Messages in this thread | | | Date | Tue, 10 Dec 2013 21:04:26 +0100 | From | Oleg Nesterov <> | Subject | Re: [PATCH 0/1] uprobes: Kill __replace_page(), change uprobe_write_opcode() to rely on gup(WRITE) |
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On 12/10, Linus Torvalds wrote: > > On Tue, Dec 10, 2013 at 11:18 AM, Oleg Nesterov <oleg@redhat.com> wrote: > > > > Well, if we use get_user_pages() we have vma for free. > > No we don't. get_user_pages() is expensive as hell.
I understand, I said "if we use get_user_pages()".
> We'd be *much* better off using get_user_pages_fast() if possible - > and I bet _is_ possible in 99% of all cases.
We can't. get_user_pages_fast() takes mmap_sem.
Perhaps we can do something, say add FOLL_FAST_ONLY, but until then we need the 1st get_user_pages(write => 0) anyway.
> > But I am not sure about "and verify it for now" above. Do you mean > > that uprobe_write_opcode() should do another valid_vma() ? > > No. I think we should just do something like > > if (vma->vm_flags & (VM_SHARED | VM_HUGETLB)) > return -EINVAL;
OK, but WARN_ON(VM_SHARED | VM_HUGETLB) looks better imho. Because this should not happen.
> then just looking up the page table _once_ - instead of > three times - would make the code safer and faster.
Unlikely we can do this only once... At least I do not see how.
> It *may* be safe if callers do > all the right black magic, but it's by no means obvious.
I agree, this needs more documentation in any case.
Oleg.
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