linux-kernel - Re: [PATCHv3 04/24] rmap: add argument to charge compound page

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Date:	Wed, 04 Mar 2015 13:09:51 +0100
From:	Vlastimil Babka <vbabka@...e.cz>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
CC:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Hugh Dickins <hughd@...gle.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Christoph Lameter <cl@...two.org>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Steve Capper <steve.capper@...aro.org>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>,
	Jerome Marchand <jmarchan@...hat.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCHv3 04/24] rmap: add argument to charge compound page

On 03/04/2015 12:52 PM, Kirill A. Shutemov wrote:
> On Mon, Feb 23, 2015 at 05:21:31PM +0100, Vlastimil Babka wrote:
>> On 02/12/2015 05:18 PM, Kirill A. Shutemov wrote:
>>> @@ -1052,21 +1052,24 @@ void page_add_anon_rmap(struct page *page,
>>>    * Everybody else should continue to use page_add_anon_rmap above.
>>>    */
>>>   void do_page_add_anon_rmap(struct page *page,
>>> -	struct vm_area_struct *vma, unsigned long address, int exclusive)
>>> +	struct vm_area_struct *vma, unsigned long address, int flags)
>>>   {
>>>   	int first = atomic_inc_and_test(&page->_mapcount);
>>>   	if (first) {
>>> +		bool compound = flags & RMAP_COMPOUND;
>>> +		int nr = compound ? hpage_nr_pages(page) : 1;
>>
>> hpage_nr_pages(page) is:
>>
>> static inline int hpage_nr_pages(struct page *page)
>> {
>>          if (unlikely(PageTransHuge(page)))
>>                  return HPAGE_PMD_NR;
>>          return 1;
>> }
>>
>> and later...
>>
>>>   		/*
>>>   		 * We use the irq-unsafe __{inc|mod}_zone_page_stat because
>>>   		 * these counters are not modified in interrupt context, and
>>>   		 * pte lock(a spinlock) is held, which implies preemption
>>>   		 * disabled.
>>>   		 */
>>> -		if (PageTransHuge(page))
>>> +		if (compound) {
>>> +			VM_BUG_ON_PAGE(!PageTransHuge(page), page);
>>
>> this means that we could assume that
>> (compound == true) => (PageTransHuge(page) == true)
>>
>> and simplify above to:
>>
>> int nr = compound ? HPAGE_PMD_NR : 1;
>>
>> Right?
>
> No. HPAGE_PMD_NR is defined based on HPAGE_PMD_SHIFT which is BUILD_BUG()
> without CONFIG_TRANSPARENT_HUGEPAGE. We will get compiler error without
> the helper.

Oh, OK. But that doesn't mean there couldn't be another helper that 
would work in this case, or even open-coded #ifdefs in these functions. 
Apparently "compound" has to be always false for 
!CONFIG_TRANSPARENT_HUGEPAGE, as in that case PageTransHuge is defined 
as 0 and the VM_BUG_ON would trigger if compound was true. So without 
such ifdefs or wrappers, you are also adding dead code and pointless 
tests for !CONFIG_TRANSPARENT_HUGEPAGE?

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