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Date: Sat, 6 Jun 2015 05:15:22 +0900
From: Michał Nazarewicz <mina86@...a86.com>
To: Laura Abbott <labbott@...hat.com>
Cc: Weijie Yang <weijie.yang@...sung.com>, iamjoonsoo.kim@....com,
Marek Szyprowski <m.szyprowski@...sung.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Weijie Yang <weijie.yang.kh@...il.com>,
"linux-mm@...ck.org" <linux-mm@...ck.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] cma: allow concurrent cma pages allocation for multi-cma areas
On Fri, Jun 05 2015, Laura Abbott wrote:
> but can we put a comment explaining this here too?
Sounds good to me. I would even document why we have two locks in case
someone decides to merge them. Perhaps something like (haven’t tested
or even compiled):
-------- >8 ------------------------------------------------------------
Subject: cma: allow concurrent allocation for different CMA regions
Currently we have to hold a single cma_mutex when allocating CMA areas.
When there are multiple CMA regions, the single cma_mutex prevents
concurrent CMA allocation.
This patch replaces the single cma_mutex with a per-CMA region
alloc_lock. This allows concurrent CMA allocation for different CMA
regions while protects access to the same pageblocks.
Signed-off-by: Weijie Yang <weijie.yang@...sung.com>
Signed-off-by: Michal Nazarewiz <mina86@...a86.com>
[mina86: renamed cma->lock to cma->bitmap_lock, added locks documentation]
---
mm/cma.c | 18 +++++++++---------
mm/cma.h | 16 +++++++++++++++-
2 files changed, 24 insertions(+), 10 deletions(-)
diff --git a/mm/cma.c b/mm/cma.c
index 3a7a67b..841fe07 100644
--- a/mm/cma.c
+++ b/mm/cma.c
@@ -41,7 +41,6 @@
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)
{
@@ -89,9 +88,9 @@ static void cma_clear_bitmap(struct cma *cma,
unsigned long pfn,
bitmap_no = (pfn - cma->base_pfn) >> cma->order_per_bit;
bitmap_count = cma_bitmap_pages_to_bits(cma, count);
- mutex_lock(&cma->lock);
+ mutex_lock(&cma->bitmap_lock);
bitmap_clear(cma->bitmap, bitmap_no, bitmap_count);
- mutex_unlock(&cma->lock);
+ mutex_unlock(&cma->bitmap_lock);
}
static int __init cma_activate_area(struct cma *cma)
@@ -127,7 +126,8 @@ static int __init cma_activate_area(struct cma *cma)
init_cma_reserved_pageblock(pfn_to_page(base_pfn));
} while (--i);
- mutex_init(&cma->lock);
+ mutex_init(&cma->alloc_lock);
+ mutex_init(&cma->bitmap_lock);
#ifdef CONFIG_CMA_DEBUGFS
INIT_HLIST_HEAD(&cma->mem_head);
@@ -381,12 +381,12 @@ struct page *cma_alloc(struct cma *cma, unsigned
int count, unsigned int align)
bitmap_count = cma_bitmap_pages_to_bits(cma, count);
for (;;) {
- mutex_lock(&cma->lock);
+ mutex_lock(&cma->bitmap_lock);
bitmap_no = bitmap_find_next_zero_area_off(cma->bitmap,
bitmap_maxno, start, bitmap_count, mask,
offset);
if (bitmap_no >= bitmap_maxno) {
- mutex_unlock(&cma->lock);
+ mutex_unlock(&cma->bitmap_lock);
break;
}
bitmap_set(cma->bitmap, bitmap_no, bitmap_count);
@@ -395,12 +395,12 @@ struct page *cma_alloc(struct cma *cma, unsigned
int count, unsigned int align)
* our exclusive use. If the migration fails we will take the
* lock again and unmark it.
*/
- mutex_unlock(&cma->lock);
+ mutex_unlock(&cma->bitmap_lock);
pfn = cma->base_pfn + (bitmap_no << cma->order_per_bit);
- mutex_lock(&cma_mutex);
+ mutex_lock(&cma->alloc_lock);
ret = alloc_contig_range(pfn, pfn + count, MIGRATE_CMA);
- mutex_unlock(&cma_mutex);
+ mutex_unlock(&cma->alloc_lock);
if (ret == 0) {
page = pfn_to_page(pfn);
break;
diff --git a/mm/cma.h b/mm/cma.h
index 1132d73..b585ba1 100644
--- a/mm/cma.h
+++ b/mm/cma.h
@@ -6,7 +6,21 @@ struct cma {
unsigned long count;
unsigned long *bitmap;
unsigned int order_per_bit; /* Order of pages represented by one bit */
- struct mutex lock;
+ /*
+ * alloc_lock protects calls to alloc_contig_range. The function must
+ * not be called concurrently on the same pageblock which is why it has
+ * to be synchronised. On the other hand, because each CMA region is
+ * pageblock-aligned we can have per-region alloc_locks since they never
+ * share a pageblock.
+ */
+ struct mutex alloc_lock;
+ /*
+ * As the name suggests, bitmap_lock protects the bitmap. It has to be
+ * a separate lock from alloc_lock so that we can free CMA areas (which
+ * only requires bitmap_lock) while allocating pages (which requires
+ * alloc_lock) is ongoing.
+ */
+ struct mutex bitmap_lock;
#ifdef CONFIG_CMA_DEBUGFS
struct hlist_head mem_head;
spinlock_t mem_head_lock;
--
Best regards, _ _
.o. | Liege of Serenely Enlightened Majesty of o' \,=./ `o
..o | Computer Science, Michał “mina86” Nazarewicz (o o)
ooo +--<mpn@...gle.com>--<xmpp:mina86@...ber.org>--ooO--(_)--Ooo--
--
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