linux-kernel - Re: [PATCH 1/5] mm: Check if mmu notifier callbacks are allowed to fail

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Date:   Fri, 16 Aug 2019 14:19:04 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Daniel Vetter <daniel.vetter@...ll.ch>
Cc:     LKML <linux-kernel@...r.kernel.org>, 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>,
        Christian König <christian.koenig@....com>,
        David Rientjes <rientjes@...gle.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Daniel Vetter <daniel.vetter@...el.com>
Subject: Re: [PATCH 1/5] mm: Check if mmu notifier callbacks are allowed to
 fail

On Wed, Aug 14, 2019 at 10:20:23PM +0200, Daniel Vetter wrote:
> Just a bit of paranoia, since if we start pushing this deep into
> callchains it's hard to spot all places where an mmu notifier
> implementation might fail when it's not allowed to.
> 
> Inspired by some confusion we had discussing i915 mmu notifiers and
> whether we could use the newly-introduced return value to handle some
> corner cases. Until we realized that these are only for when a task
> has been killed by the oom reaper.
> 
> An alternative approach would be to split the callback into two
> versions, one with the int return value, and the other with void
> return value like in older kernels. But that's a lot more churn for
> fairly little gain I think.
> 
> Summary from the m-l discussion on why we want something at warning
> level: This allows automated tooling in CI to catch bugs without
> humans having to look at everything. If we just upgrade the existing
> pr_info to a pr_warn, then we'll have false positives. And as-is, no
> one will ever spot the problem since it's lost in the massive amounts
> of overall dmesg noise.
> 
> v2: Drop the full WARN_ON backtrace in favour of just a pr_warn for
> the problematic case (Michal Hocko).
> 
> v3: Rebase on top of Glisse's arg rework.
> 
> v4: More rebase on top of Glisse reworking everything.
> 
> v5: Fixup rebase damage and also catch failures != EAGAIN for
> !blockable (Jason). Also go back to WARN_ON as requested by Jason, so
> automatic checkers can easily catch bugs by setting panic_on_warn.
> 
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Michal Hocko <mhocko@...e.com>
> Cc: "Christian König" <christian.koenig@....com>
> Cc: David Rientjes <rientjes@...gle.com>
> Cc: Daniel Vetter <daniel.vetter@...ll.ch>
> Cc: "Jérôme Glisse" <jglisse@...hat.com>
> Cc: linux-mm@...ck.org
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Jason Gunthorpe <jgg@...pe.ca>
> Signed-off-by: Daniel Vetter <daniel.vetter@...el.com>
> ---
>  mm/mmu_notifier.c | 2 ++
>  1 file changed, 2 insertions(+)

Applied to hmm.git, thanks

Jason