Re: x86 ptep_get_and_clear question

[Ben LaHaise <bcrl@redhat.com>]

On Thu, 15 Feb 2001, Kanoj Sarcar wrote:

> continue with my previous example, instead of pulling new examples.
>
> Look in mm/mprotect.c. Look at the call sequence change_protection() -> ...
> change_pte_range(). Specifically at the sequence:
>
> 	entry = ptep_get_and_clear(pte);
> 	set_pte(pte, pte_modify(entry, newprot));
>
> Go ahead and pull your x86 specs, and prove to me that between the
> ptep_get_and_clear(), which zeroes out the pte (specifically, when the
> dirty bit is not set), processor 2 can not come in and set the dirty
> bit on the in-memory pte. Which immediately gets overwritten by the
> set_pte(). For an example of how this can happen, look at my previous
> postings.

Look at the specs.  The processor uses read-modify-write cycles to update
the accessed and dirty bits.  If the in memory pte is either not present
or writable, the processor will take a page fault.

> Jamie's example misses the point in the sense that at the very beginning,
> when he says "Processor 2 has recently done some writes", processor 2 has
> made sure that the dirty bit is set in the in-memory pte. So, although
> processor 1 clears the entire pte, the set_pte() will set the dirty bit,
> and no information is lost. Even if processor 2 tries writing between
> the ptep_get_and_clear() and set_pte(). Whether Jamie was trying to
> illustrate a different problem, I am not sure. All I am trying to say
> is that the "dirty bit lost on smp x86" still exists, ptep_get_and_clear
> does not do anything to fix it.

Yes it does.  Write a test program like I did.  The processor does take a
page fault.

		-ben

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