linux-ext4 - Re: [PATCH v3 2/3] fs: record I_DIRTY_TIME even if inode already has I_DIRTY

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Date:   Sat, 20 Aug 2022 23:14:37 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Jan Kara <jack@...e.cz>
Cc:     Eric Biggers <ebiggers@...nel.org>,
        Lukas Czerner <lczerner@...hat.com>,
        linux-ext4@...r.kernel.org, tytso@....edu, jlayton@...nel.org,
        linux-fsdevel@...r.kernel.org, david@...morbit.com,
        Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH v3 2/3] fs: record I_DIRTY_TIME even if inode already has
 I_DIRTY_INODE

On Tue, Aug 16, 2022 at 01:21:24PM +0200, Jan Kara wrote:
> 2) I_DIRTY_TIME flag passed to ->dirty_inode() callback. This is admittedly
> bit of a hack. Currently XFS relies on the fact that the only time its
> ->dirty_inode() callback needs to do anything is when VFS decides it is
> time to writeback timestamps and XFS detects this situation by checking for
> I_DIRTY_TIME in inode->i_state. Now to fix the race, we need to first clear
> I_DIRTY_TIME in inode->i_state and only then call the ->dirty_inode()
> callback (otherwise timestamp update can get lost). So the solution I've
> suggested was to propagate the information "timestamp update needed" to XFS
> through I_DIRTY_TIME in flags passed to ->dirty_inode().

Maybe we should just add a separate update_lazy_time method to make this
a little more clear?