ext4: handle symlink properly with inline_data

From: Zheng Liu <[email protected]>

This commit tries to fix a bug that we can't read symlink properly with inline data feature when the length of symlink is greater than 60 bytes but less than extra space.

The key issue is in ext4_inode_is_fast_symlink() that it doesn't check whether or not an inode has inline data. When the user creates a new symlink, an inode will be allocated with MAY_INLINE_DATA flag. Then symlink will be stored in ->i_block and extended attribute space. In the mean time, this inode is with inline data flag. After remounting it, ext4_inode_is_fast_symlink() function thinks that this inode is a fast symlink so that the data in ->i_block is copied to the user, and the data in extra space is trimmed. In fact this inode should be as a normal symlink.

The following script can hit this bug.

#!/bin/bash

  cd ${MNT}
  filename=ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789
  rm -rf test
  mkdir test
  cd test
  echo "hello" >$filename
  ln -s $filename symlinkfile
  cd
  sudo umount /mnt/sda1
  sudo mount -t ext4 /dev/sda1 /mnt/sda1
  readlink /mnt/sda1/test/symlinkfile

After applying this patch, it will break the assumption in e2fsck because the original implementation doesn't want to support symlink with inline data.

---
 fs/ext4/inode.c |    3 +++
 1 file changed, 3 insertions(+)

index 55f999a..bc5e4c1 100644
@@ -148,6 +148,9 @@ static int ext4_inode_is_fast_symlink(struct inode *inode)
         int ea_blocks = EXT4_I(inode)->i_file_acl ?
 		EXT4_CLUSTER_SIZE(inode->i_sb) >> 9 : 0;
 
+	if (ext4_has_inline_data(inode))
+		return 0;
+
 	return (S_ISLNK(inode->i_mode) && inode->i_blocks - ea_blocks == 0);
 }
 

1.7.9.7

Applied, thanks.

Subject: [PATCH] ext4: make ext4_has_inline_data() as a inline function

From: Zheng Liu <[email protected]>

Now ext4_has_inline_data() is used in wide spread codepaths. So we need to make it as a inline function to avoid burning some CPU cycles.

text size (bytes):
vanilla: 10350562
patched: 10384933 (+0.33%)

I use the following script to measure the CPU usage.

#!/bin/bash

  shm_base='/dev/shm'
  img=${shm_base}/ext4-img
  mnt=/mnt/loop

  e2fsprgs_base=$HOME/e2fsprogs
  mkfs=${e2fsprgs_base}/misc/mke2fs
  fsck=${e2fsprgs_base}/e2fsck/e2fsck

  sudo umount $mnt
  dd if=/dev/zero of=$img bs=4k count=3145728
  ${mkfs} -t ext4 -O inline_data -F $img
  sudo mount -t ext4 -o loop $img $mnt

  # start testing...
  testdir="${mnt}/testdir"
  mkdir $testdir
  cd $testdir

echo "start testing..."
for ((cnt=0;cnt<100;cnt++)); do

for ((i=0;i<5;i++)); do
for ((j=0;j<5;j++)); do
for ((k=0;k<5;k++)); do
for ((l=0;l<5;l++)); do
mkdir -p $i/$j/$k/$l
echo "$i-$j-$k-$l" > $i/$j/$k/$l/testfile
done
done
done
done

ls -R $testdir > /dev/null
rm -rf $testdir/*

done

The result of perf top -G -U is as below.

vanilla:

 13.92%  [ext4]  [k] ext4_do_update_inode
  9.36%  [ext4]  [k] __ext4_get_inode_loc
  4.07%  [ext4]  [k] ftrace_define_fields_ext4_writepages
  3.83%  [ext4]  [k] __ext4_handle_dirty_metadata
  3.42%  [ext4]  [k] ext4_get_inode_flags
  2.71%  [ext4]  [k] ext4_mark_iloc_dirty
  2.46%  [ext4]  [k] ftrace_define_fields_ext4_direct_IO_enter
  2.26%  [ext4]  [k] ext4_get_inode_loc
  2.22%  [ext4]  [k] ext4_has_inline_data

[...]

After applied the patch, we don't see ext4_has_inline_data() because it has been inlined and perf couldn't sample it. Although it doesn't mean that the CPU cycles can be saved but at least the overhead of function calls can be eliminated. So IMHO we'd better inline this function.

---
 fs/ext4/ext4.h   |    7 ++++++-
 fs/ext4/inline.c |    6 ------
 2 files changed, 6 insertions(+), 7 deletions(-)

index 0519715..0531df5 100644
@@ -2564,7 +2564,6 @@ extern const struct file_operations ext4_file_operations;
 extern loff_t ext4_llseek(struct file *file, loff_t offset, int origin);
 
 /* inline.c */
-extern int ext4_has_inline_data(struct inode *inode);
 extern int ext4_get_max_inline_size(struct inode *inode);
 extern int ext4_find_inline_data_nolock(struct inode *inode);
 extern int ext4_init_inline_data(handle_t *handle, struct inode *inode,

@@ -2630,6 +2629,12 @@ extern void ext4_inline_data_truncate(struct inode *inode, int *has_inline);
 
 extern int ext4_convert_inline_data(struct inode *inode);
 
+static inline int ext4_has_inline_data(struct inode *inode)
+{
+	return ext4_test_inode_flag(inode, EXT4_INODE_INLINE_DATA) &&
+	       EXT4_I(inode)->i_inline_off;
+}
+
 /* namei.c */
 extern const struct inode_operations ext4_dir_inode_operations;
 extern const struct inode_operations ext4_special_inode_operations;

index 645205d..141b6ac 100644
@@ -120,12 +120,6 @@ int ext4_get_max_inline_size(struct inode *inode)
 	return max_inline_size + EXT4_MIN_INLINE_DATA_SIZE;
 }
 
-int ext4_has_inline_data(struct inode *inode)
-{
-	return ext4_test_inode_flag(inode, EXT4_INODE_INLINE_DATA) &&
-	       EXT4_I(inode)->i_inline_off;
-}

This represents a difference of 34k, which was over 10% the text size of ext4.o --- so this surprised me greatly. I just did my own testing, and difference I see is 117 bytes:

         text	  data	    bss     dec	    hex	filename
before: 326110	  19258	   5528	 350896	  55ab0	fs/ext4/ext4.o
after:  326227	  19258	   5528	 351013	  55b25	fs/ext4/ext4.o

So I'm going to adjust the patch include these numbers, which are much less likely to cause embedded people to complain about a large increase in code bloat. :-)

Thanks, applied.

- Ted