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

    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)

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