Messages in this thread | | | Date | Fri, 5 Oct 2007 16:33:16 +0100 (BST) | From | Hugh Dickins <> | Subject | Re: race with page_referenced_one->ptep_test_and_clear_young and pagetable setup/pulldown |
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On Fri, 5 Oct 2007, Keir Fraser wrote: > On 5/10/07 10:05, "Jeremy Fitzhardinge" <jeremy@goop.org> wrote: > > Andi says: > >> Do I misread that patch or does it really walk the complete address > >> space and try to take all possible locks? Isn't that very slow? > > > > That's pretty much what it has to do. Pinning/unpinning walks the whole > > pagetable anyway, so it shouldn't be much more expensive. And they're > > relatively rare operations (fork, exec, exit). > > It is a shame to do 3x walks per pin or unpin, rather than 1x, though. > > One way to improve this, possibly, is to pin the pte tables individually as > you go, rather than doing one big pin/unpin just at the root pgd. Then you > can lock/unlock the pte's as you go. I'd suggest that as a possible post > 2.6.23 improvement, however. Jan's patch has actually had some testing.
A few points come to mind looking at Jan's patch:
The comment about nested pagetable locks is wrong: mm/mremap.c does nest pagetable locks; but you wouldn't (as I understand it) be doing this pinning/unpinning anywhere which could race with an mremap() on the same mm, so that's a niggle about a comment, not a real issue.
Yes, it would be greatly preferable to take the locks one by one as needed. As it stands, I think you're in danger of overflowing the PREEMPT_BITS 8 area of preempt_count(), venturing into the SOFTIRQ area: I don't know the real-life consequence of that.
I don't see any protection against hugetlb areas, where the pmd entry may indicate a hugetlb page rather than a pagetable page. I guess you'll be needing to test pte_huge(). I don't know if you want to lock those or skip them: locking is usually just with page_table_lock, but beware there's also sharing of huge page pmds between mms - Ken Chen should be able to help on that.
If a 2.6.23 fix is needed, I suggest simply excluding split ptlocks in the Xen case, as shown by the mm/Kconfig - line in Jan's patch.
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