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

Ben Hutchings
2017-02-15T23:42:06Z

From: Daniel Mentz <danielmentz@google.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@google.com>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Ben Hutchings <ben@decadent.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,