[PATCH v5 5/7] proc/task_mmu: change proc_get_vma() to stop returning gate VMA at the end

[Suren Baghdasaryan <surenb@google.com>]

proc_get_vma() returning gate VMA at the end is desirable for the its
current m_start/m_next callers, as they need to report a gate VMA at the
end of the address space. This behavior is very specific to these callers
and makes proc_get_vma() hard to use for other purposes.

Move this usage-specific behavior into the callers themselves so that
proc_get_vma() returns either a valid VMA, an error or a NULL when no
more VMAs are available. This makes it more generic, simpler and usable
in the later patches.

Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
---
 fs/proc/task_mmu.c | 28 +++++++++++++++++++++++-----
 1 file changed, 23 insertions(+), 5 deletions(-)

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 8f2f302d4563..2c90b91d271b 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -240,9 +240,6 @@ static struct vm_area_struct *proc_get_vma(struct seq_file *m, loff_t *ppos)
 		 * found the extended vma with the same vm_start.
 		 */
 		*ppos = vma->vm_end;
-	} else {
-		*ppos = SENTINEL_VMA_GATE;
-		vma = get_gate_vma(priv->lock_ctx.mm);
 	}
 
 	return vma;
@@ -252,6 +249,7 @@ static void *m_start(struct seq_file *m, loff_t *ppos)
 {
 	struct proc_maps_private *priv = m->private;
 	struct proc_maps_locking_ctx *lock_ctx;
+	struct vm_area_struct *vma;
 	loff_t last_addr = *ppos;
 	struct mm_struct *mm;
 
@@ -281,19 +279,39 @@ static void *m_start(struct seq_file *m, loff_t *ppos)
 		*ppos = last_addr = priv->last_pos;
 	vma_iter_init(&priv->iter, mm, (unsigned long)last_addr);
 	hold_task_mempolicy(priv);
+	/*
+	 * If seq_file had to flush its collected data right after m_next() set
+	 * position to SENTINEL_VMA_GATE, m_start() will get that sentinel and
+	 * should return gate_vma without calling proc_get_vma().
+	 */
 	if (last_addr == SENTINEL_VMA_GATE)
 		return get_gate_vma(mm);
 
-	return proc_get_vma(m, ppos);
+	vma = proc_get_vma(m, ppos);
+	if (vma)
+		return vma;
+
+	/* Return gate VMA at the end */
+	*ppos = SENTINEL_VMA_GATE;
+	return get_gate_vma(mm);
 }
 
 static void *m_next(struct seq_file *m, void *v, loff_t *ppos)
 {
+	struct proc_maps_private *priv = m->private;
+	struct vm_area_struct *vma;
+
 	if (*ppos == SENTINEL_VMA_GATE) {
 		*ppos = SENTINEL_VMA_END;
 		return NULL;
 	}
-	return proc_get_vma(m, ppos);
+	vma = proc_get_vma(m, ppos);
+	if (vma)
+		return vma;
+
+	/* Return gate VMA at the end */
+	*ppos = SENTINEL_VMA_GATE;
+	return get_gate_vma(priv->lock_ctx.mm);
 }
 
 static void m_stop(struct seq_file *m, void *v)
-- 
2.55.0.1082.g2b9226bbc0-goog