[PATCHv2 1/1] fs: ext4: Don't use CMA for buffer_head

[zhaoyang.huang]

From: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx>

cma_alloc() keep failed in our system which thanks to a jh->bh->b_page
can not be migrated out of CMA area[1] as the jh has one cp_transaction
pending on it because of j_free > j_max_transaction_buffers[2][3][4][5][6].
We temporarily solve this by launching jbd2_log_do_checkpoint forcefully
somewhere. Since journal is common mechanism to all JFSs and
cp_transaction has a little fewer opportunity to be launched, the
cma_alloc() could be affected under the same scenario. This patch
would like to have buffer_head of ext4 not use CMA pages when doing
sb_getblk.

[1]
crash_arm64_v8.0.4++> kmem -p|grep ffffff808f0aa150(sb->s_bdev->bd_inode->i_mapping)
fffffffe01a51c00  e9470000 ffffff808f0aa150        3  2 8000000008020 lru,private
fffffffe03d189c0 174627000 ffffff808f0aa150        4  2 2004000000008020 lru,private
fffffffe03d88e00 176238000 ffffff808f0aa150      3f9  2 2008000000008020 lru,private
fffffffe03d88e40 176239000 ffffff808f0aa150        6  2 2008000000008020 lru,private
fffffffe03d88e80 17623a000 ffffff808f0aa150        5  2 2008000000008020 lru,private
fffffffe03d88ec0 17623b000 ffffff808f0aa150        1  2 2008000000008020 lru,private
fffffffe03d88f00 17623c000 ffffff808f0aa150        0  2 2008000000008020 lru,private
fffffffe040e6540 183995000 ffffff808f0aa150      3f4  2 2004000000008020 lru,private

[2] page -> buffer_head
crash_arm64_v8.0.4++> struct page.private fffffffe01a51c00 -x
      private = 0xffffff802fca0c00

[3] buffer_head -> journal_head
crash_arm64_v8.0.4++> struct buffer_head.b_private 0xffffff802fca0c00
  b_private = 0xffffff8041338e10,

[4] journal_head -> b_cp_transaction
crash_arm64_v8.0.4++> struct journal_head.b_cp_transaction 0xffffff8041338e10 -x
  b_cp_transaction = 0xffffff80410f1900,

[5] transaction_t -> journal
crash_arm64_v8.0.4++> struct transaction_t.t_journal 0xffffff80410f1900 -x
  t_journal = 0xffffff80e70f3000,

[6] j_free & j_max_transaction_buffers
crash_arm64_v8.0.4++> struct journal_t.j_free,j_max_transaction_buffers 0xffffff80e70f3000 -x
  j_free = 0x3f1,
  j_max_transaction_buffers = 0x100,

Suggested-by: Theodore Ts'o <tytso@xxxxxxx>
Signed-off-by: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx>
---
v2: switch to use getblk_unmoveable as suggested by Theodore Ts'o
---
---
 fs/ext4/inode.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 941c1c0d5c6e..a0f48840c5c1 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -869,7 +869,14 @@ struct buffer_head *ext4_getblk(handle_t *handle, struct inode *inode,
 	if (nowait)
 		return sb_find_get_block(inode->i_sb, map.m_pblk);
 
-	bh = sb_getblk(inode->i_sb, map.m_pblk);
+	/*
+	 * Since bh could introduce extra ref count such as referred by
+	 * journal_head etc. Try to avoid using __GFP_MOVABLE here
+	 * as it may fail the migration when journal_head remains.
+	 */
+	bh = getblk_unmovable(inode->i_sb->s_bdev, map.m_pblk,
+				inode->i_sb->s_blocksize);
+
 	if (unlikely(!bh))
 		return ERR_PTR(-ENOMEM);
 	if (map.m_flags & EXT4_MAP_NEW) {
-- 
2.25.1