linux-kernel - Re: [PATCH v8 3/7] mm, devm_memremap

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Date:   Thu, 29 Nov 2018 10:50:46 -0700
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>,
        Jérôme Glisse <jglisse@...hat.com>,
        Christoph Hellwig <hch@....de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Maling list - DRI developers 
        <dri-devel@...ts.freedesktop.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Stephen Bates <sbates@...thlin.com>
Subject: Re: [PATCH v8 3/7] mm, devm_memremap_pages: Fix shutdown handling

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

>> 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.

Logan