From: Larry Woodman <lwoodman@redhat.com>
Date: Thu, 2 Dec 2010 18:18:47 -0500
Subject: [mm] backport upstream stack guard page /proc reporting
Message-id: <1291313927.3214.13.camel@dhcp-100-19-198.bos.redhat.com>
Patchwork-id: 29823
O-Subject: [RHEL5.6 Patch] Backport upstream/RHEL6 stack guard page reporting
	through /proc/self/maps
Bugzilla: 643426
RH-Acked-by: Johannes Weiner <jweiner@redhat.com>

RHEL5 is missing the logic to properly report the size of the user stack
and omit only one guard page when the user stack spans multiple vma's
marked as VM_GROWSDOWN.  The RHEL5 show_map_internal() routine blindly
omits the first page from any VM_GROWSDOWN vma even if the previous vma
is also marked VM_GROWSDOWN and is adjacent.  This causes the user stack
size and bounds reported via /proc/self/maps to be incorrect and
programs(like LVM) which mlock()/munlock() the address space to do the
wrong thing.

In RHEL6 /proc/self/map uses show_map_vma() to display VMAs:

--------------------------------------------------------------------------------------
static void show_map_vma(struct seq_file *m, struct vm_area_struct *vma)
{
...
        /* We don't show the stack guard page in /proc/maps */
        start = vma->vm_start;
        if (vma->vm_flags & VM_GROWSDOWN)
                if (!vma_stack_continue(vma->vm_prev, vma->vm_start))
                        start += PAGE_SIZE;
...
static inline int vma_stack_continue(struct vm_area_struct *vma,
unsigned long addr)
{
        return vma && (vma->vm_end == addr) && (vma->vm_flags &
VM_GROWSDOWN);
}
---------------------------------------------------------------------------------------

In RHEL5 /proc/self/map uses show_map_vma() to display VMAs but it
doesnt consider adjacent VM_GROWSDOWN VMAs:

---------------------------------------------------------------------------------------
static int show_map_internal(struct seq_file *m, void *v, struct
mem_size_stats *mss)

        /* We don't show the stack guard page in /proc/maps */
        start = vma->vm_start;
        if (vma->vm_flags & VM_GROWSDOWN) {
                start += PAGE_SIZE;
---------------------------------------------------------------------------------------

Since RHEL5 does not have doubly linked VMAs I had to call
find_vma_prev() in the backport.

Upstream commit 39aa3cb3e8250db9188a6f1e3fb62ffa1a717678, fixes
BZ643426.

Signed-off-by: Jarod Wilson <jarod@redhat.com>

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 707cb78..6767d7a 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -182,8 +182,14 @@ static int show_map_internal(struct seq_file *m, void *v, struct mem_size_stats
 
 	/* We don't show the stack guard page in /proc/maps */
 	start = vma->vm_start;
-	if (vma->vm_flags & VM_GROWSDOWN)
-		start += PAGE_SIZE;
+	if (vma->vm_flags & VM_GROWSDOWN) {
+		struct vm_area_struct *prev;
+
+		/* Is the vma a continuation of the stack vma above it? */
+		find_vma_prev(vma->vm_mm, start, &prev);
+		if (!(prev && prev->vm_end == start && prev->vm_flags & VM_GROWSDOWN))
+			start += PAGE_SIZE;
+	}
 
 	seq_printf(m, "%08lx-%08lx %c%c%c%c %08lx %02x:%02x %lu %n",
 			start,