linux-kernel - Re: [PATCH 06/19] mm: store mapcount for compound page separate

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Message-ID: <20141118095811.GA21774@node.dhcp.inet.fi>
Date:	Tue, 18 Nov 2014 11:58:11 +0200
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
Cc:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Hugh Dickins <hughd@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Rik van Riel <riel@...hat.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Christoph Lameter <cl@...two.org>,
	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>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH 06/19] mm: store mapcount for compound page separate

On Tue, Nov 18, 2014 at 08:43:00AM +0000, Naoya Horiguchi wrote:
> > @@ -1837,6 +1839,9 @@ static void __split_huge_page_refcount(struct page *page,
> >  	atomic_sub(tail_count, &page->_count);
> >  	BUG_ON(atomic_read(&page->_count) <= 0);
> >  
> > +	page->_mapcount = *compound_mapcount_ptr(page);
> 
> Is atomic_set() necessary?

Do you mean
	atomic_set(&page->_mapcount, atomic_read(compound_mapcount_ptr(page)));
?

I don't see why we would need this. Simple assignment should work just
fine. Or we have archs which will break?

> > @@ -6632,10 +6637,12 @@ static void dump_page_flags(unsigned long flags)
> >  void dump_page_badflags(struct page *page, const char *reason,
> >  		unsigned long badflags)
> >  {
> > -	printk(KERN_ALERT
> > -	       "page:%p count:%d mapcount:%d mapping:%p index:%#lx\n",
> > +	pr_alert("page:%p count:%d mapcount:%d mapping:%p index:%#lx",
> >  		page, atomic_read(&page->_count), page_mapcount(page),
> >  		page->mapping, page->index);
> > +	if (PageCompound(page))
> 
> > +		printk(" compound_mapcount: %d", compound_mapcount(page));
> > +	printk("\n");
> 
> These two printk() should be pr_alert(), too?

No. It will split the line into several messages in dmesg.

> > @@ -986,9 +986,30 @@ void page_add_anon_rmap(struct page *page,
> >  void do_page_add_anon_rmap(struct page *page,
> >  	struct vm_area_struct *vma, unsigned long address, int flags)
> >  {
> > -	int first = atomic_inc_and_test(&page->_mapcount);
> > +	bool compound = flags & RMAP_COMPOUND;
> > +	bool first;
> > +
> > +	VM_BUG_ON_PAGE(!PageLocked(compound_head(page)), page);
> > +
> > +	if (PageTransCompound(page)) {
> > +		struct page *head_page = compound_head(page);
> > +
> > +		if (compound) {
> > +			VM_BUG_ON_PAGE(!PageTransHuge(page), page);
> > +			first = atomic_inc_and_test(compound_mapcount_ptr(page));
> 
> Is compound_mapcount_ptr() well-defined for tail pages?

The page is head page, otherwise VM_BUG_ON on the line above would trigger.

> > @@ -1032,10 +1052,19 @@ void page_add_new_anon_rmap(struct page *page,
> >  
> >  	VM_BUG_ON(address < vma->vm_start || address >= vma->vm_end);
> >  	SetPageSwapBacked(page);
> > -	atomic_set(&page->_mapcount, 0); /* increment count (starts at -1) */
> >  	if (compound) {
> > +		atomic_t *compound_mapcount;
> > +
> >  		VM_BUG_ON_PAGE(!PageTransHuge(page), page);
> > +		compound_mapcount = (atomic_t *)&page[1].mapping;
> 
> You can use compound_mapcount_ptr() here.

Right, thanks.

-- 
 Kirill A. Shutemov
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