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

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Date:   Tue, 22 May 2018 09:56:48 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Logan Gunthorpe <logang@...tatee.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        stable <stable@...r.kernel.org>, Christoph Hellwig <hch@....de>,
        Jérôme Glisse <jglisse@...hat.com>,
        Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/5] mm, devm_memremap_pages: handle errors allocating
 final devres action

On Tue, May 22, 2018 at 9:42 AM, Logan Gunthorpe <logang@...tatee.com> wrote:
> 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?

devm_add_action() does not call the release function,
devm_add_action_or_reset() does.

> 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...

Ah, true, good catch!

We should manually kill in the !registered case. I think this means we
need to pass in the custom kill routine, because for the pmem driver
it's blk_freeze_queue_start().