Re: [RFC][PATCH 8/10] memcg: clean up charge/uncharge anon

[Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>]

Just to make sure.

> > Maybe there is something I don't understand..
> > IIUC, when page_remove_rmap() is called by do_wp_page(),
> > there must be pte(s) which points to the page and a pte is guarded by
> > page table lock. So, I think page_mapcount() > 0 before calling page_remove_rmap()
> > because there must be a valid pte, at least.
> > 
> > Can this scenario happen ?
> I think so. I intended to mention this case :)
> I'm sorry for my vague explanation.
> 
> > ==
> >     Thread A.                                      Thread B.
> > 
> >     do_wp_page()                                 do_swap_page()
> >        PageAnon(oldpage)                         
> >          lock_page()                             lock_page()=> wait.
> >          reuse = false.
> >          unlock_page()                           get lock.      
> >        do copy-on-write
> >        pte_same() == true
> >          page_remove_rmap(oldpage) (mapcount goes to -1)
> >                                                  page_set_anon_rmap() (new anon rmap again)
> > ==
> > Then, oldpage's mapcount goes down to 0 and up to 1 immediately.
> > 
I meant "process" not "thread".
I think this cannot happen in the case of threads, because these page_remove_rmap()
and page_set_anon_rmap() are called under pte lock(they share the pte).


Thanks,
Daisuke Nishimura.

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