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Subject: Re: [PATCH v2 1/6] ext4: introduce direct IO read path using iomap
 infrastructure
To:     Matthew Bobrowski <mbobrowski@mbobrowski.org>, tytso@mit.edu,
        jack@suse.cz, adilger.kernel@dilger.ca
Cc:     linux-ext4@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        david@fromorbit.com, hch@infradead.org, darrick.wong@oracle.com
References: <cover.1567978633.git.mbobrowski@mbobrowski.org>
 <75a6ead09a10e362526a849af482510a0090f82a.1567978633.git.mbobrowski@mbobrowski.org>
From:   Ritesh Harjani <riteshh@linux.ibm.com>
Date:   Mon, 9 Sep 2019 13:18:39 +0530
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On 9/9/19 4:48 AM, Matthew Bobrowski wrote:
> This patch introduces a new direct IO read path that makes use of the
> iomap infrastructure.
> 
> The new function ext4_dio_read_iter() is responsible for calling into
> the iomap infrastructure via iomap_dio_rw(). If the inode in question
> does not pass preliminary checks in ext4_dio_checks(), then we simply
> fallback to buffered IO and take that path to fulfil the request. It's
> imperative that we drop the IOCB_DIRECT flag from iocb->ki_flags in
> order to prevent generic_file_read_iter() from trying to take the
> direct IO code path again.
> 
> Signed-off-by: Matthew Bobrowski <mbobrowski@mbobrowski.org>

Looks good to me.

Reviewed-by: Ritesh Harjani <riteshh@linux.ibm.com>

> ---
>   fs/ext4/file.c | 58 ++++++++++++++++++++++++++++++++++++++++++++++----
>   1 file changed, 54 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/ext4/file.c b/fs/ext4/file.c
> index 70b0438dbc94..e52e3928dc25 100644
> --- a/fs/ext4/file.c
> +++ b/fs/ext4/file.c
> @@ -34,6 +34,53 @@
>   #include "xattr.h"
>   #include "acl.h"
> 
> +static bool ext4_dio_checks(struct inode *inode)
> +{
> +#if IS_ENABLED(CONFIG_FS_ENCRYPTION)
> +	if (IS_ENCRYPTED(inode))
> +		return false;
> +#endif
> +	if (ext4_should_journal_data(inode))
> +		return false;
> +	if (ext4_has_inline_data(inode))
> +		return false;
> +	return true;
> +}
> +
> +static ssize_t ext4_dio_read_iter(struct kiocb *iocb, struct iov_iter *to)
> +{
> +	ssize_t ret;
> +	struct inode *inode = file_inode(iocb->ki_filp);
> +
> +	/*
> +	 * Get exclusion from truncate and other inode operations.
> +	 */
> +	if (!inode_trylock_shared(inode)) {
> +		if (iocb->ki_flags & IOCB_NOWAIT)
> +			return -EAGAIN;
> +		inode_lock_shared(inode);
> +	}
> +
> +	if (!ext4_dio_checks(inode)) {
> +		inode_unlock_shared(inode);
> +		/*
> +		 * Fallback to buffered IO if the operation being
> +		 * performed on the inode is not supported by direct
> +		 * IO. The IOCB_DIRECT flag flags needs to be cleared
> +		 * here to ensure that the direct IO code path within
> +		 * generic_file_read_iter() is not taken again.
> +		 */
> +		iocb->ki_flags &= ~IOCB_DIRECT;
> +		return generic_file_read_iter(iocb, to);
> +	}
> +
> +	ret = iomap_dio_rw(iocb, to, &ext4_iomap_ops, NULL);
> +	inode_unlock_shared(inode);
> +
> +	file_accessed(iocb->ki_filp);
> +	return ret;
> +}
> +
>   #ifdef CONFIG_FS_DAX
>   static ssize_t ext4_dax_read_iter(struct kiocb *iocb, struct iov_iter *to)
>   {
> @@ -64,16 +111,19 @@ static ssize_t ext4_dax_read_iter(struct kiocb *iocb, struct iov_iter *to)
> 
>   static ssize_t ext4_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
>   {
> -	if (unlikely(ext4_forced_shutdown(EXT4_SB(file_inode(iocb->ki_filp)->i_sb))))
> +	struct inode *inode = file_inode(iocb->ki_filp);
> +
> +	if (unlikely(ext4_forced_shutdown(EXT4_SB(inode->i_sb))))
>   		return -EIO;
> 
>   	if (!iov_iter_count(to))
>   		return 0; /* skip atime */
> 
> -#ifdef CONFIG_FS_DAX
> -	if (IS_DAX(file_inode(iocb->ki_filp)))
> +	if (IS_DAX(inode))
>   		return ext4_dax_read_iter(iocb, to);
> -#endif
> +
> +	if (iocb->ki_flags & IOCB_DIRECT)
> +		return ext4_dio_read_iter(iocb, to);
>   	return generic_file_read_iter(iocb, to);
>   }
>