linux-kernel - [PATCH v2] iommu/iova: change IOVA_MAG

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Date:   Sun,  3 Jul 2022 19:44:50 +0800
From:   Feng Tang <feng.tang@...el.com>
To:     Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        John Garry <john.garry@...wei.com>,
        iommu@...ts.linux-foundation.org, iommu@...ts.linux.dev,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Lameter <cl@...ux.com>,
        Vlastimil Babka <vbabka@...e.cz>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Feng Tang <feng.tang@...el.com>
Subject: [PATCH v2] iommu/iova: change IOVA_MAG_SIZE to 127 to save memory

kmalloc will round up the request size to power of 2, and current
iova_magazine's size is 1032 (1024+8) bytes, so each instance
allocated will get 2048 bytes from kmalloc, causing around 1KB
waste.

Change IOVA_MAG_SIZE from 128 to 127 to make size of 'iova_magazine'
1024 bytes so that no memory will be wasted.

Signed-off-by: Feng Tang <feng.tang@...el.com>
Acked-by: Robin Murphy <robin.murphy@....com>
---
Changelog:
  
  since v1:
    * update commit log per the comments from Robin and John 

 drivers/iommu/iova.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/iommu/iova.c b/drivers/iommu/iova.c
index db77aa675145b..27634ddd9b904 100644
--- a/drivers/iommu/iova.c
+++ b/drivers/iommu/iova.c
@@ -614,7 +614,12 @@ EXPORT_SYMBOL_GPL(reserve_iova);
  * dynamic size tuning described in the paper.
  */
 
-#define IOVA_MAG_SIZE 128
+/*
+ * As kmalloc's buffer size is fixed to power of 2, 127 is chosen to
+ * assure size of 'iova_magazine' to be 1024 bytes, so that no memory
+ * will be wasted.
+ */
+#define IOVA_MAG_SIZE 127
 #define MAX_GLOBAL_MAGS 32	/* magazines per bin */
 
 struct iova_magazine {
-- 
2.27.0