Re: [RFC PATCH v2 2/8] ovl: implement ->writepages operation

From: Jan Kara <jack@suse.cz>
To: Chengguang Xu <cgxu519@mykernel.net>
Cc: miklos@szeredi.hu, amir73il@gmail.com, jack@suse.cz,
	linux-unionfs@vger.kernel.org, linux-fsdevel@vger.kernel.org
Subject: Re: [RFC PATCH v2 2/8] ovl: implement ->writepages operation
Date: Mon, 2 Nov 2020 18:17:41 +0100	[thread overview]
Message-ID: <20201102171741.GE23988@quack2.suse.cz> (raw)
In-Reply-To: <20201025034117.4918-3-cgxu519@mykernel.net>

On Sun 25-10-20 11:41:11, Chengguang Xu wrote:
> Implement overlayfs' ->writepages operation so that
> we can sync dirty data/metadata to upper filesystem.
> 
> Signed-off-by: Chengguang Xu <cgxu519@mykernel.net>
> ---
>  fs/overlayfs/inode.c | 26 ++++++++++++++++++++++++++
>  1 file changed, 26 insertions(+)
> 
> diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
> index b584dca845ba..f27fc5be34df 100644
> --- a/fs/overlayfs/inode.c
> +++ b/fs/overlayfs/inode.c
> @@ -11,6 +11,7 @@
>  #include <linux/posix_acl.h>
>  #include <linux/ratelimit.h>
>  #include <linux/fiemap.h>
> +#include <linux/writeback.h>
>  #include "overlayfs.h"
>  
>  
> @@ -516,7 +517,32 @@ static const struct inode_operations ovl_special_inode_operations = {
>  	.update_time	= ovl_update_time,
>  };
>  
> +static int ovl_writepages(struct address_space *mapping,
> +			  struct writeback_control *wbc)
> +{
> +	struct inode *upper_inode = ovl_inode_upper(mapping->host);
> +	struct ovl_fs *ofs =  mapping->host->i_sb->s_fs_info;
> +	struct writeback_control tmp_wbc = *wbc;
> +
> +	if (!ovl_should_sync(ofs))
> +		return 0;
> +
> +	/*
> +	 * for sync(2) writeback, it has a separate external IO
> +	 * completion path by checking PAGECACHE_TAG_WRITEBACK
> +	 * in pagecache, we have to set for_sync to 0 in thie case,
> +	 * let writeback waits completion after syncing individual
> +	 * dirty inode, because we haven't implemented overlayfs'
> +	 * own pagecache yet.
> +	 */
> +	if (wbc->for_sync && (wbc->sync_mode == WB_SYNC_ALL))
> +		tmp_wbc.for_sync = 0;

This looks really hacky as it closely depends on the internal details of
writeback implementation. I'd be more open to say export wait_sb_inodes()
for overlayfs use... Because that's what I gather you need in your
overlayfs ->syncfs() implementation.

								Honza

> +
> +	return sync_inode(upper_inode, &tmp_wbc);
> +}
> +
>  static const struct address_space_operations ovl_aops = {
> +	.writepages		= ovl_writepages,
>  	/* For O_DIRECT dentry_open() checks f_mapping->a_ops->direct_IO */
>  	.direct_IO		= noop_direct_IO,
>  };
> -- 
> 2.26.2
> 
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR