Re: [PATCH 1/3] mm: Add __page_cache_alloc_order

[Song Liu <songliubraving@fb.com>]

> On Sep 5, 2019, at 12:02 PM, Matthew Wilcox <willy@infradead.org> wrote:
> 
> On Thu, Sep 05, 2019 at 06:58:53PM +0000, Song Liu wrote:
>>> On Sep 5, 2019, at 11:23 AM, Matthew Wilcox <willy@infradead.org> wrote:
>>> This new function allows page cache pages to be allocated that are
>>> larger than an order-0 page.
>>> 
>>> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
>>> ---
>>> include/linux/pagemap.h | 14 +++++++++++---
>>> mm/filemap.c            | 11 +++++++----
>>> 2 files changed, 18 insertions(+), 7 deletions(-)
>>> 
>>> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
>>> index 103205494ea0..d2147215d415 100644
>>> --- a/include/linux/pagemap.h
>>> +++ b/include/linux/pagemap.h
>>> @@ -208,14 +208,22 @@ static inline int page_cache_add_speculative(struct page *page, int count)
>>> }
>>> 
>>> #ifdef CONFIG_NUMA
>>> -extern struct page *__page_cache_alloc(gfp_t gfp);
>>> +extern struct page *__page_cache_alloc_order(gfp_t gfp, unsigned int order);
>> 
>> I guess we need __page_cache_alloc(gfp_t gfp) here for CONFIG_NUMA. 
> 
> ... no?  The __page_cache_alloc() below is outside the ifdef/else/endif, so
> it's the same for both NUMA and non-NUMA.

You are right. I misread this one. 

> 
>>> #else
>>> -static inline struct page *__page_cache_alloc(gfp_t gfp)
>>> +static inline
>>> +struct page *__page_cache_alloc_order(gfp_t gfp, unsigned int order)
>>> {
>>> -	return alloc_pages(gfp, 0);
>>> +	if (order > 0)
>>> +		gfp |= __GFP_COMP;
>>> +	return alloc_pages(gfp, order);
>>> }
>>> #endif
>>> 
>>> +static inline struct page *__page_cache_alloc(gfp_t gfp)
>>> +{
>>> +	return __page_cache_alloc_order(gfp, 0);
>> 
>> Maybe "return alloc_pages(gfp, 0);" here to avoid checking "order > 0"?
> 
> For non-NUMA cases, the __page_cache_alloc_order() will be inlined into
> __page_cache_alloc() and the copiler will eliminate the test.  Or you
> need a better compiler ;-)
> 
>>> -struct page *__page_cache_alloc(gfp_t gfp)
>>> +struct page *__page_cache_alloc_order(gfp_t gfp, unsigned int order)
>>> {
>>> 	int n;
>>> 	struct page *page;
>>> 
>>> +	if (order > 0)
>>> +		gfp |= __GFP_COMP;
>>> +
>> 
>> I think it will be good to have separate __page_cache_alloc() for order 0, 
>> so that we avoid checking "order > 0", but that may require too much 
>> duplication. So I am on the fence for this one. 
> 
> We're about to dive into the page allocator ... two extra instructions
> here aren't going to be noticable.

True. Thanks for the explanation. 

Acked-by: Song Liu <songliubraving@fb.com>