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

[Blaisorblade]

On Friday 02 September 2005 22:17, 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.
Thanks for noticing - I had really this doubt when writing some code (in the 
patch, I've added a dirty PTEs on read accesses because I was unsure, and 
even because of my warning).

> This seems somewhat bogus to me.  If we set the pte protection to its
> limits, then the maybe_mkwrite is unneccesary.

> 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.

No, just replace pte_write() with is_write, as below. They might not coincide, 
but if on a write fault we return with a clean PTE, we'll loop indefinitely 
(experienced while hacking on remap_f_p), so the warning above is definitely 
correct.

       WARN_ON(!pte_young(*pte) || is_write && !pte_dirty(*pte));
> 				Heff

-- 
Inform me of my mistakes, so I can keep imitating Homer Simpson's "Doh!".
Paolo Giarrusso, aka Blaisorblade (Skype ID "PaoloGiarrusso", ICQ 215621894)
http://www.user-mode-linux.org/~blaisorblade

		
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