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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