linux-kernel - [PATCH 3.16 176/306] lib/genalloc.c: start search from start of chunk

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Message-ID: <lsq.1487198500.830985269@decadent.org.uk>
Date:   Wed, 15 Feb 2017 22:41:40 +0000
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org,
        "Mathieu Desnoyers" <mathieu.desnoyers@...icios.com>,
        "Linus Torvalds" <torvalds@...ux-foundation.org>,
        "Daniel Mentz" <danielmentz@...gle.com>,
        "Will Deacon" <will.deacon@....com>
Subject: [PATCH 3.16 176/306] lib/genalloc.c: start search from start of chunk

3.16.40-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Daniel Mentz <danielmentz@...gle.com>

commit 62e931fac45b17c2a42549389879411572f75804 upstream.

gen_pool_alloc_algo() iterates over the chunks of a pool trying to find
a contiguous block of memory that satisfies the allocation request.

The shortcut

	if (size > atomic_read(&chunk->avail))
		continue;

makes the loop skip over chunks that do not have enough bytes left to
fulfill the request.  There are two situations, though, where an
allocation might still fail:

(1) The available memory is not contiguous, i.e.  the request cannot
    be fulfilled due to external fragmentation.

(2) A race condition.  Another thread runs the same code concurrently
    and is quicker to grab the available memory.

In those situations, the loop calls pool->algo() to search the entire
chunk, and pool->algo() returns some value that is >= end_bit to
indicate that the search failed.  This return value is then assigned to
start_bit.  The variables start_bit and end_bit describe the range that
should be searched, and this range should be reset for every chunk that
is searched.  Today, the code fails to reset start_bit to 0.  As a
result, prefixes of subsequent chunks are ignored.  Memory allocations
might fail even though there is plenty of room left in these prefixes of
those other chunks.

Fixes: 7f184275aa30 ("lib, Make gen_pool memory allocator lockless")
Link: http://lkml.kernel.org/r/1477420604-28918-1-git-send-email-danielmentz@google.com
Signed-off-by: Daniel Mentz <danielmentz@...gle.com>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Acked-by: Will Deacon <will.deacon@....com>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 lib/genalloc.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- a/lib/genalloc.c
+++ b/lib/genalloc.c
@@ -273,7 +273,7 @@ unsigned long gen_pool_alloc(struct gen_
 	struct gen_pool_chunk *chunk;
 	unsigned long addr = 0;
 	int order = pool->min_alloc_order;
-	int nbits, start_bit = 0, end_bit, remain;
+	int nbits, start_bit, end_bit, remain;

 #ifndef CONFIG_ARCH_HAVE_NMI_SAFE_CMPXCHG
 	BUG_ON(in_nmi());
@@ -288,6 +288,7 @@ unsigned long gen_pool_alloc(struct gen_
 		if (size > atomic_read(&chunk->avail))
 			continue;

+		start_bit = 0;
 		end_bit = chunk_size(chunk) >> order;
 retry:
 		start_bit = pool->algo(chunk->bits, end_bit, start_bit, nbits,