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

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Message-Id: <20061230.212338.92583434.davem@davemloft.net>
Date:	Sat, 30 Dec 2006 21:23:38 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	rmk+lkml@....linux.org.uk
Cc:	torvalds@...l.org, miklos@...redi.hu, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org, akpm@...l.org
Subject: Re: fuse, get_user_pages, flush_anon_page, aliasing caches and all
 that again

From: Russell King <rmk+lkml@....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.
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