From: Lecopzer Chen <lecopzer.chen@xxxxxxxxxxxx>
Subject: mm/cma.c: remove redundant cma_mutex lock
The cma_mutex which protects alloc_contig_range() was first appeared in
commit 7ee793a62fa8c ("cma: Remove potential deadlock situation"), at that
time, there is no guarantee the behavior of concurrency inside
alloc_contig_range().
After commit 2c7452a075d4db2dc ("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@xxxxxxxxxxxx
Signed-off-by: Lecopzer Chen <lecopzer.chen@xxxxxxxxxxxx>
Acked-by: David Hildenbrand <david@xxxxxxxxxx>
Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Matthias Brugger <matthias.bgg@xxxxxxxxx>
Cc: YJ Chiang <yj.chiang@xxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/cma.c | 4 +---
1 file changed, 1 insertion(+), 3 deletions(-)
--- a/mm/cma.c~mm-cmac-remove-redundant-cma_mutex-lock
+++ a/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,
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;
_
