[patch] mm: mlock: document scary-looking stack expansion mlock chain

[Johannes Weiner <hannes@cmpxchg.org>]

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@ZenIV.linux.org.uk>
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
---
 mm/mlock.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/mm/mlock.c b/mm/mlock.c
index b1647fb..78c4924 100644
--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -185,6 +185,10 @@ 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

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