linux-kernel - Re: [PATCH 4/4] mm, notifier: Catch sleeping/blocking for !blockable

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Message-ID: <CAKMK7uERsmgFqDVHMCWs=4s_3fHM0eRr7MV6A8Mdv7xVouyxJw@mail.gmail.com>
Date:   Thu, 22 Aug 2019 10:42:39 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     LKML <linux-kernel@...r.kernel.org>, Linux MM <linux-mm@...ck.org>,
        DRI Development <dri-devel@...ts.freedesktop.org>,
        Intel Graphics Development <intel-gfx@...ts.freedesktop.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        David Rientjes <rientjes@...gle.com>,
        Christian König <christian.koenig@....com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Daniel Vetter <daniel.vetter@...el.com>
Subject: Re: [PATCH 4/4] mm, notifier: Catch sleeping/blocking for !blockable

On Thu, Aug 22, 2019 at 10:16 AM Jason Gunthorpe <jgg@...pe.ca> wrote:
>
> On Wed, Aug 21, 2019 at 05:41:51PM +0200, Daniel Vetter wrote:
>
> > > Hm, I thought the page table locks we're holding there already prevent any
> > > sleeping, so would be redundant? But reading through code I think that's
> > > not guaranteed, so yeah makes sense to add it for invalidate_range_end
> > > too. I'll respin once I have the ack/nack from scheduler people.
> >
> > So I started to look into this, and I'm a bit confused. There's no
> > _nonblock version of this, so does this means blocking is never allowed,
> > or always allowed?
>
> RDMA has a mutex:
>
> ib_umem_notifier_invalidate_range_end
>   rbt_ib_umem_for_each_in_range
>    invalidate_range_start_trampoline
>     ib_umem_notifier_end_account
>       mutex_lock(&umem_odp->umem_mutex);
>
> I'm working to delete this path though!
>
> nonblocking or not follows the start, the same flag gets placed into
> the mmu_notifier_range struct passed to end.

Ok, makes sense.

I guess that also means the might_sleep (I started on that) in
invalidate_range_end also needs to be conditional? Or not bother with
a might_sleep in invalidate_range_end since you're working on removing
the last sleep in there?

> > From a quick look through implementations I've only seen spinlocks, and
> > one up_read. So I guess I should wrape this callback in some unconditional
> > non_block_start/end, but I'm not sure.
>
> For now, we should keep it the same as start, conditionally blocking.
>
> Hopefully before LPC I can send a RFC series that eliminates most
> invalidate_range_end users in favor of common locking..

Thanks, Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch