Re: fuse, get_user_pages, flush_anon_page, aliasing caches and all that again

[David Miller <davem@davemloft.net>]

From: Russell King <rmk+lkml@arm.linux.org.uk>
Date: Sat, 30 Dec 2006 22:46:04 +0000

> iirc, flush_anon_page() was introduced to fix non-working fuse on parisc,
> which occurs because fuse wants to use get_user_pages() to read data from
> the current processes memory space.
> 
> get_user_pages() contains a call to flush_dcache_page(), whose behaviour
> is defined for shared mappings.  Anonymous pages are unspecified.  It
> appears that flush_anon_page() was introduced to correct this oversight.

Sparc64 flushes anonymous pages too when flush_dcache_page() is
called on such pages.  It only tries to "defer" flushes for
pages which have a non-NULL page_mapping().  For NULL page_mapping()
we just flush immediately.

This works on sparc64, as sort of hinted by Linus, because we can
flush by physical page on sparc64.  I guess the lack of ability to
do that is why PARISC and ARM don't do that too.

For the ptrace() cases we created the copy_{to,from}_user_page()
interfaces.  So that when you access data inside of pages obtained via
a get_user_pages() call, you are supposed to use those two interfaces
so that everything works out right.

Therefore, FUSE probably could have been fixed by judicious use
of copy_{to,from}_user_page() calls instead of adding this new
ad-hoc flush_anon_page() thing.