linux-kernel - Re: [RFC PATCH V6 4/7] ovl: set 'DONTCACHE' flag for overlayfs inode

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Message-ID: <20211126092023.GD13004@quack2.suse.cz>
Date:   Fri, 26 Nov 2021 10:20:23 +0100
From:   Jan Kara <jack@...e.cz>
To:     Chengguang Xu <cgxu519@...ernel.net>
Cc:     miklos@...redi.hu, jack@...e.cz, amir73il@...il.com,
        linux-unionfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Chengguang Xu <charliecgxu@...cent.com>
Subject: Re: [RFC PATCH V6 4/7] ovl: set 'DONTCACHE' flag for overlayfs inode

On Mon 22-11-21 11:00:35, Chengguang Xu wrote:
> From: Chengguang Xu <charliecgxu@...cent.com>
> 
> Set 'DONTCACHE' flag to overlayfs inode so that
> upper inode to be always synced before eviction.
> 
> Signed-off-by: Chengguang Xu <charliecgxu@...cent.com>
> ---
>  fs/overlayfs/inode.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
> index 027ffc0a2539..c4472299d5df 100644
> --- a/fs/overlayfs/inode.c
> +++ b/fs/overlayfs/inode.c
> @@ -791,6 +791,7 @@ void ovl_inode_init(struct inode *inode, struct ovl_inode_params *oip,
>  	ovl_copyattr(realinode, inode);
>  	ovl_copyflags(realinode, inode);
>  	ovl_map_ino(inode, ino, fsid);
> +	d_mark_dontcache(inode);
>  }

Doesn't this effectively disable dcache for overlayfs dentries? I mean e.g.
whenever overlayfs file is closed, we will drop its dentry & inode from the
cache. Upper and lower inodes / dentries stay in cache so no disk access
should be needed to reconstruct overlayfs dentry & inode but still it may
be a bit costly? I guess others more familiar with overlayfs have to judge.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR