Re: [PATCH 3.18 32/32] mm: larger stack guard gap, between vmas

[Willy Tarreau <w@1wt.eu>]

On Tue, Jun 20, 2017 at 10:49:16PM -0700, Hugh Dickins wrote:
> On Mon, 19 Jun 2017, Greg Kroah-Hartman wrote:
> 
> > 3.18-stable review patch.  If anyone has any objections, please let me know.
> > 
> > ------------------
> > 
> > From: Hugh Dickins <hughd@google.com>
> > 
> > commit 1be7107fbe18eed3e319a6c3e83c78254b693acb upstream.
> 
> Here's a few adjustments to the 3.18 patch: no doubt you'll have
> already sorted out any build errors (and I have to confess that
> I haven't even tried to build this); and the VM_WARN_ON line (as
> in 4.4) only fixes a highly unlikely error; but those FOLL_MLOCK
> lines in mm/gup.c were mistaken, and do need to be deleted.

Are you sure ? The test on the FOLL_MLOCK flag remains present in
4.11 and mainline :

 	/* mlock all present pages, but do not fault in new pages */
 	if ((*flags & (FOLL_POPULATE | FOLL_MLOCK)) == FOLL_MLOCK)
 		return -ENOENT;

And this test was present although different in 3.18 as well :

	/* For mlock, just skip the stack guard page. */
	if ((*flags & FOLL_MLOCK) &&
			(stack_guard_page_start(vma, address) ||
			 stack_guard_page_end(vma, address + PAGE_SIZE)))

So by removing it we're totally removing any test on FOLL_MLOCK. That
might be the correct fix, but I'm just a bit surprized since the mainline
patch doesn't remove it, and only removes the test on FOLL_POPULATE.

Thanks,
Willy