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Date:   Wed, 25 May 2022 14:41:43 -0400
From:   Vivek Goyal <vgoyal@redhat.com>
To:     Dharmendra Singh <dharamhans87@gmail.com>
Cc:     miklos@szeredi.hu, linux-fsdevel@vger.kernel.org,
        fuse-devel@lists.sourceforge.net, linux-kernel@vger.kernel.org,
        bschubert@ddn.com, Dharmendra Singh <dsingh@ddn.com>
Subject: Re: [PATCH v3 1/1] FUSE: Allow non-extending parallel direct writes
 on the same file.
Message-ID: <Yo54Z4EQjzsQbMTp@redhat.com>
References: <20220520043443.17439-1-dharamhans87@gmail.com>
 <20220520043443.17439-2-dharamhans87@gmail.com>
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On Fri, May 20, 2022 at 10:04:43AM +0530, Dharmendra Singh wrote:
> From: Dharmendra Singh <dsingh@ddn.com>
> 
> In general, as of now, in FUSE, direct writes on the same file are
> serialized over inode lock i.e we hold inode lock for the full duration
> of the write request. I could not found in fuse code a comment which
> clearly explains why this exclusive lock is taken for direct writes.
> Our guess is some USER space fuse implementations might be relying
> on this lock for seralization and also it protects for the issues
> arising due to file size assumption or write failures.  This patch
> relaxes this exclusive lock in some cases of direct writes.
> 
> With these changes, we allows non-extending parallel direct writes
> on the same file with the help of a flag called FOPEN_PARALLEL_WRITES.
> If this flag is set on the file (flag is passed from libfuse to fuse
> kernel as part of file open/create), we do not take exclusive lock instead
> use shared lock so that all non-extending writes can run in parallel.
> 
> Best practise would be to enable parallel direct writes of all kinds
> including extending writes as well but we see some issues such as
> when one write completes and other fails, how we should truncate(if
> needed) the file if underlying file system does not support holes
> (For file systems which supports holes, there might be a possibility
> of enabling parallel writes for all cases).
> 
> FUSE implementations which rely on this inode lock for serialisation
> can continue to do so and this is default behaviour i.e no parallel
> direct writes.
> 
> Signed-off-by: Dharmendra Singh <dsingh@ddn.com>
> Signed-off-by: Bernd Schubert <bschubert@ddn.com>
> ---
>  fs/fuse/file.c            | 33 ++++++++++++++++++++++++++++++---
>  include/uapi/linux/fuse.h |  2 ++
>  2 files changed, 32 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/fuse/file.c b/fs/fuse/file.c
> index 829094451774..1a93fd80a6ce 100644
> --- a/fs/fuse/file.c
> +++ b/fs/fuse/file.c
> @@ -1541,14 +1541,37 @@ static ssize_t fuse_direct_read_iter(struct kiocb *iocb, struct iov_iter *to)
>  	return res;
>  }
>  
> +static bool fuse_direct_write_extending_i_size(struct kiocb *iocb,
> +					       struct iov_iter *iter)
> +{
> +	struct inode *inode = file_inode(iocb->ki_filp);
> +
> +	return (iocb->ki_flags & IOCB_APPEND ||
> +		iocb->ki_pos + iov_iter_count(iter) > i_size_read(inode));
> +}

Hi Dharmendra,

I have a question. What makes i_size stable. This is being read outside
the inode_lock(). Can it race with truncate. I mean we checked
i_size and decided to take shared lock. In the mean time another thread
truncated the file and now our decision to take shared lock is wrong
as file will be extended due to direct write?

Thanks
Vivek

> +
>  static ssize_t fuse_direct_write_iter(struct kiocb *iocb, struct iov_iter *from)
>  {
>  	struct inode *inode = file_inode(iocb->ki_filp);
> +	struct file *file = iocb->ki_filp;
> +	struct fuse_file *ff = file->private_data;
>  	struct fuse_io_priv io = FUSE_IO_PRIV_SYNC(iocb);
>  	ssize_t res;
> +	bool exclusive_lock = !(ff->open_flags & FOPEN_PARALLEL_WRITES) ||
> +			       fuse_direct_write_extending_i_size(iocb, from);
> +
> +	/*
> +	 * Take exclusive lock if
> +	 * - parallel writes are disabled.
> +	 * - parallel writes are enabled and i_size is being extended
> +	 * Take shared lock if
> +	 * - parallel writes are enabled but i_size does not extend.
> +	 */
> +	if (exclusive_lock)
> +		inode_lock(inode);
> +	else
> +		inode_lock_shared(inode);
>  
> -	/* Don't allow parallel writes to the same file */
> -	inode_lock(inode);
>  	res = generic_write_checks(iocb, from);
>  	if (res > 0) {
>  		if (!is_sync_kiocb(iocb) && iocb->ki_flags & IOCB_DIRECT) {
> @@ -1559,7 +1582,10 @@ static ssize_t fuse_direct_write_iter(struct kiocb *iocb, struct iov_iter *from)
>  			fuse_write_update_attr(inode, iocb->ki_pos, res);
>  		}
>  	}
> -	inode_unlock(inode);
> +	if (exclusive_lock)
> +		inode_unlock(inode);
> +	else
> +		inode_unlock_shared(inode);
>  
>  	return res;
>  }
> @@ -2901,6 +2927,7 @@ fuse_direct_IO(struct kiocb *iocb, struct iov_iter *iter)
>  
>  	if (iov_iter_rw(iter) == WRITE) {
>  		fuse_write_update_attr(inode, pos, ret);
> +		/* For extending writes we already hold exclusive lock */
>  		if (ret < 0 && offset + count > i_size)
>  			fuse_do_truncate(file);
>  	}
> diff --git a/include/uapi/linux/fuse.h b/include/uapi/linux/fuse.h
> index d6ccee961891..ee5379d41906 100644
> --- a/include/uapi/linux/fuse.h
> +++ b/include/uapi/linux/fuse.h
> @@ -301,6 +301,7 @@ struct fuse_file_lock {
>   * FOPEN_CACHE_DIR: allow caching this directory
>   * FOPEN_STREAM: the file is stream-like (no file position at all)
>   * FOPEN_NOFLUSH: don't flush data cache on close (unless FUSE_WRITEBACK_CACHE)
> + * FOPEN_PARALLEL_WRITES: Allow concurrent writes on the same inode
>   */
>  #define FOPEN_DIRECT_IO		(1 << 0)
>  #define FOPEN_KEEP_CACHE	(1 << 1)
> @@ -308,6 +309,7 @@ struct fuse_file_lock {
>  #define FOPEN_CACHE_DIR		(1 << 3)
>  #define FOPEN_STREAM		(1 << 4)
>  #define FOPEN_NOFLUSH		(1 << 5)
> +#define FOPEN_PARALLEL_WRITES	(1 << 6)
>  
>  /**
>   * INIT request/reply flags
> -- 
> 2.17.1
>