linux-kernel - Re: [PATCH] ext4: Fix handle refcount leak in ext4_write

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Date:   Thu, 7 Oct 2021 16:06:40 +0200
From:   Jan Kara <jack@...e.cz>
To:     Chenyuan Mi <cymi20@...an.edu.cn>
Cc:     yuanxzhang@...an.edu.cn, Xiyu Yang <xiyuyang19@...an.edu.cn>,
        Xin Tan <tanxin.ctf@...il.com>, Theodore Ts'o <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ext4: Fix handle refcount leak in ext4_write_begin()

On Wed 08-09-21 15:06:40, Chenyuan Mi wrote:
> The reference counting issue happens when ret is zero,
> the function forgets to decrease the refcount of handle
> increased by ext4_journal_start().
> 
> Fix this issue by using ext4_journal_stop() to decrease
> the refcount of handle.
> 
> Signed-off-by: Chenyuan Mi <cymi20@...an.edu.cn>
> Signed-off-by: Xiyu Yang <xiyuyang19@...an.edu.cn>
> Signed-off-by: Xin Tan <tanxin.ctf@...il.com>

Yes, but that is deliberate. We want to keep the transaction running until
the write is done - i.e., until ->write_end() callback where we call
ext4_journal_stop(). As a side note, please test your patches. You clearly
didn't test this patch because if you did, you would notice that your patch
breaks writing to ext4.

                                                                Honza

> ---
>  fs/ext4/inode.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index d18852d6029c..90c57d8e3de1 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -1249,6 +1249,7 @@ static int ext4_write_begin(struct file *file, struct address_space *mapping,
>  		put_page(page);
>  		return ret;
>  	}
> +	ext4_journal_stop(handle);
>  	*pagep = page;
>  	return ret;
>  }
> -- 
> 2.17.1
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR