linux-kernel - Re: [PATCH v4 02/12] dax: introduce IS_DEVDAX() and IS

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Message-ID: <20180227165449.abbhpu7gwqpxcqst@quack2.suse.cz>
Date:   Tue, 27 Feb 2018 17:54:49 +0100
From:   Jan Kara <jack@...e.cz>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     linux-nvdimm@...ts.01.org, Theodore Ts'o <tytso@....edu>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        "supporter:XFS FILESYSTEM" <linux-xfs@...r.kernel.org>,
        linux-mm@...ck.org, Andreas Dilger <adilger.kernel@...ger.ca>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, Jan Kara <jack@...e.cz>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>
Subject: Re: [PATCH v4 02/12] dax: introduce IS_DEVDAX() and IS_FSDAX()

On Mon 26-02-18 20:20:05, Dan Williams wrote:
> The current IS_DAX() helper that checks the S_DAX inode flag is
> ambiguous, and currently has the broken assumption that the S_DAX flag

I don't think S_DAX flag is really ambiguous. It is just that in
CONFIG_FS_DAX=n + CONFIG_DEV_DAX=y case, the compiler is not able to figure
out some calls behind IS_DAX() are dead code and so the kernel won't
compile / link. Or is there any other problem I'm missing?

If I'm indeed right, then please tell this in the changelog and don't talk
about abstract ambiguity of S_DAX flag.

As much as I'd prefer to solve link-time problems with stubs instead of
relying on dead-code elimination, I can live with split macros so once the
changelog is settled, feel free to add:

Reviewed-by: Jan Kara <jack@...e.cz>

								Honza

> is only non-zero in the CONFIG_FS_DAX=y case. In preparation for
> defining S_DAX to non-zero in the  CONFIG_FS_DAX=n + CONFIG_DEV_DAX=y
> case, introduce two explicit helpers to replace IS_DAX().
> 
> Cc: "Theodore Ts'o" <tytso@....edu>
> Cc: Andreas Dilger <adilger.kernel@...ger.ca>
> Cc: Alexander Viro <viro@...iv.linux.org.uk>
> Cc: "Darrick J. Wong" <darrick.wong@...cle.com>
> Cc: linux-xfs@...r.kernel.org (supporter:XFS FILESYSTEM)
> Cc: Matthew Wilcox <mawilcox@...rosoft.com>
> Cc: Ross Zwisler <ross.zwisler@...ux.intel.com>
> Cc: <stable@...r.kernel.org>
> Fixes: dee410792419 ("/dev/dax, core: file operations and dax-mmap")
> Reported-by: Jan Kara <jack@...e.cz>
> Signed-off-by: Dan Williams <dan.j.williams@...el.com>
> ---
>  include/linux/fs.h |   22 ++++++++++++++++++++++
>  1 file changed, 22 insertions(+)
> 
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 79c413985305..bd0c46880572 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -1909,6 +1909,28 @@ static inline bool sb_rdonly(const struct super_block *sb) { return sb->s_flags
>  #define IS_WHITEOUT(inode)	(S_ISCHR(inode->i_mode) && \
>  				 (inode)->i_rdev == WHITEOUT_DEV)
>  
> +static inline bool IS_DEVDAX(struct inode *inode)
> +{
> +	if (!IS_ENABLED(CONFIG_DEV_DAX))
> +		return false;
> +	if ((inode->i_flags & S_DAX) == 0)
> +		return false;
> +	if (!S_ISCHR(inode->i_mode))
> +		return false;
> +	return true;
> +}
> +
> +static inline bool IS_FSDAX(struct inode *inode)
> +{
> +	if (!IS_ENABLED(CONFIG_FS_DAX))
> +		return false;
> +	if ((inode->i_flags & S_DAX) == 0)
> +		return false;
> +	if (S_ISCHR(inode->i_mode))
> +		return false;
> +	return true;
> +}
> +
>  static inline bool HAS_UNMAPPED_ID(struct inode *inode)
>  {
>  	return !uid_valid(inode->i_uid) || !gid_valid(inode->i_gid);
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR