linux-kernel - Re: [PATCH v4 04/23] mm: devmap: refactor 1-based refcounting for ZONE

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Date:   Wed, 13 Nov 2019 14:56:58 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     Dan Williams <dan.j.williams@...el.com>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Alex Williamson <alex.williamson@...hat.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Björn Töpel <bjorn.topel@...el.com>,
        Christoph Hellwig <hch@...radead.org>,
        Daniel Vetter <daniel@...ll.ch>,
        Dave Chinner <david@...morbit.com>,
        David Airlie <airlied@...ux.ie>,
        "David S . Miller" <davem@...emloft.net>,
        Ira Weiny <ira.weiny@...el.com>, Jan Kara <jack@...e.cz>,
        Jason Gunthorpe <jgg@...pe.ca>, Jens Axboe <axboe@...nel.dk>,
        Jonathan Corbet <corbet@....net>,
        Jérôme Glisse <jglisse@...hat.com>,
        Magnus Karlsson <magnus.karlsson@...el.com>,
        "Mauro Carvalho Chehab" <mchehab@...nel.org>,
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        Ralph Campbell <rcampbell@...dia.com>
Subject: Re: [PATCH v4 04/23] mm: devmap: refactor 1-based refcounting for
 ZONE_DEVICE pages

On 11/13/19 2:55 PM, Dan Williams wrote:
> On Wed, Nov 13, 2019 at 2:49 PM John Hubbard <jhubbard@...dia.com> wrote:
>>
>> On 11/13/19 2:00 PM, Dan Williams wrote:
>> ...
>>>> Ugh, when did all this HMM specific manipulation sneak into the
>>>> generic ZONE_DEVICE path? It used to be gated by pgmap type with its
>>>> own put_zone_device_private_page(). For example it's certainly
>>>> unnecessary and might be broken (would need to check) to call
>>>> mem_cgroup_uncharge() on a DAX page. ZONE_DEVICE users are not a
>>>> monolith and the HMM use case leaks pages into code paths that DAX
>>>> explicitly avoids.
>>>
>>> It's been this way for a while and I did not react previously,
>>> apologies for that. I think __ClearPageActive, __ClearPageWaiters, and
>>> mem_cgroup_uncharge, belong behind a device-private conditional. The
>>> history here is:
>>>
>>> Move some, but not all HMM specifics to hmm_devmem_free():
>>>       2fa147bdbf67 mm, dev_pagemap: Do not clear ->mapping on final put
>>>
>>> Remove the clearing of mapping since no upstream consumers needed it:
>>>       b7a523109fb5 mm: don't clear ->mapping in hmm_devmem_free
>>>
>>> Add it back in once an upstream consumer arrived:
>>>       7ab0ad0e74f8 mm/hmm: fix ZONE_DEVICE anon page mapping reuse
>>>
>>> We're now almost entirely free of ->page_free callbacks except for
>>> that weird nouveau case, can that FIXME in nouveau_dmem_page_free()
>>> also result in killing the ->page_free() callback altogether? In the
>>> meantime I'm proposing a cleanup like this:
>>
>>
>> OK, assuming this is acceptable (no obvious problems jump out at me,
>> and we can also test it with HMM), then how would you like to proceed, as
>> far as patches go: add such a patch as part of this series here, or as a
>> stand-alone patch either before or after this series? Or something else?
>> And did you plan on sending it out as such?
> 
> I think it makes sense to include it in your series since you're
> looking to refactor the implementation. I can send you one based on
> current linux-next as a lead-in cleanup before the refactor. Does that
> work for you?
> 

That would be perfect!

>>
>> Also, the diffs didn't quite make it through intact to my "git apply", so
>> I'm re-posting the diff in hopes that this time it survives:
> 
> Apologies for that. For quick "how about this" patch examples, I just
> copy and paste into gmail and it sometimes clobbers it.
> 

No problem at all, I do the same thing and *usually* it works. ha. And
as you say, it's good enough for discussions.


thanks,
-- 
John Hubbard
NVIDIA