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    SubjectRe: [PATCH] ppc64: Fix possible race with set_pte on a present PTE
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    On Wed, 2004-05-26 at 14:08, Linus Torvalds wrote:

    > You're right. We do use it on the do_wp_page() path, and there we actually
    > use a whole new page in the "break_cow()" case. That case is in fact
    > fundamentally different from the other ones.
    >
    > So we should probably break up the "ptep_establish()" into its two pieces,
    > since the callers don't actually want to do the same thing. One really
    > wants to do a "clear old one, set a totally new one", and the two other
    > places want to actually update just the dirty and accessed bits.

    The first one could still be called "pte_establish" ... I mean, it makes
    little sense to continue calling "pte_establish" something that just
    does set one of those 2 bits... And the flush done by pte_establish in
    this case is useless on ppc... so I'd rather kill pte_establish
    completely instead and define it's arch (or generic) impl. of
    ptep_set_dirty_accessed() responsibility to do the TLB flush if
    necessary, no ? (well, a call to it on ppc isn't expensive if we didn't
    add anything to the batch anyway...)

    > In fact, the only non-generic user of "ptep_establish()" (s390) didn't
    > want to use the generic version exactly because of this very conceptual
    > bug. It uses "ptep_clear_flush()" for the replacement case, which actually
    > makes sense.
    >
    > So does it work if you do this appended patch first? This is a real
    > cleanup, and I think it will allow us to get rid of the s390-specific code
    > in ptep_establish(). Along with hopefully fixing your problem too.
    >
    > After this, we should be able to have a BUG() in "set_pte()" if the entry
    > wasn't clear before (assuming the arch doesn't use set_pte() for the dirty
    > updates etc).

    Ok, I'll give it a spin.

    > Linus
    >
    > ---
    > ===== mm/memory.c 1.177 vs edited =====
    > --- 1.177/mm/memory.c Tue May 25 12:37:09 2004
    > +++ edited/mm/memory.c Tue May 25 21:04:49 2004
    > @@ -1004,7 +1004,10 @@
    > flush_cache_page(vma, address);
    > entry = maybe_mkwrite(pte_mkdirty(mk_pte(new_page, vma->vm_page_prot)),
    > vma);
    > - ptep_establish(vma, address, page_table, entry, 1);
    > +
    > + /* Get rid of the old entry, replace with new */
    > + ptep_clear_flush(vma, address, page_table);
    > + set_pte(page_table, entry);
    > update_mmu_cache(vma, address, entry);
    > }
    >
    --
    Benjamin Herrenschmidt <benh@kernel.crashing.org>

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