Messages in this thread | | | Date | Tue, 2 Aug 2005 17:03:12 +0100 (BST) | From | Hugh Dickins <> | Subject | Re: [patch 2.6.13-rc4] fix get_user_pages bug |
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On Tue, 2 Aug 2005, Linus Torvalds wrote: > > On the other hand, this being s390, maybe nobody cares?
You have a cruel streak.
But have I just realized a non-s390 problem with your pte_dirty technique? The ptep_set_wrprotect in fork's copy_one_pte.
That's specifically write-protecting the pte to force COW, but leaving the dirty bit: so now get_user_pages will skip COW-ing it (in all write cases, not just the peculiar ptrace force one).
We really do need to COW those in get_user_pages, don't we? And although we could handle it by clearing dirty and doing a set_page_dirty, it's a path we don't want to clutter further.
(Yes, there's another issue of a fork occurring while the pages are already under get_user_pages, which is a significant issue for InfiniBand; but I see that as a separate kind of race, which we can reasonably disregard in this discussion - though it will need proper attention, perhaps through Michael Tsirkin's PROT_DONTCOPY patch.)
The simple answer to Robin's pagetable update race is to say that anyone using ptrace should be prepared for a pt race ;)
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