Re: [PATCH 11/15] btrfs: Use inode_lock_shared() for direct writes within EOF

[Josef Bacik <josef@toxicpanda.com>]

On 9/21/20 10:43 AM, Goldwyn Rodrigues wrote:
> From: Goldwyn Rodrigues <rgoldwyn@suse.com>
> 
> Direct writes within EOF are safe to be performed with inode shared lock
> to improve parallelization with other direct writes or reads because EOF
> is not changed and there is no race with truncate().
> 
> Direct reads are already performed under shared inode lock.
> 
> This patch is precursor to removing btrfs_inode->dio_sem.
> 
> Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
> ---
>   fs/btrfs/file.c | 33 +++++++++++++++++++++------------
>   1 file changed, 21 insertions(+), 12 deletions(-)
> 
> diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
> index d9c3be19d7b3..50092d24eee2 100644
> --- a/fs/btrfs/file.c
> +++ b/fs/btrfs/file.c
> @@ -1977,7 +1977,6 @@ static ssize_t btrfs_direct_write(struct kiocb *iocb, struct iov_iter *from)
>   	struct btrfs_fs_info *fs_info = btrfs_sb(inode->i_sb);
>   	loff_t pos;
>   	ssize_t written = 0;
> -	bool relock = false;
>   	ssize_t written_buffered;
>   	loff_t endbyte;
>   	int err;
> @@ -1986,6 +1985,15 @@ static ssize_t btrfs_direct_write(struct kiocb *iocb, struct iov_iter *from)
>   	if (iocb->ki_flags & IOCB_NOWAIT)
>   		ilock_flags |= BTRFS_ILOCK_TRY;
>   
> +	/*
> +	 * If the write DIO within EOF,  use a shared lock
> +	 */
> +	if (iocb->ki_pos + iov_iter_count(from) <= i_size_read(inode))
> +		ilock_flags |= BTRFS_ILOCK_SHARED;
> +	else if (iocb->ki_flags & IOCB_NOWAIT)
> +		return -EAGAIN;
> +
> +relock:

Huh?  Why are you making it so EOF extending NOWAIT writes now fail?  We are 
still using ILOCK_TRY here, so we may still not block, am I missing something? 
Thanks,

Josef