Messages in this thread | | | From | Blaisorblade <> | Subject | Re: [uml-devel] Re: [patch 1/3] uml: share page bits handling between 2 and 3 level pagetables | Date | Sun, 4 Sep 2005 13:33:30 +0200 |
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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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