Re: [PATCHv4 04/25] rmap: support file thp

From: Kirill A. Shutemov
Date: Fri Mar 18 2016 - 21:01:16 EST

On Fri, Mar 18, 2016 at 03:10:06PM +0530, Aneesh Kumar K.V wrote:
> "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx> writes:
> > [ text/plain ]
> > Naive approach: on mapping/unmapping the page as compound we update
> > ->_mapcount on each 4k page. That's not efficient, but it's not obvious
> > how we can optimize this. We can look into optimization later.
> >
> > PG_double_map optimization doesn't work for file pages since lifecycle
> > of file pages is different comparing to anon pages: file page can be
> > mapped again at any time.
> >
> Can you explain this more ?. We added PG_double_map so that we can keep
> page_remove_rmap simpler. So if it isn't a compound page we still can do
> if (!atomic_add_negative(-1, &page->_mapcount))
> I am trying to understand why we can't use that with file pages ?

The first thing: for non-compound pages we still have simple
atomic_inc_and_test() / atomic_add_negative(-1), nothing changed here.

About compound pages:

For anon-THP PG_double_map allowed to not touch _mapcount in all subpages
until a PMD which maps the page is split. This way we significantly lower
overhead on refcounting as long as we have the page mapped with PMD-only,
since we only need to increment compound_mapcount().

The optimization is possible due to relatively simple lifecycle of
anonymous THP page:

- anon-THPs always mapped with PMD first;

- new mapping of THP can only be created via fork();

- the page only can get mapped with PTEs via split_huge_pmd();

For file-THP the situation is different. Once we allocated a huge page and
put it on radix tree, the page can be mapped with PTEs or PMDs at any
time. It makes the same optimization inapplicable there.

I think there *can* be some room for optimization, but I don't want to
invest more time here, until it's identified as bottleneck. It can lead to
more complex code on rmap side.

Kirill A. Shutemov