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Date:   Wed, 4 May 2022 14:20:06 -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 v4 2/3] FUSE: Implement atomic lookup + open
Message-ID: <YnLD1ibtU0FM5nlS@redhat.com>
References: <20220502102521.22875-1-dharamhans87@gmail.com>
 <20220502102521.22875-3-dharamhans87@gmail.com>
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On Mon, May 02, 2022 at 03:55:20PM +0530, Dharmendra Singh wrote:
> From: Dharmendra Singh <dsingh@ddn.com>
> 
> We can optimize aggressive lookups which are triggered when
> there is normal open for file/dir (dentry is new/negative).
> 
> Here in this case since we are anyway going to open the file/dir
> with USER SPACE, avoid this separate lookup call into libfuse
> and combine it with open call into libfuse.
> 
> This lookup + open in single call to libfuse has been named
> as atomic open. It is expected that USER SPACE opens the file
> and fills in the attributes, which are then used to make inode
> stand/revalidate in the kernel cache.
> 
> Signed-off-by: Dharmendra Singh <dsingh@ddn.com>
> ---
>  fs/fuse/dir.c             | 78 ++++++++++++++++++++++++++++++---------
>  fs/fuse/fuse_i.h          |  3 ++
>  fs/fuse/inode.c           |  4 +-
>  include/uapi/linux/fuse.h |  2 +
>  4 files changed, 69 insertions(+), 18 deletions(-)
> 
> diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c
> index cad3322a007f..6879d3a86796 100644
> --- a/fs/fuse/dir.c
> +++ b/fs/fuse/dir.c
> @@ -516,18 +516,18 @@ static int get_security_context(struct dentry *entry, umode_t mode,
>  }
>  
>  /*
> - * Atomic create+open operation
> - *
> - * If the filesystem doesn't support this, then fall back to separate
> - * 'mknod' + 'open' requests.
> + * This is common function for initiating atomic operations into libfuse.
> + * Currently being used by Atomic create(atomic lookup + create) and
> + * Atomic open(atomic lookup + open).
>   */
> -static int fuse_create_open(struct inode *dir, struct dentry *entry,
> -			    struct file *file, unsigned int flags,
> -			    umode_t mode, uint32_t opcode)
> +static int fuse_atomic_do_common(struct inode *dir, struct dentry *entry,
> +				 struct dentry **alias, struct file *file,
> +				 unsigned int flags, umode_t mode, uint32_t opcode)

If this common function is really useful for atomic create + atomic open,
I will first add a patch which adds a common function and makes FUSE_CREATE_EXT use that function. And in later patch add functionality to do atomic open.
Just makes code more readable.

