Re: [PATCH] ext4: mballoc.c: fix ac_g_ex and ac_f_ex misuse bug in EXT4_MB_HINT_TRY_GOAL path

From: Lin Feng
Date: Sun Jun 05 2016 - 22:25:17 EST


Am I missing something or it's not important at all?

Other things:
[linfeng@localhost ext4]$ grep EXT4_MB_HINT_MERGE ./* -rHn
./ext4.h:111:#define EXT4_MB_HINT_MERGE 0x0001
./mballoc.c:1871: } else if (max > 0 && (ac->ac_flags & EXT4_MB_HINT_MERGE)) {

EXT4_MB_HINT_MERGE is only tested once and nowhere teaches how to use it.
IIUC it also should be folded into EXT4_MB_HINT_TRY_GOAL path or simply skip
EXT4_MB_HINT_MERGE test at -L1871.


On 06/02/2016 08:01 PM, Lin Feng wrote:
ext4 block allocation core stack:
mb_find_extent(e4b, ac->ac_g_ex.fe_start, ac->ac_g_ex.fe_len, &ex);

The start block searching hint for merging(use EXT4_MB_HINT_TRY_GOAL flag)
set in ext4_mb_normalize_request is stored in ac_f_ex, while in
EXT4_MB_HINT_TRY_GOAL path which falls in ext4_mb_find_by_goal always use
ac_g_ex as a hint and the hint set in ext4_mb_normalize_request is never

We could hit this bug by writing a sparse file from backward mode and the
file may get fragments even if the physical blocks in the hole is free,
which is expected to be merged into a single extent.

Signed-off-by: Lin Feng <linf@xxxxxxxxxxxxxxxxxx>
fs/ext4/mballoc.c | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index c1ab3ec..e31fc63 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -3198,15 +3198,15 @@ ext4_mb_normalize_request(struct ext4_allocation_context *ac,
if (ar->pright && (ar->lright == (start + size))) {
/* merge to the right */
ext4_get_group_no_and_offset(ac->ac_sb, ar->pright - size,
- &ac->ac_f_ex.fe_group,
- &ac->ac_f_ex.fe_start);
+ &ac->ac_g_ex.fe_group,
+ &ac->ac_g_ex.fe_start);
ac->ac_flags |= EXT4_MB_HINT_TRY_GOAL;
if (ar->pleft && (ar->lleft + 1 == start)) {
/* merge to the left */
ext4_get_group_no_and_offset(ac->ac_sb, ar->pleft + 1,
- &ac->ac_f_ex.fe_group,
- &ac->ac_f_ex.fe_start);
+ &ac->ac_g_ex.fe_group,
+ &ac->ac_g_ex.fe_start);
ac->ac_flags |= EXT4_MB_HINT_TRY_GOAL;