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 19:30:11 +0100
From:	Miklos Szeredi <miklos@...redi.hu>
To:	rmk+lkml@....linux.org.uk
CC:	miklos@...redi.hu, 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

> > > > 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.

> > > 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?

Right, I missed that.

Miklos
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