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

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Date:   Tue, 6 Nov 2018 16:47:46 +0800
From:   Aaron Lu <aaron.lu@...el.com>
To:     Vlastimil Babka <vbabka@...e.cz>
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 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.

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.

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);
> >  
> > 
>