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 11:03:13 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Dan Williams <dan.j.williams@...el.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 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... Though, I don't fully
understand how the nvdimm pmem driver cleans up the percpu counter.

Logan