netdev - Re: [PATCH v11 04/25] mm: devmap: refactor 1-based refcounting for ZONE

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Message-ID: <ddd08105-f48a-dbe1-7de1-f2fa2c5772a9@nvidia.com>
Date:   Wed, 18 Dec 2019 16:32:28 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
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>,
        Dan Williams <dan.j.williams@...el.com>,
        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>,
        Michael Ellerman <mpe@...erman.id.au>,
        Michal Hocko <mhocko@...e.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Paul Mackerras <paulus@...ba.org>,
        Shuah Khan <shuah@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>, <bpf@...r.kernel.org>,
        <dri-devel@...ts.freedesktop.org>, <kvm@...r.kernel.org>,
        <linux-block@...r.kernel.org>, <linux-doc@...r.kernel.org>,
        <linux-fsdevel@...r.kernel.org>, <linux-kselftest@...r.kernel.org>,
        <linux-media@...r.kernel.org>, <linux-rdma@...r.kernel.org>,
        <linuxppc-dev@...ts.ozlabs.org>, <netdev@...r.kernel.org>,
        <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v11 04/25] mm: devmap: refactor 1-based refcounting for
 ZONE_DEVICE pages

On 12/18/19 8:04 AM, Kirill A. Shutemov wrote:
> On Mon, Dec 16, 2019 at 02:25:16PM -0800, John Hubbard wrote:
>> An upcoming patch changes and complicates the refcounting and
>> especially the "put page" aspects of it. In order to keep
>> everything clean, refactor the devmap page release routines:
>>
>> * Rename put_devmap_managed_page() to page_is_devmap_managed(),
>>    and limit the functionality to "read only": return a bool,
>>    with no side effects.
>>
>> * Add a new routine, put_devmap_managed_page(), to handle checking
>>    what kind of page it is, and what kind of refcount handling it
>>    requires.
>>
>> * Rename __put_devmap_managed_page() to free_devmap_managed_page(),
>>    and limit the functionality to unconditionally freeing a devmap
>>    page.
> 
> What the reason to separate put_devmap_managed_page() from
> free_devmap_managed_page() if free_devmap_managed_page() has exacly one
> caller? Is it preparation for the next patches?


Yes. A later patch, #23, adds another caller: __unpin_devmap_managed_user_page().

...
>> @@ -971,7 +971,14 @@ static inline bool put_devmap_managed_page(struct page *page)
>>   	return false;
>>   }
>>   
>> +bool put_devmap_managed_page(struct page *page);
>> +
>>   #else /* CONFIG_DEV_PAGEMAP_OPS */
>> +static inline bool page_is_devmap_managed(struct page *page)
>> +{
>> +	return false;
>> +}
>> +
>>   static inline bool put_devmap_managed_page(struct page *page)
>>   {
>>   	return false;
>> @@ -1028,8 +1035,10 @@ static inline void put_page(struct page *page)
>>   	 * need to inform the device driver through callback. See
>>   	 * include/linux/memremap.h and HMM for details.
>>   	 */
>> -	if (put_devmap_managed_page(page))
>> +	if (page_is_devmap_managed(page)) {
>> +		put_devmap_managed_page(page);
> 
> put_devmap_managed_page() has yet another page_is_devmap_managed() check
> inside. It looks strange.
> 

Good point, it's an extra unnecessary check. So to clean it up, I'll note
that the "if" check is required here in put_page(), in order to stay out of
non-inlined function calls in the hot path (put_page()). So I'll do the
following:

* Leave the above code as it is here

* Simplify put_devmap_managed_page(), it was trying to do two separate things,
   and those two things have different requirements. So change it to a void
   function, with a WARN_ON_ONCE to assert that page_is_devmap_managed() is true,

* And change the other caller (release_pages()) to do that check.

...
>> @@ -1102,3 +1102,27 @@ void __init swap_setup(void)
>>   	 * _really_ don't want to cluster much more
>>   	 */
>>   }
>> +
>> +#ifdef CONFIG_DEV_PAGEMAP_OPS
>> +bool put_devmap_managed_page(struct page *page)
>> +{
>> +	bool is_devmap = page_is_devmap_managed(page);
>> +
>> +	if (is_devmap) {
> 
> Reversing the condition would save you an indentation level.

Yes. Done.

I'll also git-reply with an updated patch so you can see what it looks like.


thanks,
-- 
John Hubbard
NVIDIA