linux-kernel - Re: [PATCH v2 2/2] mm/page_alloc: use a single function to free page

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Date:   Tue, 6 Nov 2018 10:32:00 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Aaron Lu <aaron.lu@...el.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>,
        Paweł Staszewski <pstaszewski@...are.pl>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Tariq Toukan <tariqt@...lanox.com>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        Yoel Caspersen <yoel@...knet.dk>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Saeed Mahameed <saeedm@...lanox.com>,
        Michal Hocko <mhocko@...e.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>
Subject: Re: [PATCH v2 2/2] mm/page_alloc: use a single function to free page

On 11/6/18 9:47 AM, Aaron Lu wrote:
> On Tue, Nov 06, 2018 at 09:16:20AM +0100, Vlastimil Babka wrote:
>> On 11/6/18 6:30 AM, Aaron Lu wrote:
>>> We have multiple places of freeing a page, most of them doing similar
>>> things and a common function can be used to reduce code duplicate.
>>>
>>> It also avoids bug fixed in one function but left in another.
>>>
>>> Signed-off-by: Aaron Lu <aaron.lu@...el.com>
>>
>> Acked-by: Vlastimil Babka <vbabka@...e.cz>
> 
> Thanks.
> 
>> I assume there's no arch that would run page_ref_sub_and_test(1) slower
>> than put_page_testzero(), for the critical __free_pages() case?
> 
> Good question.
> 
> I followed the non-arch specific calls and found that:
> page_ref_sub_and_test() ends up calling atomic_sub_return(i, v) while
> put_page_testzero() ends up calling atomic_sub_return(1, v). So they
> should be same for archs that do not have their own implementations.

x86 seems to distinguish between DECL and SUBL, see
arch/x86/include/asm/atomic.h although I could not figure out where does
e.g. arch_atomic_dec_and_test become atomic_dec_and_test to override the
generic implementation.
I don't know if the CPU e.g. executes DECL faster, but objectively it
has one parameter less. Maybe it doesn't matter?

> Back to your question: I don't know either.
> If this is deemed unsafe, we can probably keep the ref modify part in
> their original functions and only take the free part into a common
> function.

I guess you could also employ  if (__builtin_constant_p(nr)) in
free_the_page(), but the result will be ugly I guess, and maybe not
worth it :)

> Regards,
> Aaron
> 
>>> ---
>>> v2: move comments close to code as suggested by Dave.
>>>
>>>  mm/page_alloc.c | 36 ++++++++++++++++--------------------
>>>  1 file changed, 16 insertions(+), 20 deletions(-)
>>>
>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>>> index 91a9a6af41a2..4faf6b7bf225 100644
>>> --- a/mm/page_alloc.c
>>> +++ b/mm/page_alloc.c
>>> @@ -4425,9 +4425,17 @@ unsigned long get_zeroed_page(gfp_t gfp_mask)
>>>  }
>>>  EXPORT_SYMBOL(get_zeroed_page);
>>>  
>>> -void __free_pages(struct page *page, unsigned int order)
>>> +static inline void free_the_page(struct page *page, unsigned int order, int nr)
>>>  {
>>> -	if (put_page_testzero(page)) {
>>> +	VM_BUG_ON_PAGE(page_ref_count(page) == 0, page);
>>> +
>>> +	/*
>>> +	 * Free a page by reducing its ref count by @nr.
>>> +	 * If its refcount reaches 0, then according to its order:
>>> +	 * order0: send to PCP;
>>> +	 * high order: directly send to Buddy.
>>> +	 */
>>> +	if (page_ref_sub_and_test(page, nr)) {
>>>  		if (order == 0)
>>>  			free_unref_page(page);
>>>  		else
>>> @@ -4435,6 +4443,10 @@ void __free_pages(struct page *page, unsigned int order)
>>>  	}
>>>  }
>>>  
>>> +void __free_pages(struct page *page, unsigned int order)
>>> +{
>>> +	free_the_page(page, order, 1);
>>> +}
>>>  EXPORT_SYMBOL(__free_pages);
>>>  
>>>  void free_pages(unsigned long addr, unsigned int order)
>>> @@ -4481,16 +4493,7 @@ static struct page *__page_frag_cache_refill(struct page_frag_cache *nc,
>>>  
>>>  void __page_frag_cache_drain(struct page *page, unsigned int count)
>>>  {
>>> -	VM_BUG_ON_PAGE(page_ref_count(page) == 0, page);
>>> -
>>> -	if (page_ref_sub_and_test(page, count)) {
>>> -		unsigned int order = compound_order(page);
>>> -
>>> -		if (order == 0)
>>> -			free_unref_page(page);
>>> -		else
>>> -			__free_pages_ok(page, order);
>>> -	}
>>> +	free_the_page(page, compound_order(page), count);
>>>  }
>>>  EXPORT_SYMBOL(__page_frag_cache_drain);
>>>  
>>> @@ -4555,14 +4558,7 @@ void page_frag_free(void *addr)
>>>  {
>>>  	struct page *page = virt_to_head_page(addr);
>>>  
>>> -	if (unlikely(put_page_testzero(page))) {
>>> -		unsigned int order = compound_order(page);
>>> -
>>> -		if (order == 0)
>>> -			free_unref_page(page);
>>> -		else
>>> -			__free_pages_ok(page, order);
>>> -	}
>>> +	free_the_page(page, compound_order(page), 1);
>>>  }
>>>  EXPORT_SYMBOL(page_frag_free);
>>>  
>>>
>>