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Date:   Thu, 15 Aug 2019 11:37:59 -0300
From:   Jason Gunthorpe <jgg@ziepe.ca>
To:     Daniel Vetter <daniel.vetter@ffwll.ch>
Cc:     Michal Hocko <mhocko@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux MM <linux-mm@kvack.org>,
        DRI Development <dri-devel@lists.freedesktop.org>,
        Intel Graphics Development <intel-gfx@lists.freedesktop.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        David Rientjes <rientjes@google.com>,
        Christian =?utf-8?B?S8O2bmln?= <christian.koenig@amd.com>,
        =?utf-8?B?SsOpcsO0bWU=?= Glisse <jglisse@redhat.com>,
        Masahiro Yamada <yamada.masahiro@socionext.com>,
        Wei Wang <wvw@google.com>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Jann Horn <jannh@google.com>, Feng Tang <feng.tang@intel.com>,
        Kees Cook <keescook@chromium.org>,
        Randy Dunlap <rdunlap@infradead.org>,
        Daniel Vetter <daniel.vetter@intel.com>
Subject: Re: [PATCH 2/5] kernel.h: Add non_block_start/end()
Message-ID: <20190815143759.GG21596@ziepe.ca>
References: <20190814202027.18735-1-daniel.vetter@ffwll.ch>
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 <20190814134558.fe659b1a9a169c0150c3e57c@linux-foundation.org>
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On Thu, Aug 15, 2019 at 03:12:11PM +0200, Daniel Vetter wrote:
> On Thu, Aug 15, 2019 at 3:04 PM Jason Gunthorpe <jgg@ziepe.ca> wrote:
> >
> > On Thu, Aug 15, 2019 at 10:44:29AM +0200, Michal Hocko wrote:
> >
> > > As the oom reaper is the primary guarantee of the oom handling forward
> > > progress it cannot be blocked on anything that might depend on blockable
> > > memory allocations. These are not really easy to track because they
> > > might be indirect - e.g. notifier blocks on a lock which other context
> > > holds while allocating memory or waiting for a flusher that needs memory
> > > to perform its work.
> >
> > But lockdep *does* track all this and fs_reclaim_acquire() was created
> > to solve exactly this problem.
> >
> > fs_reclaim is a lock and it flows through all the usual lockdep
> > schemes like any other lock. Any time the page allocator wants to do
> > something the would deadlock with reclaim it takes the lock.
> >
> > Failure is expressed by a deadlock cycle in the lockdep map, and
> > lockdep can handle arbitary complexity through layers of locks, work
> > queues, threads, etc.
> >
> > What is missing?
> 
> Lockdep doens't seen everything by far. E.g. a wait_event will be
> caught by the annotations here, but not by lockdep. 

Sure, but the wait_event might be OK if its progress isn't contingent
on fs_reclaim, ie triggered from interrupt, so why ban it?

> And since we're talking about mmu notifiers here and gpus/dma engines.
> We have dma_fence_wait, which can wait for any hw/driver in the system
> that takes part in shared/zero-copy buffer processing. Which at least
> on the graphics side is everything. This pulls in enormous amounts of
> deadlock potential that lockdep simply is blind about and will never
> see.

It seems very risky to entagle a notifier widely like that.

It looks pretty sure that notifiers are fs_reclaim, so at a minimum
that wait_event can't be contingent on anything that is doing
GFP_KERNEL or it will deadlock - and blockable doesn't make that sleep
safe.

Avoiding an uncertain wait_event under notifiers would seem to be the
only reasonable design here..

Jason