linux-ext4 - Re: [PATCH 1/2] libext2fs: use ZERO_RANGE before PUNCH_HOLE in unix

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Date:   Wed, 18 Apr 2018 09:46:40 -0600
From:   Andreas Dilger <adilger@...ger.ca>
To:     "Darrick J. Wong" <darrick.wong@...cle.com>
Cc:     tytso@....edu, linux-ext4@...r.kernel.org
Subject: Re: [PATCH 1/2] libext2fs: use ZERO_RANGE before PUNCH_HOLE in
 unix_zeroout

On Apr 17, 2018, at 1:19 PM, Darrick J. Wong <darrick.wong@...cle.com> wrote:
> 
> From: Darrick J. Wong <darrick.wong@...cle.com>
> 
> In unix_zeroout() for files, we should try a ZERO_RANGE before we try
> PUNCH_HOLE because the former will not cause us to lose preallocated
> blocks.  Since block devices have supported fallocate for a few years
> now, refactor the fallocate calls into a helper and call it from either
> case.
> 
> Reported-by: Andreas Dilger <adilger@...ger.ca>
> Signed-off-by: Darrick J. Wong <darrick.wong@...cle.com>

The patch was a bit confusing to read (I thought it skipped zero-out
for block devices), but looking at the whole code it is clear that the
__unix_zeroout() is always being called, and the truncate() code block
is only being called for regular files.

Reviewed-by: Andreas Dilger <adilger@...ger.ca>

> ---
> lib/ext2fs/unix_io.c |   53 ++++++++++++++++++++++++++++----------------------
> 1 file changed, 30 insertions(+), 23 deletions(-)
> 
> 
> diff --git a/lib/ext2fs/unix_io.c b/lib/ext2fs/unix_io.c
> index c6b058d..1fb6279 100644
> --- a/lib/ext2fs/unix_io.c
> +++ b/lib/ext2fs/unix_io.c
> @@ -1118,6 +1118,31 @@ static errcode_t unix_discard(io_channel channel, unsigned long long block,
> 	return EXT2_ET_UNIMPLEMENTED;
> }
> 
> +/*
> + * If we know about ZERO_RANGE, try that before we try PUNCH_HOLE because
> + * ZERO_RANGE doesn't unmap preallocated blocks.  We prefer fallocate because
> + * it always invalidates page cache, and libext2fs requires that reads after
> + * ZERO_RANGE return zeroes.
> + */
> +static int __unix_zeroout(int fd, off_t offset, off_t len)
> +{
> +	int ret = -1;
> +
> +#if defined(HAVE_FALLOCATE) && defined(FALLOC_FL_ZERO_RANGE)
> +	ret = fallocate(fd, FALLOC_FL_ZERO_RANGE, offset, len);
> +	if (ret == 0)
> +		return 0;
> +#endif
> +#if defined(HAVE_FALLOCATE) && defined(FALLOC_FL_PUNCH_HOLE) && defined(FALLOC_FL_KEEP_SIZE)
> +	ret = fallocate(fd, FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE,
> +			offset,  len);
> +	if (ret == 0)
> +		return 0;
> +#endif
> +	errno = EOPNOTSUPP;
> +	return ret;
> +}
> +
> /* parameters might not be used if OS doesn't support zeroout */
> #pragma GCC diagnostic push
> #pragma GCC diagnostic ignored "-Wunused-parameter"
> @@ -1134,10 +1159,7 @@ static errcode_t unix_zeroout(io_channel channel, unsigned long long block,
> 	if (safe_getenv("UNIX_IO_NOZEROOUT"))
> 		goto unimplemented;
> 
> -	if (channel->flags & CHANNEL_FLAGS_BLOCK_DEVICE) {
> -		/* Not implemented until the BLKZEROOUT mess is fixed */
> -		goto unimplemented;
> -	} else {
> +	if (!(channel->flags & CHANNEL_FLAGS_BLOCK_DEVICE)) {
> 		/* Regular file, try to use truncate/punch/zero. */
> 		struct stat statbuf;
> 
> @@ -1157,26 +1179,11 @@ static errcode_t unix_zeroout(io_channel channel, unsigned long long block,
> 			if (ret)
> 				goto err;
> 		}
> -#if defined(HAVE_FALLOCATE) && (defined(FALLOC_FL_ZERO_RANGE) || \
> -	(defined(FALLOC_FL_PUNCH_HOLE) && defined(FALLOC_FL_KEEP_SIZE)))
> -#if defined(FALLOC_FL_PUNCH_HOLE) && defined(FALLOC_FL_KEEP_SIZE)
> -		ret = fallocate(data->dev,
> -				FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE,
> -				(off_t)(block) * channel->block_size + data->offset,
> -				(off_t)(count) * channel->block_size);
> -		if (ret == 0)
> -			goto err;
> -#endif
> -#ifdef FALLOC_FL_ZERO_RANGE
> -		ret = fallocate(data->dev,
> -				FALLOC_FL_ZERO_RANGE,
> -				(off_t)(block) * channel->block_size + data->offset,
> -				(off_t)(count) * channel->block_size);
> -#endif
> -#else
> -		goto unimplemented;
> -#endif /* HAVE_FALLOCATE && (ZERO_RANGE || (PUNCH_HOLE && KEEP_SIZE)) */
> 	}
> +
> +	ret = __unix_zeroout(data->dev,
> +			(off_t)(block) * channel->block_size + data->offset,
> +			(off_t)(count) * channel->block_size);
> err:
> 	if (ret < 0) {
> 		if (errno == EOPNOTSUPP)
> 


Cheers, Andreas






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