mm/cma.c: remove redundant cma_mutex lock

The cma_mutex which protects alloc_contig_range() was first appeared in
commit 7ee793a62f ("cma: Remove potential deadlock situation"), at that
time, there is no guarantee the behavior of concurrency inside
alloc_contig_range().

After commit 2c7452a075 ("mm/page_isolation.c: make
start_isolate_page_range() fail if already isolated")

  > However, two subsystems (CMA and gigantic
  > huge pages for example) could attempt operations on the same range.  If
  > this happens, one thread may 'undo' the work another thread is doing.
  > This can result in pageblocks being incorrectly left marked as
  > MIGRATE_ISOLATE and therefore not available for page allocation.

The concurrency inside alloc_contig_range() was clarified.

Now we can find that hugepage and virtio call alloc_contig_range() without
any lock, thus cma_mutex is "redundant" in cma_alloc() now.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Lecopzer Chen <[email protected]>
Acked-by: David Hildenbrand <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
Cc: Matthias Brugger <[email protected]>
Cc: YJ Chiang <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/cma.c

@@ -38,7 +38,6 @@
 
 struct cma cma_areas[MAX_CMA_AREAS];
 unsigned cma_area_count;
-static DEFINE_MUTEX(cma_mutex);
 
 phys_addr_t cma_get_base(const struct cma *cma)
 {
@@ -454,10 +453,9 @@ struct page *cma_alloc(struct cma *cma, size_t count, unsigned int align,
 		mutex_unlock(&cma->lock);
 
 		pfn = cma->base_pfn + (bitmap_no << cma->order_per_bit);
-		mutex_lock(&cma_mutex);
 		ret = alloc_contig_range(pfn, pfn + count, MIGRATE_CMA,
 				     GFP_KERNEL | (no_warn ? __GFP_NOWARN : 0));
-		mutex_unlock(&cma_mutex);
+
 		if (ret == 0) {
 			page = pfn_to_page(pfn);
 			break;