Re: [PATCH 2/5] mm, devm_memremap_pages: handle errors allocating final devres action

From: Logan Gunthorpe <logang@deltatee.com>
To: Dan Williams <dan.j.williams@intel.com>,
	Andrew Morton <akpm@linux-foundation.org>
Cc: stable <stable@vger.kernel.org>, "Christoph Hellwig" <hch@lst.de>,
	"Jérôme Glisse" <jglisse@redhat.com>,
	"Linux MM" <linux-mm@kvack.org>,
	"Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 2/5] mm, devm_memremap_pages: handle errors allocating final devres action
Date: Tue, 22 May 2018 10:42:33 -0600	[thread overview]
Message-ID: <860a8c46-5171-78ac-0255-ee1d21b16ce8@deltatee.com> (raw)
In-Reply-To: <CAPcyv4hMwMefMu3La+hZvN6r+Q6_N5t+eOgGE0bqVou=Cjpfwg@mail.gmail.com>

Hey Dan,

On 21/05/18 06:07 PM, Dan Williams wrote:
> Without this change we could fail to register the teardown of
> devm_memremap_pages(). The likelihood of hitting this failure is tiny
> as small memory allocations almost always succeed. However, the impact
> of the failure is large given any future reconfiguration, or
> disable/enable, of an nvdimm namespace will fail forever as subsequent
> calls to devm_memremap_pages() will fail to setup the pgmap_radix
> since there will be stale entries for the physical address range.

Sorry, I don't follow this. The change only seems to prevent a warning
from occurring in this situation. Won't pgmap_radix_release() still be
called regardless of whether this patch is applied?

But it looks to me like this patch doesn't quite solve the issue -- at
least when looking at dax/pmem.c: If devm_add_action_or_reset() fails,
then dax_pmem_percpu_kill() won't be registered as an action and the
percpu_ref will never get killed. Thus, dax_pmem_percpu_release() would
not get called and dax_pmem_percpu_exit() will hang waiting for a
completion that will never occur. So we probably need to add a kill call
somewhere on the failing path...

Logan