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Date:   Tue, 22 Dec 2020 12:23:11 +1100
From:   NeilBrown <neilb@...e.de>
To:     Vivek Goyal <vgoyal@...hat.com>, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-unionfs@...r.kernel.org
Cc:     jlayton@...nel.org, vgoyal@...hat.com, amir73il@...il.com,
        sargun@...gun.me, miklos@...redi.hu, willy@...radead.org,
        jack@...e.cz, neilb@...e.com, viro@...iv.linux.org.uk, hch@....de
Subject: Re: [PATCH 1/3] vfs: Do not ignore return code from s_op->sync_fs

On Mon, Dec 21 2020, Vivek Goyal wrote:

> Current implementation of __sync_filesystem() ignores the
> return code from ->sync_fs(). I am not sure why that's the case.
>
> Ignoring ->sync_fs() return code is problematic for overlayfs where
> it can return error if sync_filesystem() on upper super block failed.
> That error will simply be lost and sycnfs(overlay_fd), will get
> success (despite the fact it failed).
>
> Al Viro noticed that there are other filesystems which can sometimes
> return error in ->sync_fs() and these errors will be ignored too.
>
> fs/btrfs/super.c:2412:  .sync_fs        = btrfs_sync_fs,
> fs/exfat/super.c:204:   .sync_fs        = exfat_sync_fs,
> fs/ext4/super.c:1674:   .sync_fs        = ext4_sync_fs,
> fs/f2fs/super.c:2480:   .sync_fs        = f2fs_sync_fs,
> fs/gfs2/super.c:1600:   .sync_fs                = gfs2_sync_fs,
> fs/hfsplus/super.c:368: .sync_fs        = hfsplus_sync_fs,
> fs/nilfs2/super.c:689:  .sync_fs        = nilfs_sync_fs,
> fs/ocfs2/super.c:139:   .sync_fs        = ocfs2_sync_fs,
> fs/overlayfs/super.c:399:	.sync_fs        = ovl_sync_fs,
> fs/ubifs/super.c:2052:  .sync_fs       = ubifs_sync_fs,
>
> Hence, this patch tries to fix it and capture error returned
> by ->sync_fs() and return to caller. I am specifically interested
> in syncfs() path and return error to user.
>
> I am assuming that we want to continue to call __sync_blockdev()
> despite the fact that there have been errors reported from
> ->sync_fs(). So this patch continues to call __sync_blockdev()
> even if ->sync_fs() returns an error.
>
> Al noticed that there are few other callsites where ->sync_fs() error
> code is being ignored.
>
> sync_fs_one_sb(): For this it seems desirable to ignore the return code.
>
> dquot_disable(): Jan Kara mentioned that ignoring return code here is fine
> 		 because we don't want to fail dquot_disable() just beacuse
> 		 caches might be incoherent.
>
> dquot_quota_sync(): Jan thinks that it might make some sense to capture
> 		    return code here. But I am leaving it untouched for
> 		   now. When somebody needs it, they can easily fix it.
>
> Signed-off-by: Vivek Goyal <vgoyal@...hat.com>
> ---
>  fs/sync.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/fs/sync.c b/fs/sync.c
> index 1373a610dc78..b5fb83a734cd 100644
> --- a/fs/sync.c
> +++ b/fs/sync.c
> @@ -30,14 +30,18 @@
>   */
>  static int __sync_filesystem(struct super_block *sb, int wait)
>  {
> +	int ret, ret2;
> +
>  	if (wait)
>  		sync_inodes_sb(sb);
>  	else
>  		writeback_inodes_sb(sb, WB_REASON_SYNC);
>  
>  	if (sb->s_op->sync_fs)
> -		sb->s_op->sync_fs(sb, wait);
> -	return __sync_blockdev(sb->s_bdev, wait);
> +		ret = sb->s_op->sync_fs(sb, wait);
> +	ret2 = __sync_blockdev(sb->s_bdev, wait);
> +
> +	return ret ? ret : ret2;

I'm surprised that the compiler didn't complain that 'ret' might be used
uninitialized.

NeilBrown

>  }
>  
>  /*
> -- 
> 2.25.4

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