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

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Date:	Mon, 16 Feb 2015 17:20:56 +0200
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Rik van Riel <riel@...hat.com>
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>, Vlastimil Babka <vbabka@...e.cz>,
	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 Thu, Feb 12, 2015 at 04:10:21PM -0500, Rik van Riel wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
> 
> On 02/12/2015 11:18 AM, Kirill A. Shutemov wrote:
> 
> > +++ b/include/linux/rmap.h @@ -168,16 +168,24 @@ static inline void
> > anon_vma_merge(struct vm_area_struct *vma,
> > 
> > struct anon_vma *page_get_anon_vma(struct page *page);
> > 
> > +/* flags for do_page_add_anon_rmap() */ +enum { +	RMAP_EXCLUSIVE =
> > 1, +	RMAP_COMPOUND = 2, +};
> 
> Always a good idea to name things. However, "exclusive" is
> not that clear to me. Given that the argument is supposed
> to indicate whether we map a single or a compound page,
> maybe the names in the enum could just be SINGLE and COMPOUND?
> 
> Naming the enum should make it clear enough what it does:
> 
>  enum rmap_page {
>       SINGLE = 0,
>       COMPOUND
>  }

Okay, this is probably confusing: do_page_add_anon_rmap() already had one
of arguments called 'exclusive'. It indicates if the page is exclusively
owned by the current process. And I needed also to indicate if we need to
handle the page as a compound or not. I've reused the same argument and
converted it to set bit-flags: bit 0 is exclusive, bit 1 - compound.

> 
> > +++ b/kernel/events/uprobes.c @@ -183,7 +183,7 @@ static int
> > __replace_page(struct vm_area_struct *vma, unsigned long addr, goto
> > unlock;
> > 
> > get_page(kpage); -	page_add_new_anon_rmap(kpage, vma, addr); +
> > page_add_new_anon_rmap(kpage, vma, addr, false); 
> > mem_cgroup_commit_charge(kpage, memcg, false); 
> > lru_cache_add_active_or_unevictable(kpage, vma);
> 
> Would it make sense to use the name in the argument to that function,
> too?
> 
> I often find it a lot easier to see what things do if they use symbolic
> names, rather than by trying to remember what each boolean argument to
> a function does.

I can convert these compound booleans to enums if you want. I'm personally
not sure that if will bring much value.

-- 
 Kirill A. Shutemov
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