[uml-devel] Re: [patch 1/3] uml: share page bits handling between 2 and 3 level pagetables

[Hugh Dickins <hugh@veritas.com>]

On Fri, 2 Sep 2005, Jeff Dike wrote:
> On Wed, Aug 10, 2005 at 09:37:28PM +0200, Blaisorblade wrote:
> > Also look, on the "set_pte" theme, at the attached patch. 
> 
> +       WARN_ON(!pte_young(*pte) || pte_write(*pte) && !pte_dirty(*pte));
> 
> This one has been firing on me, and I decided to figure out why.  The
> culprit is this code in do_no_page:
> 
> 	if (pte_none(*page_table)) {
> 		if (!PageReserved(new_page))
> 			inc_mm_counter(mm, rss);
> 
> 		flush_icache_page(vma, new_page);
> 		entry = mk_pte(new_page, vma->vm_page_prot);
> 		if (write_access)
> 			entry = maybe_mkwrite(pte_mkdirty(entry), vma);
> 		set_pte_at(mm, address, page_table, entry);
> 
> The first mk_pte immediately sets the pte to the protection limits of
> the VMA, regardless of the access type.  So, if it's a read access on
> a writeable page, we get a writeable, but not dirty pte, since the
> mkdirty never happens.  The exercises the warning you added.
> 
> This seems somewhat bogus to me.  If we set the pte protection to its
> limits, then the maybe_mkwrite is unneccesary.  
> 
> If we are the process in this address space, and we have a write
> access, then the maybe_mkwrite doesn't do anything because the pte is
> already writeable because the VMA has to be writeable, or we would
> have been faulted already.

Not at all.  The private, COW areas.  They may be writeable in the sense
that VM_WRITE is set, but pte_write permission cannot be in vm_page_prot,
or we'd never get the fault when to Copy On Write.

What is bogus, I think, are those places where we do the reverse:
like do_anonymous_page's pte_wrprotect of its mkpte of the ZERO_PAGE.

> If we are a debugger changing the process memory, then the vma may be
> read-only, and maybe_mkwrite is explicitly a no-op in this case.
> 
> This doesn't seem to harm our dirty bit emulation.  fix_range_common
> checks the dirty and accessed bits and disables read and write
> protection as appropriate.
> 
> So, it seems like the warning could be dropped, or perhaps made more
> selective, like checking for is_write == 0 and VM_WRITE, but then the
> test is getting complicated.
> 
> 				Heff

Jugh


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