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SubjectRe: [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@infradead.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@suse.cz>
>
> after fixing these two.

Thanks!

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