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From: Dan Williams <dan.j.williams@intel.com>
To: Logan Gunthorpe <logang@deltatee.com>
Cc: "Andrew Morton" <akpm@linux-foundation.org>,
	stable <stable@vger.kernel.org>,
	"Jérôme Glisse" <jglisse@redhat.com>,
	"Christoph Hellwig" <hch@lst.de>,
	"Linus Torvalds" <torvalds@linux-foundation.org>,
	"Linux MM" <linux-mm@kvack.org>,
	"Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
	"Maling list - DRI developers" <dri-devel@lists.freedesktop.org>,
	"Bjorn Helgaas" <bhelgaas@google.com>,
	"Stephen Bates" <sbates@raithlin.com>
Subject: Re: [PATCH v8 3/7] mm, devm_memremap_pages: Fix shutdown handling
Date: Thu, 29 Nov 2018 10:51:06 -0800	[thread overview]
Message-ID: <CAPcyv4hJV71RLhBCKgqc=nQ_D_upySD=ZZk0y=5Qk69kKPHFog@mail.gmail.com> (raw)
In-Reply-To: <43778343-6d43-eb43-0de0-3db6828902d0@deltatee.com>

On Thu, Nov 29, 2018 at 9:51 AM Logan Gunthorpe <logang@deltatee.com> wrote:
>
>
>
> On 2018-11-29 10:30 a.m., Dan Williams wrote:
> > Oh! Yes, nice find. We need to wait for the percpu-ref to be dead and
> > all outstanding references dropped before we can proceed to
> > arch_remove_memory(), and I think this problem has been there since
> > day one because the final exit was always after devm_memremap_pages()
> > release which means arch_remove_memory() was always racing any final
> > put_page(). I'll take a look, it seems the arch_remove_pages() call
> > needs to be moved out-of-line to its own context and wait for the
> > final exit of the percpu-ref.
>
> Ok, well I thought moving the wait_for_completion() into the kill() call
> was a pretty good solution to this.

True, it is...

> Though, if we move the
> arch_remove_pages() into a different context, it *may* help with the
> problem below...

Glad to see my over-engineered proposal in this case might be good for
something...

>
> >> Though, now that I look at it, the current change in question will be
> >> wrong if there are two devm_memremap_pages_release()s to call. Both need
> >> to drop their references before we can wait_for_completion() ;(. I guess
> >> I need multiple percpu_refs or more complex changes to
> >> devm_memremap_pages_release().
> >
> > Can you just have a normal device-level kref for this case? On final
> > device-level kref_put then kill the percpu_ref? I guess the problem is
> > devm semantics where p2pdma only gets one callback on a driver
> > ->remove() event. I'm not sure how to support multiple references of
> > the same pages without creating a non-devm version of
> > devm_memremap_pages(). I'm not opposed to that, but afaiu I don't
> > think p2pdma is compatible with devm as long as it supports N>1:1
> > mappings of the same range.
>
> Hmm, no I think you misunderstood what I said. I'm saying I need to have
> exactly one percpu_ref per call to devm_memremap_pages() and this is
> doable, just slightly annoying. Right now I have one percpu_ref for
> multiple calls to devm_memremap_pages() which doesn't work with the
> above fix because there will always be a wait_for_completion() before
> the last references are dropped in this way:
>
> 1) First devm_memremap_pages_release() is called which drops it's
> reference and waits_for_completion().
>
> 2) The second devm_memremap_pages_release() needs to be called to drop
> it's reference, but can't seeing the first is waiting, and therefore the
> percpu_ref never goes to zero and the wait_for_completion() never returns.
>

Got it, let me see how bad moving arch_remove_memory() turns out,
sounds like a decent approach to coordinate multiple users of a single
ref.

  reply	other threads:[~2018-11-29 18:51 UTC|newest]

Thread overview: 28+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-11-20 23:12 [PATCH v8 0/7] mm: Merge hmm into devm_memremap_pages, mark GPL-only Dan Williams
2018-11-20 23:12 ` [PATCH v8 1/7] mm, devm_memremap_pages: Mark devm_memremap_pages() EXPORT_SYMBOL_GPL Dan Williams
2018-11-22 13:30   ` Michal Hocko
2018-11-22 16:38     ` Christoph Hellwig
2018-11-22 16:40       ` Christoph Hellwig
2018-11-23  8:47       ` Michal Hocko
2018-11-20 23:13 ` [PATCH v8 2/7] mm, devm_memremap_pages: Kill mapping "System RAM" support Dan Williams
2018-11-20 23:13 ` [PATCH v8 3/7] mm, devm_memremap_pages: Fix shutdown handling Dan Williams
2018-11-27 21:43   ` Logan Gunthorpe
2018-11-29  3:10     ` Dan Williams
2018-11-29 17:06       ` Logan Gunthorpe
2018-11-29 17:30         ` Dan Williams
2018-11-29 17:50           ` Logan Gunthorpe
2018-11-29 18:51             ` Dan Williams [this message]
2018-11-30 22:19               ` Logan Gunthorpe
2018-11-30 22:28                 ` Dan Williams
2018-11-30 22:34                   ` Logan Gunthorpe
2018-11-30 22:47                     ` Dan Williams
2018-11-20 23:13 ` [PATCH v8 4/7] mm, devm_memremap_pages: Add MEMORY_DEVICE_PRIVATE support Dan Williams
2018-11-23 10:48   ` David Hildenbrand
2018-11-20 23:13 ` [PATCH v8 5/7] mm, hmm: Use devm semantics for hmm_devmem_{add, remove} Dan Williams
2018-11-20 23:13 ` [PATCH v8 6/7] mm, hmm: Replace hmm_devmem_pages_create() with devm_memremap_pages() Dan Williams
2018-11-20 23:13 ` [PATCH v8 7/7] mm, hmm: Mark hmm_devmem_{add, add_resource} EXPORT_SYMBOL_GPL Dan Williams
2018-11-22  1:20 ` [PATCH v8 0/7] mm: Merge hmm into devm_memremap_pages, mark GPL-only Andrew Morton
2018-11-25 22:04   ` Pavel Machek
2018-12-03 23:37 ` Jerome Glisse
2019-02-10 11:09 [PATCH v8 3/7] mm, devm_memremap_pages: Fix shutdown handling Krzysztof Grygiencz
2019-02-11 19:57 ` Jerome Glisse

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