linux-kernel - Re: [PATCH v3] loop: use vfs_getattr

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Message-ID: <ddaef6bd-fbf3-0d6b-e437-6f4fd9c967d2@huaweicloud.com>
Date: Wed, 13 Aug 2025 09:38:54 +0800
From: Yu Kuai <yukuai1@...weicloud.com>
To: Rajeev Mishra <rajeevm@....com>, axboe@...nel.dk, yukuai1@...weicloud.com
Cc: linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
 "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH v3] loop: use vfs_getattr_nosec() for accurate file size

Hi,

在 2025/08/13 4:07, Rajeev Mishra 写道:
> This commit includes the following changes:
>   1. Renamed get_size to lo_calculate_size.
>   2. Merged the functionality of get_size and
>      get_loop_size into lo_calculate_size.
>   3  Updated callers of the above functions
>      to use lo_calculate_size.
>   4. Replaced i_size_read with vfs_getattr_nosec()
>      to obtain a more accurate file size for
>       network filesystems where cached metadata may be stale
> 
Please split 1-3 to a seperate patch.

> Signed-off-by: Rajeev Mishra <rajeevm@....com>
> ---
>   drivers/block/loop.c | 53 +++++++++++++++++++++++++++++---------------
>   1 file changed, 35 insertions(+), 18 deletions(-)
> 
> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> index 1b6ee91f8eb9..6bfec38275b0 100644
> --- a/drivers/block/loop.c
> +++ b/drivers/block/loop.c
> @@ -137,20 +137,43 @@ static void loop_global_unlock(struct loop_device *lo, bool global)
>   static int max_part;
>   static int part_shift;
>   
> -static loff_t get_size(loff_t offset, loff_t sizelimit, struct file *file)
> +/**
> + * lo_calculate_size - calculate the effective size of a loop device
> + * @lo: loop device
> + * @file: backing file
> + *
> + * Calculate the effective size of the loop device based on backing file size,
> + * offset, and size limit. Uses vfs_getattr_nosec() for accurate file size
> + * information, particularly important for network filesystems where cached
> + * metadata may be stale.
> + *
> + * Returns: size in 512-byte sectors, or 0 on error
> + */

I feel this internal function comment is not necessary, the name can
explain itself, and you explain getattr below as well.

Otherwise this patch LGTM.

Thanks,
Kuai

> +static loff_t lo_calculate_size(struct loop_device *lo, struct file *file)
>   {
> +	struct kstat stat;
>   	loff_t loopsize;
> +	int ret;
>   
> -	/* 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 */
> +	/*
> +	 * Get the accurate file size. This provides better results than
> +	 * cached inode data, particularly for network filesystems where
> +	 * metadata may be stale.
> +	 */
> +	ret = vfs_getattr_nosec(&file->f_path, &stat, STATX_SIZE, 0);
> +	if (ret)
> +		return 0;
> +
> +	loopsize = stat.size;
> +
> +	if (lo->lo_offset > 0)
> +		loopsize -= lo->lo_offset;
> +	/* offset is beyond file size, weird but possible */
>   	if (loopsize < 0)
>   		return 0;
>   
> -	if (sizelimit > 0 && sizelimit < loopsize)
> -		loopsize = sizelimit;
> +	if (lo->lo_sizelimit > 0 && lo->lo_sizelimit < loopsize)
> +		loopsize = lo->lo_sizelimit;
>   	/*
>   	 * Unfortunately, if we want to do I/O on the device,
>   	 * the number of 512-byte sectors has to fit into a sector_t.
> @@ -158,11 +181,6 @@ static loff_t get_size(loff_t offset, loff_t sizelimit, struct file *file)
>   	return loopsize >> 9;
>   }
>   
> -static loff_t get_loop_size(struct loop_device *lo, struct file *file)
> -{
> -	return get_size(lo->lo_offset, lo->lo_sizelimit, file);
> -}
> -
>   /*
>    * We support direct I/O only if lo_offset is aligned with the logical I/O size
>    * of backing device, and the logical block size of loop is bigger than that of
> @@ -569,7 +587,7 @@ static int loop_change_fd(struct loop_device *lo, struct block_device *bdev,
>   	error = -EINVAL;
>   
>   	/* size of the new backing store needs to be the same */
> -	if (get_loop_size(lo, file) != get_loop_size(lo, old_file))
> +	if (lo_calculate_size(lo, file) != lo_calculate_size(lo, old_file))
>   		goto out_err;
>   
>   	/*
> @@ -1063,7 +1081,7 @@ static int loop_configure(struct loop_device *lo, blk_mode_t mode,
>   	loop_update_dio(lo);
>   	loop_sysfs_init(lo);
>   
> -	size = get_loop_size(lo, file);
> +	size = lo_calculate_size(lo, file);
>   	loop_set_size(lo, size);
>   
>   	/* Order wrt reading lo_state in loop_validate_file(). */
> @@ -1255,8 +1273,7 @@ loop_set_status(struct loop_device *lo, const struct loop_info64 *info)
>   	if (partscan)
>   		clear_bit(GD_SUPPRESS_PART_SCAN, &lo->lo_disk->state);
>   	if (!err && size_changed) {
> -		loff_t new_size = get_size(lo->lo_offset, lo->lo_sizelimit,
> -					   lo->lo_backing_file);
> +		loff_t new_size = lo_calculate_size(lo, lo->lo_backing_file);
>   		loop_set_size(lo, new_size);
>   	}
>   out_unlock:
> @@ -1399,7 +1416,7 @@ static int loop_set_capacity(struct loop_device *lo)
>   	if (unlikely(lo->lo_state != Lo_bound))
>   		return -ENXIO;
>   
> -	size = get_loop_size(lo, lo->lo_backing_file);
> +	size = lo_calculate_size(lo, lo->lo_backing_file);
>   	loop_set_size(lo, size);
>   
>   	return 0;
>