate_start and invalidate_end with non-blocking
Date: Thu, 8 Aug 2019 14:13:09 +0200
Message-ID: <20190808121309.GD18351@dhcp22.suse.cz> (raw)
In-Reply-To: <20190808120402.GA1975@mellanox.com>

On Thu 08-08-19 12:04:07, Jason Gunthorpe wrote:
> On Thu, Aug 08, 2019 at 10:18:27AM +0200, Michal Hocko wrote:
> > On Wed 07-08-19 19:16:32, 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.
> > > 
> > > The recent change to add non-blocking notifiers breaks this assumption
> > > when multiple notifiers are present in the list as an EAGAIN return from a
> > > later notifier causes all earlier notifiers to get their
> > > invalidate_range_end() skipped.
> > > 
> > > During the development of non-blocking each user was audited to be sure
> > > they can skip their invalidate_range_end() if their start returns -EAGAIN,
> > > so the only place that has a problem is when there are multiple
> > > subscriptions.
> > > 
> > > Due to the RCU locking we can't reliably generate a subset of the linked
> > > list representing the notifiers already called, and generate an
> > > invalidate_range_end() pairing.
> > > 
> > > Rather than design an elaborate fix, for now, just block non-blocking
> > > requests early on if there are multiple subscriptions.
> > 
> > Which means that the oom path cannot really release any memory for
> > ranges covered by these notifiers which is really unfortunate because
> > that might cover a lot of memory. Especially when the particular range
> > might not be tracked at all, right?
> 
> Yes, it is a very big hammer to avoid a bug where the locking schemes
> get corrupted and the impacted drivers deadlock.
> 
> If you really don't like it then we have to push ahead on either an
> rcu-safe undo algorithm or some locking thing. I've been looking at
> the locking thing, so we can wait a bit more and see. 

Well, I do not like it but I understand that an over reaction for OOM is
much less of a pain than a deadlock or similar misbehavior. So go ahead
with this as a stop gap with Cc: stable but please let's do not stop
there and let's come up with something of a less hamery kind.

That being said, feel free to add
Acked-by: Michal Hocko <mhocko@suse.com>
with a printk_once to explain what is going on and a TODO note that this
is just a stop gap.

Thanks!
-- 
Michal Hocko
SUSE Labs


  reply index

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-07 19:16 Jason Gunthorpe
2019-08-08  7:00 ` Christoph Hellwig
2019-08-08  8:18 ` Michal Hocko
2019-08-08 12:04   ` Jason Gunthorpe
2019-08-08 12:13     ` Michal Hocko [this message]
2019-10-24 14:15       ` Jason Gunthorpe

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