>  {
>  	int err;
>  	struct inode *inode;
>  	struct fuse_mount *fm = get_fuse_mount(dir);
> +	struct fuse_conn *fc = get_fuse_conn(dir);
>  	FUSE_ARGS(args);
>  	struct fuse_forget_link *forget;
>  	struct fuse_create_in inarg;
> @@ -539,9 +539,13 @@ static int fuse_create_open(struct inode *dir, struct dentry *entry,
>  	void *security_ctx = NULL;
>  	u32 security_ctxlen;
>  	bool atomic_create = (opcode == FUSE_ATOMIC_CREATE ? true : false);
> +	bool create_op = (opcode == FUSE_CREATE ||
> +			  opcode == FUSE_ATOMIC_CREATE) ? true : false;
> +	if (alias)
> +		*alias = NULL;
>  
>  	/* Userspace expects S_IFREG in create mode */
> -	BUG_ON((mode & S_IFMT) != S_IFREG);
> +	BUG_ON(create_op && (mode & S_IFMT) != S_IFREG);
>  
>  	forget = fuse_alloc_forget();
>  	err = -ENOMEM;
> @@ -553,7 +557,7 @@ static int fuse_create_open(struct inode *dir, struct dentry *entry,
>  	if (!ff)
>  		goto out_put_forget_req;
>  
> -	if (!fm->fc->dont_mask)
> +	if (!fc->dont_mask)
>  		mode &= ~current_umask();
>  
>  	flags &= ~O_NOCTTY;
> @@ -642,8 +646,9 @@ static int fuse_create_open(struct inode *dir, struct dentry *entry,
>  				err = PTR_ERR(res);
>  				goto out_err;
>  			}
> -			/* res is expected to be NULL since its REG file */
> -			WARN_ON(res);
> +			entry = res;
> +			if (alias)
> +				*alias = res;
>  		}
>  	}
>  	fuse_change_entry_timeout(entry, &outentry);
> @@ -681,8 +686,8 @@ static int fuse_atomic_create(struct inode *dir, struct dentry *entry,
>  	if (fc->no_atomic_create)
>  		return -ENOSYS;
>  
> -	err = fuse_create_open(dir, entry, file, flags, mode,
> -			       FUSE_ATOMIC_CREATE);
> +	err = fuse_atomic_do_common(dir, entry, NULL, file, flags, mode,
> +				    FUSE_ATOMIC_CREATE);
>  	/* If atomic create is not implemented then indicate in fc so that next
>  	 * request falls back to normal create instead of going into libufse and
>  	 * returning with -ENOSYS.
> @@ -694,6 +699,29 @@ static int fuse_atomic_create(struct inode *dir, struct dentry *entry,
>  	return err;
>  }
>  
> +static int fuse_do_atomic_open(struct inode *dir, struct dentry *entry,
> +				struct dentry **alias, struct file *file,
> +				unsigned int flags, umode_t mode)
> +{
> +	int err;
> +	struct fuse_conn *fc = get_fuse_conn(dir);
> +
> +	if (!fc->do_atomic_open)
> +		return -ENOSYS;
> +
> +	err = fuse_atomic_do_common(dir, entry, alias, file, flags, mode,
> +				    FUSE_ATOMIC_OPEN);
> +	/* Atomic open imply atomic trunc as well i.e truncate should be performed
> +	 * as part of atomic open call itself.
> +	 */
> +	if (!fc->atomic_o_trunc) {
> +		if (fc->do_atomic_open)
> +			fc->atomic_o_trunc = 1;
> +	}
> +
> +	return err;
> +}
> +
>  static int fuse_mknod(struct user_namespace *, struct inode *, struct dentry *,
>  		      umode_t, dev_t);
>  static int fuse_atomic_open(struct inode *dir, struct dentry *entry,
> @@ -702,12 +730,23 @@ static int fuse_atomic_open(struct inode *dir, struct dentry *entry,
>  {
>  	int err;
>  	struct fuse_conn *fc = get_fuse_conn(dir);
> -	struct dentry *res = NULL;
> +	struct dentry *res = NULL, *alias = NULL;
>  	bool create = flags & O_CREAT ? true : false;
>  
>  	if (fuse_is_bad(dir))
>  		return -EIO;
>  
> +	if (!create) {
> +		err = fuse_do_atomic_open(dir, entry, &alias,
> +					  file, flags, mode);

So this is the core change of behavior. As of now, if we are not doing
create operation, we just lookup and return. But now you are doing
open + lookup in a single operation. Interesting. I am not sure if
it breaks anything in VFS.

> +		res = alias;
> +		if (!err || err != -ENOSYS)
> +			goto out_dput;
> +	}
> +	 /*
> +	  * ENOSYS fall back for open- user space does not have full
> +	  * atomic open.
> +	  */

This ENOSYS stuff all the place is making these patches look very unclean
to me. A part of me says that should we negotiate these as feature bits.
Having said that feature bits are precious and should not be used for
trivial purposes. Hmm..., may be we can make handle ENOSYS little better
in general.

