6.18-stable review patch. If anyone has any objections, please let me know.
------------------
From: Yongjian Sun sunyongjian1@huawei.com
[ Upstream commit d9ee3ff810f1cc0e253c9f2b17b668b973cb0e06 ]
When the MB_CHECK_ASSERT macro is enabled, we found that the current validation logic in __mb_check_buddy has a gap in detecting certain invalid buddy states, particularly related to order-0 (bitmap) bits.
The original logic consists of three steps: 1. Validates higher-order buddies: if a higher-order bit is set, at most one of the two corresponding lower-order bits may be free; if a higher-order bit is clear, both lower-order bits must be allocated (and their bitmap bits must be 0). 2. For any set bit in order-0, ensures all corresponding higher-order bits are not free. 3. Verifies that all preallocated blocks (pa) in the group have pa_pstart within bounds and their bitmap bits marked as allocated.
However, this approach fails to properly validate cases where order-0 bits are incorrectly cleared (0), allowing some invalid configurations to pass:
corrupt integral
order 3 1 1 order 2 1 1 1 1 order 1 1 1 1 1 1 1 1 1 order 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1
Here we get two adjacent free blocks at order-0 with inconsistent higher-order state, and the right one shows the correct scenario.
The root cause is insufficient validation of order-0 zero bits. To fix this and improve completeness without significant performance cost, we refine the logic:
1. Maintain the top-down higher-order validation, but we no longer check the cases where the higher-order bit is 0, as this case will be covered in step 2. 2. Enhance order-0 checking by examining pairs of bits: - If either bit in a pair is set (1), all corresponding higher-order bits must not be free. - If both bits are clear (0), then exactly one of the corresponding higher-order bits must be free 3. Keep the preallocation (pa) validation unchanged.
This change closes the validation gap, ensuring illegal buddy states involving order-0 are correctly detected, while removing redundant checks and maintaining efficiency.
Fixes: c9de560ded61f ("ext4: Add multi block allocator for ext4") Suggested-by: Jan Kara jack@suse.cz Signed-off-by: Yongjian Sun sunyongjian1@huawei.com Reviewed-by: Baokun Li libaokun1@huawei.com Reviewed-by: Jan Kara jack@suse.cz Message-ID: 20251106060614.631382-3-sunyongjian@huaweicloud.com Signed-off-by: Theodore Ts'o tytso@mit.edu Signed-off-by: Sasha Levin sashal@kernel.org --- fs/ext4/mballoc.c | 49 +++++++++++++++++++++++++++++++---------------- 1 file changed, 32 insertions(+), 17 deletions(-)
diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c index 9087183602e44..c56b51e1162eb 100644 --- a/fs/ext4/mballoc.c +++ b/fs/ext4/mballoc.c @@ -682,6 +682,24 @@ do { \ } \ } while (0)
+/* + * Perform buddy integrity check with the following steps: + * + * 1. Top-down validation (from highest order down to order 1, excluding order-0 bitmap): + * For each pair of adjacent orders, if a higher-order bit is set (indicating a free block), + * at most one of the two corresponding lower-order bits may be clear (free). + * + * 2. Order-0 (bitmap) validation, performed on bit pairs: + * - If either bit in a pair is set (1, allocated), then all corresponding higher-order bits + * must not be free (0). + * - If both bits in a pair are clear (0, free), then exactly one of the corresponding + * higher-order bits must be free (0). + * + * 3. Preallocation (pa) list validation: + * For each preallocated block (pa) in the group: + * - Verify that pa_pstart falls within the bounds of this block group. + * - Ensure the corresponding bit(s) in the order-0 bitmap are marked as allocated (1). + */ static void __mb_check_buddy(struct ext4_buddy *e4b, char *file, const char *function, int line) { @@ -723,15 +741,6 @@ static void __mb_check_buddy(struct ext4_buddy *e4b, char *file, continue; }
- /* both bits in buddy2 must be 1 */ - MB_CHECK_ASSERT(mb_test_bit(i << 1, buddy2)); - MB_CHECK_ASSERT(mb_test_bit((i << 1) + 1, buddy2)); - - for (j = 0; j < (1 << order); j++) { - k = (i * (1 << order)) + j; - MB_CHECK_ASSERT( - !mb_test_bit(k, e4b->bd_bitmap)); - } count++; } MB_CHECK_ASSERT(e4b->bd_info->bb_counters[order] == count); @@ -747,15 +756,21 @@ static void __mb_check_buddy(struct ext4_buddy *e4b, char *file, fragments++; fstart = i; } - continue; + } else { + fstart = -1; } - fstart = -1; - /* check used bits only */ - for (j = 0; j < e4b->bd_blkbits + 1; j++) { - buddy2 = mb_find_buddy(e4b, j, &max2); - k = i >> j; - MB_CHECK_ASSERT(k < max2); - MB_CHECK_ASSERT(mb_test_bit(k, buddy2)); + if (!(i & 1)) { + int in_use, zero_bit_count = 0; + + in_use = mb_test_bit(i, buddy) || mb_test_bit(i + 1, buddy); + for (j = 1; j < e4b->bd_blkbits + 2; j++) { + buddy2 = mb_find_buddy(e4b, j, &max2); + k = i >> j; + MB_CHECK_ASSERT(k < max2); + if (!mb_test_bit(k, buddy2)) + zero_bit_count++; + } + MB_CHECK_ASSERT(zero_bit_count == !in_use); } } MB_CHECK_ASSERT(!EXT4_MB_GRP_NEED_INIT(e4b->bd_info));