linux-kernel - Re: [PATCHv4 14/39] thp, mm: rewrite delete_from_page

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Message-Id: <20130528122812.0D624E0090@blue.fi.intel.com>
Date:	Tue, 28 May 2013 15:28:12 +0300 (EEST)
From:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:	Dave Hansen <dave@...1.net>
Cc:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Hugh Dickins <hughd@...gle.com>,
	Wu Fengguang <fengguang.wu@...el.com>, Jan Kara <jack@...e.cz>,
	Mel Gorman <mgorman@...e.de>, linux-mm@...ck.org,
	Andi Kleen <ak@...ux.intel.com>,
	Matthew Wilcox <matthew.r.wilcox@...el.com>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Hillf Danton <dhillf@...il.com>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCHv4 14/39] thp, mm: rewrite delete_from_page_cache() to
 support huge pages

Dave Hansen wrote:
> On 05/11/2013 06:23 PM, Kirill A. Shutemov wrote:
> > From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
> > 
> > As with add_to_page_cache_locked() we handle HPAGE_CACHE_NR pages a
> > time.
> > 
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> > ---
> >  mm/filemap.c |   31 +++++++++++++++++++++++++------
> >  1 file changed, 25 insertions(+), 6 deletions(-)
> > 
> > diff --git a/mm/filemap.c b/mm/filemap.c
> > index b0c7c8c..657ce82 100644
> > --- a/mm/filemap.c
> > +++ b/mm/filemap.c
> > @@ -115,6 +115,9 @@
> >  void __delete_from_page_cache(struct page *page)
> >  {
> >  	struct address_space *mapping = page->mapping;
> > +	bool thp = PageTransHuge(page) &&
> > +		IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE_PAGECACHE);
> > +	int nr;
> 
> Is that check for the config option really necessary?  How would we get
> a page with PageTransHuge() set without it being enabled?

I'll drop it and use hpagecache_nr_page() instead.

> I like to rewrite your code. :)

It's nice. Thanks.

> Which reminds me...  Why do we handle their reference counts differently? :)
> 
> It seems like we could easily put a for loop in delete_from_page_cache()
> that will release their reference counts along with the head page.
> Wouldn't that make the code less special-cased for tail pages?

delete_from_page_cache() is not the only user of
__delete_from_page_cache()...

It seems I did it wrong in add_to_page_cache_locked(). We shouldn't take
references on tail pages there, only one on head. On split it will be
distributed properly.

> >  	/* Leave page->index set: truncation lookup relies upon it */
> > -	mapping->nrpages--;
> > -	__dec_zone_page_state(page, NR_FILE_PAGES);
> > +	mapping->nrpages -= nr;
> > +	__mod_zone_page_state(page_zone(page), NR_FILE_PAGES, -nr);
> >  	if (PageSwapBacked(page))
> > -		__dec_zone_page_state(page, NR_SHMEM);
> > +		__mod_zone_page_state(page_zone(page), NR_SHMEM, -nr);
> >  	BUG_ON(page_mapped(page));
> 
> Man, we suck:
> 
> 	__dec_zone_page_state()
> and
> 	__mod_zone_page_state()
> 
> take a differently-typed first argument.  <sigh>
> 
> Would there be any good to making __dec_zone_page_state() check to see
> if the page we passed in _is_ a compound page, and adjusting its
> behaviour accordingly?

Yeah, it would be better but I think it outside the scope of the patchset.
Probably, later.

-- 
 Kirill A. Shutemov
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