Re: [PATCH 1/1] Document get_user_pages_fast()

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

> > There is one problem.
> > gup and get_page() don't provide any page pinning.
> > 
> > get_page() only gurantee not freed. but don't gurantee to prevent page
> > reclaim
> 
> is_page_cache_freeable() measures page_count()...

sorry, my explanation was too short. 

reclaim can unmap the page. but page migration assume unmap successing mean
page migratable, then user-address associate another page and lost writing data
to old page.

IOW

     CPU0              CPU1                 note
------------------------------------------------------
    gup()
                    page-migration         user-address attach another page
 write data to                             write to old page.
 gup()ed page                              userland can't see it.

   put_page()                              free old page.


> 
> > and Cow change owner process.
> 
> hm.  buggy userspace? Why is that a problem from a kernel perspective?

We already started to discuttion in "[PATCH] fork vs gup(-fast) fix" thread.
but this is very difficult problem. it is still in progress ;)



> > Yes, current all gup caller have serious bug. especially direct-io, aio, bio
> > are broken.