Re: [PATCH v2 2/3] fs: record I_DIRTY_TIME even if inode already has I_DIRTY_INODE

[Lukas Czerner <lczerner@redhat.com>]

On Mon, Aug 08, 2022 at 09:08:10AM +1000, Dave Chinner wrote:
> 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@fromorbit.com>
> > Cc: Christoph Hellwig <hch@infradead.org>
> > Signed-off-by: Lukas Czerner <lczerner@redhat.com>
> > Suggested-by: Jan Kara <jack@suse.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

Hi Dave,

please see the other thready for this patch with Eric Biggers, where I
try to explain and give some suggestion to change the doc. Does it make
sense to you, or am I missing something?

https://marc.info/?l=linux-ext4&m=165970194205621&w=2

> out what the condition this is new code is supposed to be doing. I
> *can't verify it is correct* by reading the code.

The ->dirty_inode() needed to be changed to clear I_DIRTY_TIME from
i_state *before* we call ->dirty_inode() to avoid race where we would
lose timestamp update that comes just a little later, after
-dirty_inode() call with I_DRITY_INODE.

But that would break xfs, so I decided to keep the condition and loosen
the requirement so that I_DIRTY_TIME can also be se in 'flag', not just
the i_state. Hence the abomination.

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

How about something like this?

Only do the timestamp update if the inode is dirty (I_DIRTY_SYNC) and
has dirty timestamp (I_DIRTY_TIME). I_DIRTY_TIME can be either already
set in i_state, or passed in flags possibly together with I_DIRTY_SYNC.

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

Sure, I can do that.

Thanks!
-Lukas

> 
> Cheers,
> 
> Dave.
> 
> -- 
> Dave Chinner
> david@fromorbit.com
>