linux-kernel - Re: [RFC PATCH v5 07/10] ovl: cache dirty overlayfs' inode

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Date:   Thu, 7 Oct 2021 13:09:46 +0200
From:   Miklos Szeredi <miklos@...redi.hu>
To:     Chengguang Xu <cgxu519@...ernel.net>
Cc:     Jan Kara <jack@...e.cz>, Amir Goldstein <amir73il@...il.com>,
        linux-fsdevel@...r.kernel.org,
        overlayfs <linux-unionfs@...r.kernel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v5 07/10] ovl: cache dirty overlayfs' inode

On Thu, 23 Sept 2021 at 15:08, Chengguang Xu <cgxu519@...ernel.net> wrote:
>
> Now drop overlayfs' inode will sync dirty data,
> so we change to only drop clean inode.
>
> The purpose of doing this is to keep compatible
> behavior with before because without this change
> dropping overlayfs inode will not trigger syncing
> of underlying dirty inode.
>
> Signed-off-by: Chengguang Xu <cgxu519@...ernel.net>
> ---
>  fs/overlayfs/super.c | 16 +++++++++++++++-
>  1 file changed, 15 insertions(+), 1 deletion(-)
>
> diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
> index cddae3ca2fa5..bf4000eb9be8 100644
> --- a/fs/overlayfs/super.c
> +++ b/fs/overlayfs/super.c
> @@ -441,11 +441,25 @@ static int ovl_write_inode(struct inode *inode,
>         return ret;
>  }
>
> +/*
> + * In iput_final(), clean inode will drop directly and dirty inode will
> + * keep in the cache until write back to sync dirty data then add to lru
> + * list to wait reclaim.
> + */
> +static int ovl_drop_inode(struct inode *inode)
> +{
> +       struct inode *upper = ovl_inode_upper(inode);
> +
> +       if (!upper || !(inode->i_state & I_DIRTY_ALL))

Could we check upper dirtyness here? That would give a more precise result.

Alternatively don't set .drop_inode (i.e. use generic_drop_inode())
and set I_DONTCACHE on overlay inodes.  That would cause the upper
inode to be always written back before eviction.

The latter would result in simpler logic, and I think performance-wise
it wouldn't matter.   But I may be missing something.

Thanks,
Miklos