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Date:   Mon, 13 Jan 2020 14:19:57 -0800
From:   "Darrick J. Wong" <darrick.wong@...cle.com>
To:     ira.weiny@...el.com
Cc:     linux-kernel@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Chinner <david@...morbit.com>,
        Christoph Hellwig <hch@....de>,
        "Theodore Y. Ts'o" <tytso@....edu>, Jan Kara <jack@...e.cz>,
        linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [RFC PATCH V2 08/12] fs/xfs: Add lock/unlock mode to xfs

On Fri, Jan 10, 2020 at 11:29:38AM -0800, ira.weiny@...el.com wrote:
> From: Ira Weiny <ira.weiny@...el.com>
> 
> XFS requires regular files to be locked while changing to/from DAX mode.
> 
> Define a new DAX lock type and implement the [un]lock_mode() inode
> operation callbacks.
> 
> We define a new XFS_DAX_* lock type to carry the lock through the
> transaction because we don't want to use IOLOCK as that would cause
> performance issues with locking of the inode itself.
> 
> Signed-off-by: Ira Weiny <ira.weiny@...el.com>
> ---
>  fs/xfs/xfs_icache.c |  2 ++
>  fs/xfs/xfs_inode.c  | 37 +++++++++++++++++++++++++++++++++++--
>  fs/xfs/xfs_inode.h  | 12 ++++++++++--
>  fs/xfs/xfs_iops.c   | 24 +++++++++++++++++++++++-
>  4 files changed, 70 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/xfs/xfs_icache.c b/fs/xfs/xfs_icache.c
> index 8dc2e5414276..0288672e8902 100644
> --- a/fs/xfs/xfs_icache.c
> +++ b/fs/xfs/xfs_icache.c
> @@ -74,6 +74,8 @@ xfs_inode_alloc(
>  	INIT_LIST_HEAD(&ip->i_ioend_list);
>  	spin_lock_init(&ip->i_ioend_lock);
>  
> +	percpu_init_rwsem(&ip->i_dax_sem);
> +
>  	return ip;
>  }
>  
> diff --git a/fs/xfs/xfs_inode.c b/fs/xfs/xfs_inode.c
> index 401da197f012..e8fd95b75e5b 100644
> --- a/fs/xfs/xfs_inode.c
> +++ b/fs/xfs/xfs_inode.c
> @@ -142,12 +142,12 @@ xfs_ilock_attr_map_shared(
>   *
>   * Basic locking order:
>   *
> - * i_rwsem -> i_mmap_lock -> page_lock -> i_ilock
> + * i_rwsem -> i_dax_sem -> i_mmap_lock -> page_lock -> i_ilock

Mmmmmm, more locks.  Can we skip the extra lock if CONFIG_FSDAX=n or if
the filesystem devices don't support DAX at all?

Also, I don't think we're actually following the i_rwsem -> i_daxsem
order in fallocate, and possibly elsewhere too?

Does the vfs have to take the i_dax_sem to do remapping things like
reflink?  (Pretend that reflink and dax are compatible for the moment)

>   * mmap_sem locking order:
>   *
>   * i_rwsem -> page lock -> mmap_sem
> - * mmap_sem -> i_mmap_lock -> page_lock
> + * mmap_sem -> i_dax_sem -> i_mmap_lock -> page_lock
>   *
>   * The difference in mmap_sem locking order mean that we cannot hold the
>   * i_mmap_lock over syscall based read(2)/write(2) based IO. These IO paths can
> @@ -181,6 +181,13 @@ xfs_ilock(
>  	ASSERT((lock_flags & (XFS_ILOCK_SHARED | XFS_ILOCK_EXCL)) !=
>  	       (XFS_ILOCK_SHARED | XFS_ILOCK_EXCL));
>  	ASSERT((lock_flags & ~(XFS_LOCK_MASK | XFS_LOCK_SUBCLASS_MASK)) == 0);
> +	ASSERT((lock_flags & (XFS_DAX_SHARED | XFS_DAX_EXCL)) !=
> +	       (XFS_DAX_SHARED | XFS_DAX_EXCL));
> +
> +	if (lock_flags & XFS_DAX_EXCL)
> +		percpu_down_write(&ip->i_dax_sem);
> +	else if (lock_flags & XFS_DAX_SHARED)
> +		percpu_down_read(&ip->i_dax_sem);
>  
>  	if (lock_flags & XFS_IOLOCK_EXCL) {
>  		down_write_nested(&VFS_I(ip)->i_rwsem,
> @@ -224,6 +231,8 @@ xfs_ilock_nowait(
>  	 * You can't set both SHARED and EXCL for the same lock,
>  	 * and only XFS_IOLOCK_SHARED, XFS_IOLOCK_EXCL, XFS_ILOCK_SHARED,
>  	 * and XFS_ILOCK_EXCL are valid values to set in lock_flags.
> +	 *
> +	 * XFS_DAX_* is not allowed
>  	 */
>  	ASSERT((lock_flags & (XFS_IOLOCK_SHARED | XFS_IOLOCK_EXCL)) !=
>  	       (XFS_IOLOCK_SHARED | XFS_IOLOCK_EXCL));
> @@ -232,6 +241,7 @@ xfs_ilock_nowait(
>  	ASSERT((lock_flags & (XFS_ILOCK_SHARED | XFS_ILOCK_EXCL)) !=
>  	       (XFS_ILOCK_SHARED | XFS_ILOCK_EXCL));
>  	ASSERT((lock_flags & ~(XFS_LOCK_MASK | XFS_LOCK_SUBCLASS_MASK)) == 0);
> +	ASSERT((lock_flags & (XFS_DAX_SHARED | XFS_DAX_EXCL)) == 0);
>  
>  	if (lock_flags & XFS_IOLOCK_EXCL) {
>  		if (!down_write_trylock(&VFS_I(ip)->i_rwsem))
> @@ -302,6 +312,8 @@ xfs_iunlock(
>  	       (XFS_ILOCK_SHARED | XFS_ILOCK_EXCL));
>  	ASSERT((lock_flags & ~(XFS_LOCK_MASK | XFS_LOCK_SUBCLASS_MASK)) == 0);
>  	ASSERT(lock_flags != 0);
> +	ASSERT((lock_flags & (XFS_DAX_SHARED | XFS_DAX_EXCL)) !=
> +	       (XFS_DAX_SHARED | XFS_DAX_EXCL));
>  
>  	if (lock_flags & XFS_IOLOCK_EXCL)
>  		up_write(&VFS_I(ip)->i_rwsem);
> @@ -318,6 +330,11 @@ xfs_iunlock(
>  	else if (lock_flags & XFS_ILOCK_SHARED)
>  		mrunlock_shared(&ip->i_lock);
>  
> +	if (lock_flags & XFS_DAX_EXCL)
> +		percpu_up_write(&ip->i_dax_sem);
> +	else if (lock_flags & XFS_DAX_SHARED)
> +		percpu_up_read(&ip->i_dax_sem);
> +
>  	trace_xfs_iunlock(ip, lock_flags, _RET_IP_);
>  }
>  
> @@ -333,6 +350,8 @@ xfs_ilock_demote(
>  	ASSERT(lock_flags & (XFS_IOLOCK_EXCL|XFS_MMAPLOCK_EXCL|XFS_ILOCK_EXCL));
>  	ASSERT((lock_flags &
>  		~(XFS_IOLOCK_EXCL|XFS_MMAPLOCK_EXCL|XFS_ILOCK_EXCL)) == 0);
> +	/* XFS_DAX_* is not allowed */
> +	ASSERT((lock_flags & (XFS_DAX_SHARED | XFS_DAX_EXCL)) == 0);
>  
>  	if (lock_flags & XFS_ILOCK_EXCL)
>  		mrdemote(&ip->i_lock);
> @@ -369,6 +388,13 @@ xfs_isilocked(
>  		return rwsem_is_locked(&VFS_I(ip)->i_rwsem);
>  	}
>  
> +	if (lock_flags & (XFS_DAX_EXCL|XFS_DAX_SHARED)) {
> +		if (!(lock_flags & XFS_DAX_SHARED))
> +			return !debug_locks ||
> +				percpu_rwsem_is_held(&ip->i_dax_sem, 0);
> +		return rwsem_is_locked(&ip->i_dax_sem);
> +	}
> +
>  	ASSERT(0);
>  	return 0;
>  }
> @@ -465,6 +491,9 @@ xfs_lock_inodes(
>  	ASSERT(!(lock_mode & XFS_ILOCK_EXCL) ||
>  		inodes <= XFS_ILOCK_MAX_SUBCLASS + 1);
>  
> +	/* XFS_DAX_* is not allowed */
> +	ASSERT((lock_mode & (XFS_DAX_SHARED | XFS_DAX_EXCL)) == 0);
> +
>  	if (lock_mode & XFS_IOLOCK_EXCL) {
>  		ASSERT(!(lock_mode & (XFS_MMAPLOCK_EXCL | XFS_ILOCK_EXCL)));
>  	} else if (lock_mode & XFS_MMAPLOCK_EXCL)
> @@ -566,6 +595,10 @@ xfs_lock_two_inodes(
>  	ASSERT(!(ip0_mode & (XFS_MMAPLOCK_SHARED|XFS_MMAPLOCK_EXCL)) ||
>  	       !(ip1_mode & (XFS_ILOCK_SHARED|XFS_ILOCK_EXCL)));
>  
> +	/* XFS_DAX_* is not allowed */
> +	ASSERT((ip0_mode & (XFS_DAX_SHARED | XFS_DAX_EXCL)) == 0);
> +	ASSERT((ip1_mode & (XFS_DAX_SHARED | XFS_DAX_EXCL)) == 0);
> +
>  	ASSERT(ip0->i_ino != ip1->i_ino);
>  
>  	if (ip0->i_ino > ip1->i_ino) {
> diff --git a/fs/xfs/xfs_inode.h b/fs/xfs/xfs_inode.h
> index 492e53992fa9..693ca66bd89b 100644
> --- a/fs/xfs/xfs_inode.h
> +++ b/fs/xfs/xfs_inode.h
> @@ -67,6 +67,9 @@ typedef struct xfs_inode {
>  	spinlock_t		i_ioend_lock;
>  	struct work_struct	i_ioend_work;
>  	struct list_head	i_ioend_list;
> +
> +	/* protect changing the mode to/from DAX */
> +	struct percpu_rw_semaphore i_dax_sem;
>  } xfs_inode_t;
>  
>  /* Convert from vfs inode to xfs inode */
> @@ -278,10 +281,13 @@ static inline void xfs_ifunlock(struct xfs_inode *ip)
>  #define	XFS_ILOCK_SHARED	(1<<3)
>  #define	XFS_MMAPLOCK_EXCL	(1<<4)
>  #define	XFS_MMAPLOCK_SHARED	(1<<5)
> +#define	XFS_DAX_EXCL		(1<<6)
> +#define	XFS_DAX_SHARED		(1<<7)
>  
>  #define XFS_LOCK_MASK		(XFS_IOLOCK_EXCL | XFS_IOLOCK_SHARED \
>  				| XFS_ILOCK_EXCL | XFS_ILOCK_SHARED \
> -				| XFS_MMAPLOCK_EXCL | XFS_MMAPLOCK_SHARED)
> +				| XFS_MMAPLOCK_EXCL | XFS_MMAPLOCK_SHARED \
> +				| XFS_DAX_EXCL | XFS_DAX_SHARED)
>  
>  #define XFS_LOCK_FLAGS \
>  	{ XFS_IOLOCK_EXCL,	"IOLOCK_EXCL" }, \
> @@ -289,7 +295,9 @@ static inline void xfs_ifunlock(struct xfs_inode *ip)
>  	{ XFS_ILOCK_EXCL,	"ILOCK_EXCL" }, \
>  	{ XFS_ILOCK_SHARED,	"ILOCK_SHARED" }, \
>  	{ XFS_MMAPLOCK_EXCL,	"MMAPLOCK_EXCL" }, \
> -	{ XFS_MMAPLOCK_SHARED,	"MMAPLOCK_SHARED" }
> +	{ XFS_MMAPLOCK_SHARED,	"MMAPLOCK_SHARED" }, \
> +	{ XFS_DAX_EXCL,   	"DAX_EXCL" }, \

Whitespace between the comma & string.

> +	{ XFS_DAX_SHARED,	"DAX_SHARED" }
>  
>  
>  /*
> diff --git a/fs/xfs/xfs_iops.c b/fs/xfs/xfs_iops.c
> index d6843cdb51d0..a2f2604c3187 100644
> --- a/fs/xfs/xfs_iops.c
> +++ b/fs/xfs/xfs_iops.c
> @@ -1158,6 +1158,16 @@ xfs_vn_tmpfile(
>  	return xfs_generic_create(dir, dentry, mode, 0, true);
>  }
>  
> +static void xfs_lock_mode(struct inode *inode)
> +{
> +	xfs_ilock(XFS_I(inode), XFS_DAX_SHARED);
> +}
> +
> +static void xfs_unlock_mode(struct inode *inode)
> +{
> +	xfs_iunlock(XFS_I(inode), XFS_DAX_SHARED);
> +}
> +
>  static const struct inode_operations xfs_inode_operations = {
>  	.get_acl		= xfs_get_acl,
>  	.set_acl		= xfs_set_acl,
> @@ -1168,6 +1178,18 @@ static const struct inode_operations xfs_inode_operations = {
>  	.update_time		= xfs_vn_update_time,
>  };
>  
> +static const struct inode_operations xfs_reg_inode_operations = {
> +	.get_acl		= xfs_get_acl,
> +	.set_acl		= xfs_set_acl,
> +	.getattr		= xfs_vn_getattr,
> +	.setattr		= xfs_vn_setattr,
> +	.listxattr		= xfs_vn_listxattr,
> +	.fiemap			= xfs_vn_fiemap,
> +	.update_time		= xfs_vn_update_time,
> +	.lock_mode              = xfs_lock_mode,
> +	.unlock_mode            = xfs_unlock_mode,
> +};
> +
>  static const struct inode_operations xfs_dir_inode_operations = {
>  	.create			= xfs_vn_create,
>  	.lookup			= xfs_vn_lookup,
> @@ -1372,7 +1394,7 @@ xfs_setup_iops(
>  
>  	switch (inode->i_mode & S_IFMT) {
>  	case S_IFREG:
> -		inode->i_op = &xfs_inode_operations;
> +		inode->i_op = &xfs_reg_inode_operations;

xfs_file_inode_operations?

--D

>  		inode->i_fop = &xfs_file_operations;
>  		if (IS_DAX(inode))
>  			inode->i_mapping->a_ops = &xfs_dax_aops;
> -- 
> 2.21.0
> 

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