[PATCH] mlock: operate on any regions with protection != PROT_NONE

[Michel Lespinasse <walken@google.com>]

As Tao Ma noticed, change 5ecfda0 breaks blktrace. This is because
blktrace mmaps a file with PROT_WRITE permissions but without PROT_READ,
so my attempt to not unnecessarity break COW during mlock ended up
causing mlock to fail with a permission problem.

I am proposing to let mlock ignore vma protection in all cases except
PROT_NONE. In particular, mlock should not fail for PROT_WRITE regions
(as in the blktrace case, which broke at 5ecfda0) or for PROT_EXEC
regions (which seem to me like they were always broken).

Please review. I am proposing this as a candidate for 2.6.38 inclusion,
because of the behavior change with blktrace.

diff --git a/mm/mlock.c b/mm/mlock.c
index 13e81ee..c3924c7f 100644
--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -178,6 +178,13 @@ static long __mlock_vma_pages_range(struct vm_area_struct *vma,
 	if ((vma->vm_flags & (VM_WRITE | VM_SHARED)) == VM_WRITE)
 		gup_flags |= FOLL_WRITE;
 
+	/*
+	 * We want mlock to succeed for regions that have any permissions
+	 * other than PROT_NONE.
+	 */
+	if (vma->vm_flags & (VM_READ | VM_WRITE | VM_EXEC))
+		gup_flags |= FOLL_FORCE;
+
 	if (vma->vm_flags & VM_LOCKED)
 		gup_flags |= FOLL_MLOCK;
 

-- 
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.

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