linux-kernel - Re: [PATCH v2 5/8] fsdax: dedupe: iter two files at the same time

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Date:   Thu, 1 Dec 2022 16:05:37 -0800
From:   "Darrick J. Wong" <djwong@...nel.org>
To:     Shiyang Ruan <ruansy.fnst@...itsu.com>
Cc:     linux-kernel@...r.kernel.org, linux-xfs@...r.kernel.org,
        nvdimm@...ts.linux.dev, linux-fsdevel@...r.kernel.org,
        david@...morbit.com, dan.j.williams@...el.com,
        akpm@...ux-foundation.org
Subject: Re: [PATCH v2 5/8] fsdax: dedupe: iter two files at the same time

On Thu, Dec 01, 2022 at 03:31:41PM +0000, Shiyang Ruan wrote:
> The iomap_iter() on a range of one file may loop more than once.  In
> this case, the inner dst_iter can update its iomap but the outer
> src_iter can't.  This may cause the wrong remapping in filesystem.  Let
> them called at the same time.
> 
> Signed-off-by: Shiyang Ruan <ruansy.fnst@...itsu.com>

Thank you for adding that explanation, it makes the problem much more
obvious. :)

Reviewed-by: Darrick J. Wong <djwong@...nel.org>

--D

> ---
>  fs/dax.c | 16 ++++++++--------
>  1 file changed, 8 insertions(+), 8 deletions(-)
> 
> diff --git a/fs/dax.c b/fs/dax.c
> index f1eb59bee0b5..354be56750c2 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -1964,15 +1964,15 @@ int dax_dedupe_file_range_compare(struct inode *src, loff_t srcoff,
>  		.len		= len,
>  		.flags		= IOMAP_DAX,
>  	};
> -	int ret;
> +	int ret, compared = 0;
>  
> -	while ((ret = iomap_iter(&src_iter, ops)) > 0) {
> -		while ((ret = iomap_iter(&dst_iter, ops)) > 0) {
> -			dst_iter.processed = dax_range_compare_iter(&src_iter,
> -						&dst_iter, len, same);
> -		}
> -		if (ret <= 0)
> -			src_iter.processed = ret;
> +	while ((ret = iomap_iter(&src_iter, ops)) > 0 &&
> +	       (ret = iomap_iter(&dst_iter, ops)) > 0) {
> +		compared = dax_range_compare_iter(&src_iter, &dst_iter, len,
> +						  same);
> +		if (compared < 0)
> +			return ret;
> +		src_iter.processed = dst_iter.processed = compared;
>  	}
>  	return ret;
>  }
> -- 
> 2.38.1
>