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Date:   Thu, 15 Aug 2019 10:04:15 -0300
From:   Jason Gunthorpe <jgg@ziepe.ca>
To:     Michal Hocko <mhocko@kernel.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Daniel Vetter <daniel.vetter@ffwll.ch>,
        LKML <linux-kernel@vger.kernel.org>, 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: <20190815130415.GD21596@ziepe.ca>
References: <20190814202027.18735-1-daniel.vetter@ffwll.ch>
 <20190814202027.18735-3-daniel.vetter@ffwll.ch>
 <20190814134558.fe659b1a9a169c0150c3e57c@linux-foundation.org>
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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?

Jason