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

[Russell King <rmk+lkml@arm.linux.org.uk>]

On Sat, Dec 30, 2006 at 10:26:20AM -0800, Linus Torvalds wrote:
> 
> 
> On Sat, 30 Dec 2006, Russell King wrote:
> > 
> > And here's the flush_anon_page() part.
> > 
> > Add flush_anon_page() for ARM, to avoid data corruption issues when using
> > fuse or other subsystems using get_user_pages().
> 
> Btw, since this doesn't actually change any code for anybody but ARM, just 
> adds a parameter that is obviously unused by everybody else, and if it 
> actually fixes a real bug for ARM, I'll obviously happily take it even 
> before 2.6.20. So go ahead put it in your ARM tree, and we'll get some 
> testing through that. And just ask me to pull at some point.
> 
> I wonder why nobody else seems to have a "flush_anon_page()"? This would 
> seem to be a potential issue for architectures like sparc too.. Although 
> maybe sparc can do a flush by physical index with "flush_dcache_page()".

Well...

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.

Looking at some of the other users of get_user_pages() which want to
access the current processes memory space, one finds the following:

some use flush_cache_page():
- binfmt_elf coredump
- ptrace (in arch code)

others don't:
- aio
- bio
- block (block_dev::blk_get_page seems to be for direct-io, there's problems
   reported with this on ARM)
- direct-io
- fuse
- vmsplice

So, anything except coredumps and ptrace are currently unsafe on ARM
without something being added to get_user_pages() to ensure coherency
of anonymous pages.

Given that we have reported corruption with direct-io, and debian bug
#402876 for nonworking fuse, it seems the correct thing to do is to
implement this function.

-- 
Russell King
 Linux kernel    2.6 ARM Linux   - http://www.arm.linux.org.uk/
 maintainer of: