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

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

On Thu, Dec 21, 2006 at 06:55:47PM +0100, Miklos Szeredi wrote:
> > > Yes, note the flush_dcache_page() call in fuse_copy_finish().  That
> > > could be replaced by the flush_kernel_dcache_page() (added by James
> > > Bottomley together with flush_anon_page()) when all relevant
> > > architectures have defined it.
> > 
> > I should say that flush_anon_page() in its current form is going to be
> > problematic for ARM.  It is passed:
> > 
> > 1. the struct page
> > 2. the virtual address in process memory for the page
> > 
> > It is not passed the mm or vma.  This means that we have no idea whether
> > the virtual address is in the currently mapped VM space or not.  The
> > common use of get_area_pages() is to get pages from other address
> > spaces.
> 
> I'm not sure I understand.  flush_anon_page() needs only to flush the
> mapping for the given virtual address, no?

Yes, but that virtual /user/ address is meaningless without knowing
which process address space it belongs to.

> It's always mapped at that address (since it was just accessed through
> that).

No.  Consider ptrace() (invoked by gdb) reading data from another
processes address space to obtain structure data or instructions.

> Any other mappings
> of the anonymous page are irrelevant, they don't need to be flushed.

Again, incorrect.  Consider if the page you're accessing is a file-
backed page, and is mapped into a process using a shared mapping.
Because you've written to the file, those shared mappings need to see
that write, and the interface for achieving that is flush_dcache_page().
If not, data loss can occur.

> > If we use the supplied virtual address to perform cache maintainence of
> > the userspace mapping, we might end up hitting a completely different
> > processes address space, which may contain some page sensitive to such
> > operations, or may not contain any page and thereby could cause a page
> > fault on some ARM CPUs.
> 
> I think calling get_user_pages() from a different process' address
> space simply doesn't make any sense.

That was it's main use - to implement ptrace() to read other processes
address spaces.  Why do you think it takes a task_struct and mm_struct?

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