linux-kernel - Re: [PATCH 1/4] rmap: introduce rmap_walk

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Date:	Wed, 3 Feb 2016 14:56:07 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:	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>,
	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>,
	Sasha Levin <sasha.levin@...cle.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 1/4] rmap: introduce rmap_walk_locked()

On Thu, 4 Feb 2016 01:45:07 +0300 "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com> wrote:

> On Wed, Feb 03, 2016 at 02:40:19PM -0800, Andrew Morton wrote:
> > On Wed,  3 Feb 2016 18:14:16 +0300 "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com> wrote:
> > 
> > > rmap_walk_locked() is the same as rmap_walk(), but caller takes care
> > > about relevant rmap lock. It only supports anonymous pages for now.
> > > 
> > > It's preparation to switch THP splitting from custom rmap walk in
> > > freeze_page()/unfreeze_page() to generic one.
> > > 
> > > ...
> > >
> > > +/* Like rmap_walk, but caller holds relevant rmap lock */
> > > +int rmap_walk_locked(struct page *page, struct rmap_walk_control *rwc)
> > > +{
> > > +	/* only for anon pages for now */
> > > +	VM_BUG_ON_PAGE(!PageAnon(page) || PageKsm(page), page);
> > > +	return rmap_walk_anon(page, rwc, true);
> > > +}
> > 
> > Should be rmap_walk_anon_locked()?
> 
> I leave interface open for further extension for file mappings, once it
> will be needed. Interface is mirroring plain rmap_walk()

hm, yes, I see.

> If you prefer to rename the function, I can do it too.

Well, what does "unlocked" mean in the context of rmap_walk_ksm() and
rmap_walk_file()?  That the caller holds totally different locks.  I
expect that sitting down and writing out the interface definition for
such an rmap_walk_locked() would reveal that we shouldn't have created
it.

I mean, if the caller is to call such an rmap_walk_locked(), he first
needs to work out if it's a ksm page or an anon page or a file page,
then take the appropriate lock and then call rmap_walk_locked(). 
That's silly - at this point he should directly call
rmap_walk_ksm_locked()?