linux-kernel - Re: [PATCH v2] loop: use vfs_getattr

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Message-ID: <45b33232-9089-4df4-b9f8-c843cb3d2d07@kernel.org>
Date: Tue, 12 Aug 2025 10:40:21 +0900
From: Damien Le Moal <dlemoal@...nel.org>
To: Rajeev Mishra <rajeevm@....com>, axboe@...nel.dk, yukuai1@...weicloud.com
Cc: linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] loop: use vfs_getattr_nosec() for accurate file size

On 8/12/25 4:03 AM, Rajeev Mishra wrote:
> The get_size() function now uses vfs_getattr_nosec() instead of
> i_size_read() to obtain file size information. This provides more
> accurate results for network filesystems where cached metadata
> may be stale, ensuring the loop device reflects the current file
> size rather than potentially outdated cached values.
> 
> Signed-off-by: Rajeev Mishra <rajeevm@....com>
> ---
>  drivers/block/loop.c | 24 ++++++++++++++++++++++--
>  1 file changed, 22 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> index 1b6ee91f8eb9..c418c47db76e 100644
> --- a/drivers/block/loop.c
> +++ b/drivers/block/loop.c
> @@ -137,12 +137,32 @@ static void loop_global_unlock(struct loop_device *lo, bool global)
>  static int max_part;
>  static int part_shift;
>  
> +/**
> + * get_size - calculate the effective size of a loop device
> + * @offset: offset into the backing file
> + * @sizelimit: user-specified size limit
> + * @file: the backing file
> + *
> + * Calculate the effective size of the loop device
> + *
> + * Returns: size in 512-byte sectors, or 0 if invalid
> + */
>  static loff_t get_size(loff_t offset, loff_t sizelimit, struct file *file)
>  {
> +	struct kstat stat;
>  	loff_t loopsize;
> +	int ret;
> +
> +	/*
> +	 * Get the accurate file size. This will prevent caching
> +	 * issue that occurs at filesystem layer.
> +	 */
> +	ret = vfs_getattr_nosec(&file->f_path, &stat, STATX_SIZE, 0);
> +	if (ret)
> +		return 0;

return 0 here is odd. Why not "return ret;" to propagate the error if any ?
An error may come from the underlying FS inode->i_op->getattr().

> +
> +	loopsize = stat.size;
>  
> -	/* Compute loopsize in bytes */
> -	loopsize = i_size_read(file->f_mapping->host);
>  	if (offset > 0)
>  		loopsize -= offset;
>  	/* offset is beyond i_size, weird but possible */


-- 
Damien Le Moal
Western Digital Research