I've recently discovered that doing infinite loop of systemctl start <ext4_on_lvm>.mount, and systemctl stop <ext4_on_lvm>.mount linearly increases percpu allocator memory consumption. In several hours, it might lead to system instability by consuming most of the memory.
During debugging it was found that most of active percpu allocations are from /system.slice/<ext4_on_lvm>.mount memory cgroups (created by systemd for each mount). All of these cgroups are in dying state with refcount equal to 2. And most interesting that each mount/umount itera- tion creates exactly one dying memory cgroup.
Tracking down the remaining refcounts showed that it was charged from ext4_fill_super(). And the page is always 0 index in the page cache mapping.
The issue was hidden behind initial super block read using logical blocksize from bdev and adjusting blocksize later after reading actual block size from superblock. If blocksizes differ, sb_set_blocksize() will kill current buffers and page cache by using kill_bdev(). And then super block will be reread again but using correct blocksize this time. sb_set_blocksize() didn't fully free superblock page and buffers as buffer pointed by bh variable remained busy. So buffer and its page remains in the memory (leak). Super block reread logic does not happen when ext4 filesystem is on physical partition as blocksize is correct for initial superblock read.
brelse(bh), where bh is a buffer head of superblock page, must be called and bh references must be released before kill_bdev(). kill_bdev() subfunctions (see callstack below) won't be able to free not released buffer (even if it's clean) and superblock page won't be freed as well.
callstack: kill_bdev() ->truncate_inode_pages() ->truncate_inode_pages_range() ->truncate_cleanup_page() ->do_invalidatepage ->block_invalidatepage() ->try_to_release_page() == fail to release ->try_to_free_buffers() == fail to free ->drop_buffers() ->buffer_busy() == yes
Incorrect order of brelse() and kill_bdev() in ext4_fill_super() was introduced by commit ce40733ce93d ("ext4: Check for return value from sb_set_blocksize") 13 years ago! Thanks to memory hungry percpu, it was easy to detect this issue now.
Fix this by moving the brelse() before sb_set_blocksize() and add a comment about the dependency.
In addition, fix similar issue under failed_mount: label (in the same function) about incorrect order of ext4_blkdev_remove() vs brelse() introduced by commit ac27a0ec112a ("ext4: initial copy of files from ext3")
Signed-off-by: Alexey Makhalov amakhalov@vmware.com Cc: stable@vger.kernel.org Fixes: ce40733ce93d ("ext4: Check for return value from sb_set_blocksize") Fixes: ac27a0ec112a ("ext4: initial copy of files from ext3") Cc: "Theodore Ts'o" tytso@mit.edu Cc: Andreas Dilger adilger.kernel@dilger.ca --- fs/ext4/super.c | 11 +++++++++-- 1 file changed, 9 insertions(+), 2 deletions(-)
diff --git a/fs/ext4/super.c b/fs/ext4/super.c index b9693680463a..6c8f68309834 100644 --- a/fs/ext4/super.c +++ b/fs/ext4/super.c @@ -4451,14 +4451,20 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent) }
if (sb->s_blocksize != blocksize) { + /* + * bh must be released before kill_bdev(), otherwise + * it won't be freed and its page also. kill_bdev() + * is called by sb_set_blocksize(). + */ + brelse(bh); /* Validate the filesystem blocksize */ if (!sb_set_blocksize(sb, blocksize)) { ext4_msg(sb, KERN_ERR, "bad block size %d", blocksize); + bh = NULL; goto failed_mount; }
- brelse(bh); logical_sb_block = sb_block * EXT4_MIN_BLOCK_SIZE; offset = do_div(logical_sb_block, blocksize); bh = ext4_sb_bread_unmovable(sb, logical_sb_block); @@ -5178,8 +5184,9 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent) kfree(get_qf_name(sb, sbi, i)); #endif fscrypt_free_dummy_policy(&sbi->s_dummy_enc_policy); - ext4_blkdev_remove(sbi); + /* ext4_blkdev_remove() calls kill_bdev(), release bh before it. */ brelse(bh); + ext4_blkdev_remove(sbi); out_fail: sb->s_fs_info = NULL; kfree(sbi->s_blockgroup_lock);