mmap locking API: convert nested write lock sites

Add API for nested write locks and convert the few call sites doing that.
Signed-off-by: Michel Lespinasse <walken@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Daniel Jordan <daniel.m.jordan@oracle.com>
Reviewed-by: Laurent Dufour <ldufour@linux.ibm.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Davidlohr Bueso <dbueso@suse.de>
Cc: David Rientjes <rientjes@google.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Jerome Glisse <jglisse@redhat.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Liam Howlett <Liam.Howlett@oracle.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ying Han <yinghan@google.com>
Link: http://lkml.kernel.org/r/20200520052908.204642-7-walken@google.comSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/um/include/asm/mmu_context.h

@@ -8,6 +8,7 @@
 
 #include <linux/sched.h>
 #include <linux/mm_types.h>
+#include <linux/mmap_lock.h>
 
 #include <asm/mmu.h>
 
@@ -47,7 +48,7 @@ static inline void activate_mm(struct mm_struct *old, struct mm_struct *new)
 	 * when the new ->mm is used for the first time.
 	 */
 	__switch_mm(&new->context.id);
-	down_write_nested(&new->mmap_sem, 1);
+	mmap_write_lock_nested(new, SINGLE_DEPTH_NESTING);
 	uml_setup_stubs(new);
 	mmap_write_unlock(new);
 }

include/linux/mmap_lock.h

@@ -11,6 +11,11 @@ static inline void mmap_write_lock(struct mm_struct *mm)
 	down_write(&mm->mmap_sem);
 }
 
+static inline void mmap_write_lock_nested(struct mm_struct *mm, int subclass)
+{
+	down_write_nested(&mm->mmap_sem, subclass);
+}
+
 static inline int mmap_write_lock_killable(struct mm_struct *mm)
 {
 	return down_write_killable(&mm->mmap_sem);

kernel/fork.c

@@ -501,7 +501,7 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
 	/*
 	 * Not linked in yet - no deadlock potential:
 	 */
-	down_write_nested(&mm->mmap_sem, SINGLE_DEPTH_NESTING);
+	mmap_write_lock_nested(mm, SINGLE_DEPTH_NESTING);
 
 	/* No ordering required: file already has been exposed. */
 	RCU_INIT_POINTER(mm->exe_file, get_mm_exe_file(oldmm));