Messages in this thread | | | From | Linus Torvalds <> | Date | Fri, 11 Aug 2017 13:50:07 -0700 | Subject | Re: [PATCH 2/2] mm,fork: introduce MADV_WIPEONFORK |
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On Fri, Aug 11, 2017 at 1:27 PM, Rik van Riel <riel@redhat.com> wrote: >> >> Yes, you don't do the page table copies. Fine. But you leave vma with >> the the anon_vma pointer - doesn't that mean that it's still >> connected >> to the original anonvma chain, and we might end up swapping something >> in? > > Swapping something in would require there to be a swap entry in > the page table entries, which we are not copying, so this should > not be a correctness issue.
Yeah, I thought the rmap code just used the offset from the start to avoid even doing swap entries, but I guess we don't actually ever populate the page tables without the swap entry being there.
> There is another test in copy_page_range already which ends up > skipping the page table copy when it should not be done.
Well, the VM_DONTCOPY test is in dup_mmap(), and I think I'd rather match up the VM_WIPEONFORK logic with VM_DONTCOPY than with the copy_page_range() tests.
Because I assume you are talking about the "if it's a shared mapping, we don't need to copy the page tables and can just do it at page fault time instead" part? That's a rather different thing, which isn't so much about semantics, as about just a trade-off about when to touch the page tables.
But yes, that one *might* make sense in dup_mmap() too. I just don't think it's really analogous to the WIPEONFORK and DONTCOPY tests.
Linus
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