On Fri, 2017-08-11 at 12:42 -0700, Linus Torvalds wrote: > On Fri, Aug 11, 2017 at 12:19 PM, <riel@xxxxxxxxxx> wrote: > > diff --git a/mm/memory.c b/mm/memory.c > > index 0e517be91a89..f9b0ad7feb57 100644 > > --- a/mm/memory.c > > +++ b/mm/memory.c > > @@ -1134,6 +1134,16 @@ int copy_page_range(struct mm_struct > > *dst_mm, struct mm_struct *src_mm, > > !vma->anon_vma) > > return 0; > > > > + /* > > + * With VM_WIPEONFORK, the child inherits the VMA from the > > + * parent, but not its contents. > > + * > > + * A child accessing VM_WIPEONFORK memory will see all > > zeroes; > > + * a child accessing VM_DONTCOPY memory receives a > > segfault. > > + */ > > + if (vma->vm_flags & VM_WIPEONFORK) > > + return 0; > > + > > Is this right? > > 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. > And even if that ends up not being an issue, I'd expect that you'd > want to break the anon_vma chain just to not make it grow > unnecessarily. This is a good point. I can send a v4 that skips the anon_vma_fork() call if VM_WIPEONFORK, and calls anon_vma_prepare(), instead. > So my gut feel is that doing this in "copy_page_range()" is wrong, > and > the logic should be moved up to dup_mmap(), where we can also > short-circuit the anon_vma chain entirely. > > No? There is another test in copy_page_range already which ends up skipping the page table copy when it should not be done. If you want, I can move that test into a should_copy_page_range() function, and call that from dup_mmap(), skipping the call to copy_page_range() if should_copy_page_range() returns false. Having only one of the two sets of tests in dup_mmap(), and the other in copy_page_range() seems wrong. Just let me know what you prefer, and I'll put that in v4. > The madvice() interface looks fine to me. That was the main reason for adding you to the thread :) kind regards, Rik -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
