Commit a4efc174 authored Dec 14, 2020 by Lecopzer Chen Committed by Linus Torvalds Dec 15, 2020

mm/cma.c: remove redundant cma_mutex lock

The cma_mutex which protects alloc_contig_range() was first appeared in
commit 7ee793a6 ("cma: Remove potential deadlock situation"), at that
time, there is no guarantee the behavior of concurrency inside
alloc_contig_range().

After commit 2c7452a0 ("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/20201020102241.3729-1-lecopzer.chen@mediatek.com


Signed-off-by: Lecopzer Chen <lecopzer.chen@mediatek.com>
Acked-by: David Hildenbrand <david@redhat.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Matthias Brugger <matthias.bgg@gmail.com>
Cc: YJ Chiang <yj.chiang@mediatek.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent d85c6db4

mm/cma.c

+1 −3

Original line number	Diff line number	Diff line
		@@ -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;

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