linux-ext4 - Re: [PATCH] ext4: Fix data corruption in inodes with journalled data

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Date:	Sat, 23 Jul 2011 16:21:55 +0300
From:	Amir Goldstein <amir73il@...il.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Ted Tso <tytso@....edu>, linux-ext4@...r.kernel.org
Subject: Re: [PATCH] ext4: Fix data corruption in inodes with journalled data

On Sat, Jul 23, 2011 at 3:39 AM, Jan Kara <jack@...e.cz> wrote:
> When journalling data for an inode (either because it is a symlink or
> because the filesystem is mounted in data=journal mode), ext4_evict_inode()
> can discard unwritten data by calling truncate_inode_pages(). This is
> because we don't mark the buffer / page dirty when journalling data but only
> add the buffer to the running transaction and thus mm does not know there
> are still unwritten data.
>
> Fix the problem by carefully tracking transaction containing inode's data,
> committing this transaction, and writing uncheckpointed buffers when inode
> should be reaped.
>
> Signed-off-by: Jan Kara <jack@...e.cz>
> ---
>  fs/ext4/inode.c |   29 +++++++++++++++++++++++++++++
>  1 files changed, 29 insertions(+), 0 deletions(-)
>
>  This is ext4 version of an ext3 fix I sent a while ago. It received only
> light testing but I figured you might want get the patch earlier rather than
> later given the merge window is open.
>
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index e3126c0..019995b 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -190,6 +190,33 @@ void ext4_evict_inode(struct inode *inode)
>
>        trace_ext4_evict_inode(inode);
>        if (inode->i_nlink) {
> +               /*
> +                * When journalling data dirty buffers are tracked only in the
> +                * journal. So although mm thinks everything is clean and
> +                * ready for reaping the inode might still have some pages to
> +                * write in the running transaction or waiting to be
> +                * checkpointed. Thus calling jbd2_journal_invalidatepage()
> +                * (via truncate_inode_pages()) to discard these buffers can
> +                * cause data loss. Also even if we did not discard these
> +                * buffers, we would have no way to find them after the inode
> +                * is reaped and thus user could see stale data if he tries to
> +                * read them before the transaction is checkpointed. So be
> +                * careful and force everything to disk here... We use
> +                * ei->i_datasync_tid to store the newest transaction
> +                * containing inode's data.
> +                *
> +                * Note that directories do not have this problem because they
> +                * don't use page cache.
> +                */
> +               if (ext4_should_journal_data(inode) &&
> +                   (S_ISLNK(inode->i_mode) || S_ISREG(inode->i_mode))) {
> +                       journal_t *journal = EXT4_SB(inode->i_sb)->s_journal;
> +                       tid_t commit_tid = EXT4_I(inode)->i_datasync_tid;
> +
> +                       jbd2_log_start_commit(journal, commit_tid);
> +                       jbd2_log_wait_commit(journal, commit_tid);
> +                       filemap_write_and_wait(&inode->i_data);
> +               }
>                truncate_inode_pages(&inode->i_data, 0);
>                goto no_delete;
>        }
> @@ -1863,6 +1890,7 @@ static int ext4_journalled_write_end(struct file *file,
>        if (new_i_size > inode->i_size)
>                i_size_write(inode, pos+copied);
>        ext4_set_inode_state(inode, EXT4_STATE_JDATA);
> +       EXT4_I(inode)->i_datasync_tid = handle->h_transaction->t_tid;
>        if (new_i_size > EXT4_I(inode)->i_disksize) {
>                ext4_update_i_disksize(inode, new_i_size);
>                ret2 = ext4_mark_inode_dirty(handle, inode);
> @@ -2571,6 +2599,7 @@ static int __ext4_journalled_writepage(struct page *page,
>                                write_end_fn);
>        if (ret == 0)
>                ret = err;
> +       EXT4_I(inode)->i_datasync_tid = handle->h_transaction->t_tid;
>        err = ext4_journal_stop(handle);
>        if (!ret)
>                ret = err;
> --
> 1.7.1
>

Hi Jan,

Patch looks correct to me, but I am uncomfortable with i_datasync_tid
being treated differently
in journalled write - that is, being set on different places in the write paths.

How about setting i_datasync_tid in a more generic place like
ext4_{,da_}write_begin()?
I know it's a bit redundant to setting dirty pages, but at least this
way i_datasync_tid can be
checked in all journal modes and have a consistent meaning.

Perhaps we can even use i_datasync_tid to optimize away things like
fiemap checks for dirty pages.
Just a though.

Amir.
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