linux-ext4 - Re: [PATCH v2 2/3] fs: record I_DIRTY_TIME even if inode already has I_DIRTY

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Date:   Mon, 8 Aug 2022 09:08:10 +1000
From:   Dave Chinner <david@...morbit.com>
To:     Lukas Czerner <lczerner@...hat.com>
Cc:     linux-ext4@...r.kernel.org, jlayton@...nel.org, tytso@....edu,
        linux-fsdevel@...r.kernel.org,
        Christoph Hellwig <hch@...radead.org>, Jan Kara <jack@...e.cz>
Subject: Re: [PATCH v2 2/3] fs: record I_DIRTY_TIME even if inode already has
 I_DIRTY_INODE

On Wed, Aug 03, 2022 at 12:53:39PM +0200, Lukas Czerner wrote:
> Currently the I_DIRTY_TIME will never get set if the inode already has
> I_DIRTY_INODE with assumption that it supersedes I_DIRTY_TIME.  That's
> true, however ext4 will only update the on-disk inode in
> ->dirty_inode(), not on actual writeback. As a result if the inode
> already has I_DIRTY_INODE state by the time we get to
> __mark_inode_dirty() only with I_DIRTY_TIME, the time was already filled
> into on-disk inode and will not get updated until the next I_DIRTY_INODE
> update, which might never come if we crash or get a power failure.
> 
> The problem can be reproduced on ext4 by running xfstest generic/622
> with -o iversion mount option.
> 
> Fix it by allowing I_DIRTY_TIME to be set even if the inode already has
> I_DIRTY_INODE. Also make sure that the case is properly handled in
> writeback_single_inode() as well. Additionally changes in
> xfs_fs_dirty_inode() was made to accommodate for I_DIRTY_TIME in flag.
> 
> Thanks Jan Kara for suggestions on how to make this work properly.
> 
> Cc: Dave Chinner <david@...morbit.com>
> Cc: Christoph Hellwig <hch@...radead.org>
> Signed-off-by: Lukas Czerner <lczerner@...hat.com>
> Suggested-by: Jan Kara <jack@...e.cz>
> ---
> v2: Reworked according to suggestions from Jan

....

> diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
> index aa977c7ea370..cff05a4771b5 100644
> --- a/fs/xfs/xfs_super.c
> +++ b/fs/xfs/xfs_super.c
> @@ -658,7 +658,8 @@ xfs_fs_dirty_inode(
>  
>  	if (!(inode->i_sb->s_flags & SB_LAZYTIME))
>  		return;
> -	if (flag != I_DIRTY_SYNC || !(inode->i_state & I_DIRTY_TIME))
> +	if ((flag & ~I_DIRTY_TIME) != I_DIRTY_SYNC ||
> +	    !((inode->i_state | flag) & I_DIRTY_TIME))
>  		return;

My eyes, they bleed. The dirty time code was already a horrid
abomination, and this makes it worse.

>From looking at the code, I cannot work out what the new semantics
for I_DIRTY_TIME and I_DIRTY_SYNC are supposed to be, nor can I work
out what the condition this is new code is supposed to be doing. I
*can't verify it is correct* by reading the code.

Can you please add a comment here explaining the conditions where we
don't have to log a new timestamp update?

Also, if "flag" now contains multiple flags, can you rename it
"flags"?

Cheers,

Dave.

-- 
Dave Chinner
david@...morbit.com