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

[Aneesh Kumar K.V]

"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 ?

-aneesh