Re: [PATCH] mm: fix possible cause of a page_mapped BUG

[Linus Torvalds <torvalds@linux-foundation.org>]

[-- Attachment #1: Type: text/plain, Size: 1701 bytes --]

On Tue, Apr 12, 2011 at 8:48 AM, Robert Święcki <robert@swiecki.net> wrote:
>>
>> Hmm. Sounds like an endless loop in kernel mode.
>>
>> Use "perf record -ag" as root, it should show up very clearly in the report.
>
> I've put some data here -
> http://groups.google.com/group/fa.linux.kernel/browse_thread/thread/4345dcc4f7750ce2
> - I think it's somewhat connected (sys_mlock appears on both cases).

Ok, so it's definitely sys_mlock.

And I suspect it's due to commit 53a7706d5ed8 somehow looping forever.

One possible cause would be how that commit made things care deeply
about the return value of __get_user_pages(), and in particular what
happens when that return value is zero. It ends up looping forever in
do_mlock_pages() for that case, because it does

                nend = nstart + ret * PAGE_SIZE;

so now the next round we'll set "nstart = nend" and start all over.

I see at least one way __get_user_pages() will return zero, and it's
if it is passed a npages of 0 to begin with. Which can easily happen
if you try to mlock() the first page of a stack segment: the code will
jump over that stack segment page, and then have nothing to do, and
return zero. So then do_mlock_pages() will just keep on trying again.

THIS IS A HACKY AND UNTESTED PATCH!

It's ugly as hell, because the real problem is do_mlock_pages() caring
too damn much about the return value, and us hiding the whole stack
page thing in that function. I wouldn't want to commit it as-is, but
if you can easily reproduce the problem, it's a good patch to test out
the theory. Assuming I didn't screw something up.

Again, TOTALLY UNTESTED!

                           Linus

[-- Attachment #2: patch.diff --]
[-- Type: text/x-patch, Size: 1066 bytes --]

 mm/mlock.c |   10 +++++++++-
 1 files changed, 9 insertions(+), 1 deletions(-)

diff --git a/mm/mlock.c b/mm/mlock.c
index 2689a08c79af..080c219973ea 100644
--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -162,6 +162,7 @@ static long __mlock_vma_pages_range(struct vm_area_struct *vma,
 	unsigned long addr = start;
 	int nr_pages = (end - start) / PAGE_SIZE;
 	int gup_flags;
+	long retval, offset;
 
 	VM_BUG_ON(start & ~PAGE_MASK);
 	VM_BUG_ON(end   & ~PAGE_MASK);
@@ -189,13 +190,20 @@ static long __mlock_vma_pages_range(struct vm_area_struct *vma,
 		gup_flags |= FOLL_MLOCK;
 
 	/* We don't try to access the guard page of a stack vma */
+	offset = 0;
 	if (stack_guard_page(vma, start)) {
 		addr += PAGE_SIZE;
 		nr_pages--;
+		offset = 1;
 	}
 
-	return __get_user_pages(current, mm, addr, nr_pages, gup_flags,
+	retval = __get_user_pages(current, mm, addr, nr_pages, gup_flags,
 				NULL, NULL, nonblocking);
+
+	/* Get the return value correct even in the face of the guard page */
+	if (retval < 0)
+		return offset ? : retval;
+	return retval + offset;
 }
 
 /*