On Sun, Dec 16, 2012 at 05:04:03PM +0000, Al Viro wrote: > BTW, the __get_user_pages()/find_extend_vma()/mlock_vma_pages_range() pile is > really asking for trouble; sure, the recursion there is limited, but it > deserves a comment. Agreed, how about this? --- From: Johannes Weiner <hannes@xxxxxxxxxxx> Subject: [patch] mm: mlock: document scary-looking stack expansion mlock chain The fact that mlock calls get_user_pages, and get_user_pages might call mlock when expanding a stack looks like a potential recursion. However, mlock makes sure the requested range is already contained within a vma, so no stack expansion will actually happen from mlock. Should this ever change: the stack expansion mlocks only the newly expanded range and so will not result in recursive expansion. Reported-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx> Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx> --- mm/mlock.c | 4 ++++ 1 file changed, 4 insertions(+) diff --git a/mm/mlock.c b/mm/mlock.c index f0b9ce5..17cf905 100644 --- a/mm/mlock.c +++ b/mm/mlock.c @@ -186,6 +186,10 @@ static long __mlock_vma_pages_range(struct vm_area_struct *vma, if (vma->vm_flags & (VM_READ | VM_WRITE | VM_EXEC)) gup_flags |= FOLL_FORCE; + /* + * We made sure addr is within a VMA, so the following will + * not result in a stack expansion that recurses back here. + */ return __get_user_pages(current, mm, addr, nr_pages, gup_flags, NULL, NULL, nonblocking); } -- 1.7.11.7 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>