Re: [PATCH] ext4: properly check for dirty state in ext4_inode_datasync_dirty()

[Ritesh Harjani]

On 10/27/20 3:58 AM, harshad shirwadkar wrote:
Thanks Andrea for catching and sending out a fix for this.

On Sat, Oct 24, 2020 at 7:01 AM Andrea Righi <andrea.ri...@canonical.com> wrote:

ext4_inode_datasync_dirty() needs to return 'true' if the inode is
dirty, 'false' otherwise, but the logic seems to be incorrectly changed
by commit aa75f4d3daae ("ext4: main fast-commit commit path").

This introduces a problem with swap files that are always failing to be
activated, showing this error in dmesg:

  [   34.406479] swapon: file is not committed


Well, I too noticed this yesterday while I was testing xfstests -g swap.
Those tests were returning _notrun, hence that could be the reason why
it didn't get notice in XFSTESTing from Ted.

- I did notice that this code was introduced in v10 only.
This wasn't there in v9 though.


Simple test case to reproduce the problem:

   # fallocate -l 8G swapfile
   # chmod 0600 swapfile
   # mkswap swapfile
   # swapon swapfile

Fix the logic to return the proper state of the inode.

Link: https://lore.kernel.org/lkml/20201024131333.GA32124@xps-13-7390
Fixes: aa75f4d3daae ("ext4: main fast-commit commit path")
Signed-off-by: Andrea Righi <andrea.ri...@canonical.com>
---
  fs/ext4/inode.c | 4 ++--
  1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 03c2253005f0..a890a17ab7e1 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -3308,8 +3308,8 @@ static bool ext4_inode_datasync_dirty(struct inode *inode)
         if (journal) {
                 if (jbd2_transaction_committed(journal,
                                         EXT4_I(inode)->i_datasync_tid))
-                       return true;
-               return atomic_read(&EXT4_SB(inode->i_sb)->s_fc_subtid) >=
+                       return false;
+               return atomic_read(&EXT4_SB(inode->i_sb)->s_fc_subtid) <
                         EXT4_I(inode)->i_fc_committed_subtid;
In addition, the above condition should only be checked if fast
commits are enabled. So, in effect this overall condition will look
like this:

if (journal) {
     if (jbd2_transaction_committed(journal, EXT4_I(inode)->i_datasync_tid))
         return false;
     if (test_opt2(sb, JOURNAL_FAST_COMMIT))
         return atomic_read(&EXT4_SB(inode->i_sb)->s_fc_subtid) <
EXT4_I(inode)->i_fc_committed_subtid;
     return true;
}

Yup - I too had made a similar patch. But then I also noticed that below
condition will always remain false. Since we never update
"i_fc_committed_subtid" other than at these 2 places
(one during init where we set it to 0 and other during ext4_fc_commit()
where we set it to sbi->s_fc_subtid).

<condition>
atomic_read(&EXT4_SB(inode->i_sb)->s_fc_subtid < 
EXT4_I(inode)->i_fc_committed_subtid


Maybe I need more reading around this.

-ritesh






Thanks,
Harshad

         }

--
2.27.0