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 10:25:13 -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 10:03 AM, Logan Gunthorpe <logang@...tatee.com> wrote:
>
>
> On 22/05/18 10:56 AM, Dan Williams wrote:
>> 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.
>
> Oh, yes. Thanks I see that now.
>
>> 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().
>
> It may be cleaner to just have the caller call the specific kill
> function if devm_memremap_pages fails...

As far as I can see by then it's too late, or we need to expose
release details to the caller which defeats the purpose of devm
semantics.

> Though, I don't fully
> understand how the nvdimm pmem driver cleans up the percpu counter.

The dev_pagemap setup for pmem is entirely too subtle and arguably a
layering violation as it reuses the block layer q_usage_counter
percpu_ref. We arrange for that counter to be shutdown before the
blk_cleanup_queue() does the same.