Re: [PATCH] fstests: fix fssum to actually ignore file holes when supposed to

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

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On 2018/10/29 下午5:43, fdmanana@kernel.org wrote:
> From: Filipe Manana <fdmanana@suse.com>
> 
> Unless the '-s' option is passed to fssum, it should not detect file holes
> and have their existence influence the computed checksum for a file. This
> tool was added to test btrfs' send/receive feature, so that it checks for
> any metadata and data differences between the original filesystem and the
> filesystem that receives send streams.
> 
> For a long time the test btrfs/007, which tests btrfs' send/receive with
> fsstress, fails sporadically reporting data differences between files.
> However the md5sum/sha1sum from the reported files in the original and
> new filesystems are the same. The reason why fssum fails is because even
> in normal mode it still accounts for number of holes that exist in the
> file and their respective lengths. This is done using the SEEK_DATA mode
> of lseek. The btrfs send feature does not preserve holes nor prealloc
> extents (not supported by the current protocol), so whenever a hole or
> prealloc (unwritten) extent is detected in the source filesystem, it
> issues a write command full of zeroes, which will translate to a regular
> (written) extent in the destination filesystem. This is why fssum reports
> a different checksum. A prealloc extent also counts as hole when using
> lseek.
> 
> For example when passing a seed of 1540592967 to fsstress in btrfs/007,
> the test fails, as file p0/d0/f7 has a prealloc extent in the original
> filesystem (in the incr snapshot).
> 
> Fix this by making fssum just read the hole file and feed its data to the
> digest calculation function when option '-s' is not given. If we ever get
> btrfs' send/receive to support holes and fallocate, we can just change
> the test and pass the '-s' option to all fssum calls.

However this is causing more problem here.

Now since fssum doesn't skip holes, for a super large sparse file, fssum
will try to fill all the holes with zero and spend tons of CPU time
calculating the result.

E.g. when seed = 1550703281.
We will have a PiB level file:
$ ls /mnt/scratch/base/p0/f3 -alh
-rw-rw-rw- 1 1441 2774 666P Feb 13 13:39 /mnt/scratch/base/p0/f3

(well, 666, no wonder this will fail)

But it only takes aroud 1M:
$ du /mnt/scratch/base/p0/f3
1044	/mnt/scratch/base/p0/f3

This is even more annoying than test failure.

Can't we just filter out the fallocate/hole punching part in the test
itself?

Thanks,
Qu

> 
> Signed-off-by: Filipe Manana <fdmanana@suse.com>
> ---
>  src/fssum.c | 65 +++++--------------------------------------------------------
>  1 file changed, 5 insertions(+), 60 deletions(-)
> 
> diff --git a/src/fssum.c b/src/fssum.c
> index 5da39abf..f1da72fb 100644
> --- a/src/fssum.c
> +++ b/src/fssum.c
> @@ -224,71 +224,16 @@ int
>  sum_file_data_permissive(int fd, sum_t *dst)
>  {
>  	int ret;
> -	off_t pos;
> -	off_t old;
> -	int i;
> -	uint64_t zeros = 0;
> -
> -	pos = lseek(fd, 0, SEEK_CUR);
> -	if (pos == (off_t)-1)
> -		return errno == ENXIO ? 0 : -2;
>  
>  	while (1) {
> -		old = pos;
> -		pos = lseek(fd, pos, SEEK_DATA);
> -		if (pos == (off_t)-1) {
> -			if (errno == ENXIO) {
> -				ret = 0;
> -				pos = lseek(fd, 0, SEEK_END);
> -				if (pos != (off_t)-1)
> -					zeros += pos - old;
> -			} else {
> -				ret = -2;
> -			}
> -			break;
> -		}
>  		ret = read(fd, buf, sizeof(buf));
> -		assert(ret); /* eof found by lseek */
> -		if (ret <= 0)
> +		if (ret < 0)
> +			return -errno;
> +		sum_add(dst, buf, ret);
> +		if (ret < sizeof(buf))
>  			break;
> -		if (old < pos) /* hole */
> -			zeros += pos - old;
> -		for (i = 0; i < ret; ++i) {
> -			for (old = i; buf[i] == 0 && i < ret; ++i)
> -				;
> -			if (old < i) /* code like a hole */
> -				zeros += i - old;
> -			if (i == ret)
> -				break;
> -			if (zeros) {
> -				if (verbose >= 2)
> -					fprintf(stderr,
> -						"adding %llu zeros to sum\n",
> -						(unsigned long long)zeros);
> -				sum_add_u64(dst, 0);
> -				sum_add_u64(dst, zeros);
> -				zeros = 0;
> -			}
> -			for (old = i; buf[i] != 0 && i < ret; ++i)
> -				;
> -			if (verbose >= 2)
> -				fprintf(stderr, "adding %u non-zeros to sum\n",
> -					i - (int)old);
> -			sum_add(dst, buf + old, i - old);
> -		}
> -		pos += ret;
>  	}
> -
> -	if (zeros) {
> -		if (verbose >= 2)
> -			fprintf(stderr,
> -				"adding %llu zeros to sum (finishing)\n",
> -				(unsigned long long)zeros);
> -		sum_add_u64(dst, 0);
> -		sum_add_u64(dst, zeros);
> -	}
> -
> -	return ret;
> +	return 0;
>  }
>  
>  int
> 


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