linux-kernel - [PATCH 03/10] dmapool: fix boundary comparison

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Message-ID: <f418f3c5-32b7-31c1-c91a-1bdb64b44db3@cybernetics.com>
Date:   Tue, 31 May 2022 14:14:23 -0400
From:   Tony Battersby <tonyb@...ernetics.com>
To:     linux-mm@...ck.org, linux-kernel@...r.kernel.org
Cc:     iommu@...ts.linux-foundation.org, kernel-team@...com,
        Matthew Wilcox <willy@...radead.org>,
        Keith Busch <kbusch@...nel.org>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        Robin Murphy <robin.murphy@....com>,
        Tony Lindgren <tony@...mide.com>
Subject: [PATCH 03/10] dmapool: fix boundary comparison

Fix the boundary comparison when constructing the list of free blocks
for the case that 'size' is a power of two.  Since 'boundary' is also a
power of two, that would make 'boundary' a multiple of 'size', in which
case a single block would never cross the boundary.  This bug would
cause some of the allocated memory to be wasted (but not leaked).

Example:

size       = 512
boundary   = 2048
allocation = 4096

Address range
   0 -  511
 512 - 1023
1024 - 1535
1536 - 2047 *
2048 - 2559
2560 - 3071
3072 - 3583
3584 - 4095 *

Prior to this fix, the address ranges marked with "*" would not have
been used even though they didn't cross the given boundary.

Fixes: e34f44b3517f ("pool: Improve memory usage for devices which can't cross boundaries")
Signed-off-by: Tony Battersby <tonyb@...ernetics.com>
---
 mm/dmapool.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/dmapool.c b/mm/dmapool.c
index d7b372248111..782143144a32 100644
--- a/mm/dmapool.c
+++ b/mm/dmapool.c
@@ -210,7 +210,7 @@ static void pool_initialise_page(struct dma_pool *pool, struct dma_page *page)
 
 	do {
 		unsigned int next = offset + pool->size;
-		if (unlikely((next + pool->size) >= next_boundary)) {
+		if (unlikely((next + pool->size) > next_boundary)) {
 			next = next_boundary;
 			next_boundary += pool->boundary;
 		}
-- 
2.25.1