percpu: fix iteration to prevent skipping over block

1fa4df3e · Dennis Zhou <[email protected]> · 2017-09-27 16:35:00 -05:00

The iterator functions pcpu_next_md_free_region and
pcpu_next_fit_region use the block offset to determine if they have
checked the area in the prior iteration. However, this causes an issue
when the block offset is greater than subsequent block contig hints. If
within the iterator it moves to check subsequent blocks, it may fail in
the second predicate due to the block offset not being cleared. Thus,
this causes the allocator to skip over blocks leading to false failures
when allocating from the reserved chunk. While this happens in the
general case as well, it will only fail if it cannot allocate a new
chunk.

This patch resets the block offset to 0 to pass the second predicate
when checking subseqent blocks within the iterator function.

Signed-off-by: Dennis Zhou <[email protected]>
Reported-and-tested-by: Luis Henriques <[email protected]>
Signed-off-by: Tejun Heo <[email protected]>

Commit: 1fa4df3e688902d033dfda796eb83ae6ad8d0488
Parents: 2e08d20d

Changes

1 files changed · +4 −0

File	Status	Add	Del	Content
mm/percpu.c	modified	+4	−0	View file

mm/percpu.c

View file

diff --git a/mm/percpu.c b/mm/percpu.c
index 59d44d6..aa121ce 100644
--- a/mm/percpu.c
+++ b/mm/percpu.c
@@ -353,6 +353,8 @@ static void pcpu_next_md_free_region(struct pcpu_chunk *chunk, int *bit_off,
 					block->contig_hint_start);
 			return;
 		}
+		/* reset to satisfy the second predicate above */
+		block_off = 0;
 
 		*bits = block->right_free;
 		*bit_off = (i + 1) * PCPU_BITMAP_BLOCK_BITS - block->right_free;
@@ -407,6 +409,8 @@ static void pcpu_next_fit_region(struct pcpu_chunk *chunk, int alloc_bits,
 			*bit_off = pcpu_block_off_to_off(i, block->first_free);
 			return;
 		}
+		/* reset to satisfy the second predicate above */
+		block_off = 0;
 
 		*bit_off = ALIGN(PCPU_BITMAP_BLOCK_BITS - block->right_free,
 				 align);