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

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Date:	Thu, 21 Dec 2006 18:55:12 +0000
From:	Russell King <rmk+lkml@....linux.org.uk>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org
Subject: Re: fuse, get_user_pages, flush_anon_page, aliasing caches and all that again

On Thu, Dec 21, 2006 at 07:30:11PM +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.
> 
> Yes, for file backed pages.  But flush_anon_page() only needs to deal
> with anonymous (not file backed) pages.

Ignore my final paragraph, it was clearly wrong; I was thinking about
the flush after you've written to the page.

However, I continue to assert that I require the VMA to implement
flush_anon_page() since a userspace address without knowing which
userspace it corresponds with is utterly useless for cache maintainence
purposes.

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