Messages in this thread | | | Date | Sun, 1 Jul 2007 09:54:45 +0100 (BST) | From | Hugh Dickins <> | Subject | Re: [patch 5/5] Optimize page_mkclean_one |
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On Sun, 1 Jul 2007, Martin Schwidefsky wrote: > > > > Expect you're right, but I _really_ don't want to comment, when I don't > > understand that "|| pte_write" in the first place, and don't know the > > consequence of pte_dirty && !pte_write or !pte_dirty && pte_write there. > > The pte_write() part is for the shared dirty page tracking. If you want > to make sure that a max of x% of your pages are dirty then you cannot > allow to have more than x% to be writable. Thats why page_mkclean_one > clears the dirty bit and makes the page read-only.
The whole of page_mkclean_one is for the dirty page tracking: so it's obvious why it tests pte_dirty, but not obvious why it tests pte_write.
> > > My suspicion is that the "|| pte_write" is precisely to cover your > > s390 case where pte is never dirty (it may even have been me who got > > Peter to put it in for that reason). In which case your patch would > > be fine - though I think it'd be improved a lot by a comment or > > rearrangement or new macro in place of the pte_dirty || pte_write > > line (perhaps adjust my pte_maybe_dirty in asm-generic/pgtable.h, > > and use that - its former use in msync has gone away now). > > No, s390 is covered by the page_test_dirty / page_clear_dirty pair in > page_mkclean.
That's where its dirty page count comes from, yes: but since the s390 pte_dirty just says no, if page_mkclean_one tested only pte_dirty, then it wouldn't do anything on s390, and in particular wouldn't write protect the ptes to re-enforce dirty counting from then on.
So in answering your denials, I grow more confident that the pte_write test is precisely for the s390 case. Though it might also be to cover some defect in the write-protection scheme on other arches.
Come to think of it, would your patch really make any difference? Although page_mkclean's "count" of dirty ptes on s390 will be nonsense, that count would anyway be unknown, and it's only used as a boolean; and now I don't think your patch changes the boolean value - if any pte is found writable (and if the scheme is working) that implies that the page was written to, and so should give the same answer as the page_test_dirty.
But I could easily be overlooking something: Peter will recall.
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