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

[Lin Feng]

Hi,

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.

thanks,
linfeng


On 06/02/2016 08:01 PM, Lin Feng wrote:

Descriptions:
ext4 block allocation core stack:
ext4_mb_new_blocks
   ext4_mb_normalize_request
   ext4_mb_regular_allocator
     ext4_mb_find_by_goal
       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
use.

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 <l...@chinanetcenter.com>
---
  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;
        }