linux-kernel - Re: [PATCH v2] page cache: Store only head pages in i

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Date:   Wed, 13 Feb 2019 12:17:15 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Jan Kara <jack@...e.cz>
Cc:     "Kirill A . Shutemov" <kirill@...temov.name>, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Hugh Dickins <hughd@...gle.com>,
        William Kucharski <william.kucharski@...cle.com>
Subject: Re: [PATCH v2] page cache: Store only head pages in i_pages

On Wed, Feb 13, 2019 at 03:41:02PM +0100, Jan Kara wrote:
> On Tue 12-02-19 10:34:54, Matthew Wilcox wrote:
> > Transparent Huge Pages are currently stored in i_pages as pointers to
> > consecutive subpages.  This patch changes that to storing consecutive
> > pointers to the head page in preparation for storing huge pages more
> > efficiently in i_pages.
> > 
> > Large parts of this are "inspired" by Kirill's patch
> > https://lore.kernel.org/lkml/20170126115819.58875-2-kirill.shutemov@linux.intel.com/
> > 
> > Signed-off-by: Matthew Wilcox <willy@...radead.org>
> 
> I like the idea!

Thanks!  It's a step towards reducing the overhead of huge pages in the
page cache from (on x86) 520 pointers to a mere 8.  Still not as good as
hugetlbfs, but I'm working on that too.

> > -		pages[ret] = page;
> > +		pages[ret] = find_subpage(page, xas.xa_index);
> >  		if (++ret == nr_pages) {
> >  			*start = page->index + 1;
> >  			goto out;
> >  		}
> 
> So this subtly changes the behavior because now we will be returning in
> '*start' a different index. So you should rather use 'pages[ret]->index'
> instead.

You're right, I made a mistake there.  However, seeing this:
https://lore.kernel.org/lkml/20190110030838.84446-1-yuzhao@google.com/

makes me think that I should be using xa_index + 1 there.

> Otherwise the patch looks good to me so feel free to add:
> 
> Acked-by: Jan Kara <jack@...e.cz>
> 
> after fixing these two.

Thanks!