>  	if ((!create || fc->no_atomic_create) && d_in_lookup(entry)) {
>  		res = fuse_lookup(dir, entry, 0);
>  		if (IS_ERR(res))
> @@ -730,9 +769,14 @@ static int fuse_atomic_open(struct inode *dir, struct dentry *entry,
>  	/* Libfuse/user space has not implemented atomic create, therefore
>  	 * fall back to normal create.
>  	 */
> -	if (err == -ENOSYS)
> -		err = fuse_create_open(dir, entry, file, flags, mode,
> -				       FUSE_CREATE);
> +	/* Atomic create+open operation
> +	 * If the filesystem doesn't support this, then fall back to separate
> +	 * 'mknod' + 'open' requests.
> +	 */
> +	if (err == -ENOSYS) {
> +		err = fuse_atomic_do_common(dir, entry, NULL, file, flags,
> +					    mode, FUSE_CREATE);
> +	}
>  	if (err == -ENOSYS) {
>  		fc->no_create = 1;
>  		goto mknod;
> diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
> index d577a591ab16..24793b82303f 100644
> --- a/fs/fuse/fuse_i.h
> +++ b/fs/fuse/fuse_i.h
> @@ -669,6 +669,9 @@ struct fuse_conn {
>  	/** Is open/release not implemented by fs? */
>  	unsigned no_open:1;
>  
> +	/** Is atomic open implemented by fs ? */
> +	unsigned do_atomic_open : 1;
> +
>  	/** Is atomic create not implemented by fs? */
>  	unsigned no_atomic_create:1;
>  
> diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
> index ee846ce371d8..5f667de69115 100644
> --- a/fs/fuse/inode.c
> +++ b/fs/fuse/inode.c
> @@ -1190,6 +1190,8 @@ static void process_init_reply(struct fuse_mount *fm, struct fuse_args *args,
>  				fc->setxattr_ext = 1;
>  			if (flags & FUSE_SECURITY_CTX)
>  				fc->init_security = 1;
> +			if (flags & FUSE_DO_ATOMIC_OPEN)
> +				fc->do_atomic_open = 1;

Hey you are negotiating FUSE_DO_ATOMIC_OPEN flag. If that's the case why
do you have to deal with -ENOSYS in fuse_do_atomic_open(). You should
be able to just check.

if (!create && fc->do_atomic_open) {
        fuse_do_atomic_open().
}

I have yet to check what happens if file does not exist.

>  		} else {
>  			ra_pages = fc->max_read / PAGE_SIZE;
>  			fc->no_lock = 1;
> @@ -1235,7 +1237,7 @@ void fuse_send_init(struct fuse_mount *fm)
>  		FUSE_ABORT_ERROR | FUSE_MAX_PAGES | FUSE_CACHE_SYMLINKS |
>  		FUSE_NO_OPENDIR_SUPPORT | FUSE_EXPLICIT_INVAL_DATA |
>  		FUSE_HANDLE_KILLPRIV_V2 | FUSE_SETXATTR_EXT | FUSE_INIT_EXT |
> -		FUSE_SECURITY_CTX;
> +		FUSE_SECURITY_CTX | FUSE_DO_ATOMIC_OPEN;
>  #ifdef CONFIG_FUSE_DAX
>  	if (fm->fc->dax)
>  		flags |= FUSE_MAP_ALIGNMENT;
> diff --git a/include/uapi/linux/fuse.h b/include/uapi/linux/fuse.h
> index e4b56004b148..ab91e391241a 100644
> --- a/include/uapi/linux/fuse.h
> +++ b/include/uapi/linux/fuse.h
> @@ -391,6 +391,7 @@ struct fuse_file_lock {
>  /* bits 32..63 get shifted down 32 bits into the flags2 field */
>  #define FUSE_SECURITY_CTX	(1ULL << 32)
>  #define FUSE_HAS_INODE_DAX	(1ULL << 33)
> +#define FUSE_DO_ATOMIC_OPEN	(1ULL << 34)
>  
>  /**
>   * CUSE INIT request/reply flags
> @@ -540,6 +541,7 @@ enum fuse_opcode {
>  	FUSE_REMOVEMAPPING	= 49,
>  	FUSE_SYNCFS		= 50,
>  	FUSE_ATOMIC_CREATE	= 51,
> +	FUSE_ATOMIC_OPEN	= 52,
>  
>  	/* CUSE specific operations */
>  	CUSE_INIT		= 4096,
> -- 
> 2.17.1
>