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Date:	Wed, 14 Oct 2015 10:51:19 +0200
From:	Jan Kara <jack@...e.cz>
To:	Ross Zwisler <ross.zwisler@...ux.intel.com>
Cc:	linux-kernel@...r.kernel.org,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Jan Kara <jack@...e.com>,
	Matthew Wilcox <willy@...ux.intel.com>,
	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dan Williams <dan.j.williams@...el.com>,
	Dave Chinner <david@...morbit.com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	linux-nvdimm@...ts.01.org,
	Matthew Wilcox <matthew.r.wilcox@...el.com>
Subject: Re: [PATCH v2 2/2] ext2: Add locking for DAX faults

On Tue 13-10-15 16:25:37, Ross Zwisler wrote:
> Add locking to ensure that DAX faults are isolated from ext2 operations
> that modify the data blocks allocation for an inode.  This is intended to
> be analogous to the work being done in XFS by Dave Chinner:
> 
> http://www.spinics.net/lists/linux-fsdevel/msg90260.html
> 
> Compared with XFS the ext2 case is greatly simplified by the fact that ext2
> already allocates and zeros new blocks before they are returned as part of
> ext2_get_block(), so DAX doesn't need to worry about getting unmapped or
> unwritten buffer heads.
> 
> This means that the only work we need to do in ext2 is to isolate the DAX
> faults from inode block allocation changes.  I believe this just means that
> we need to isolate the DAX faults from truncate operations.
> 
> The newly introduced dax_sem is intended to replicate the protection
> offered by i_mmaplock in XFS.  In addition to truncate the i_mmaplock also
> protects XFS operations like hole punching, fallocate down, extent
> manipulation IOCTLS like xfs_ioc_space() and extent swapping.  Truncate is
> the only one of these operations supported by ext2.
> 
> Signed-off-by: Ross Zwisler <ross.zwisler@...ux.intel.com>

The patch looks good to me. Feel free to add:

Reviewed-by: Jan Kara <jack@...e.com>

Or I can push the patch through my tree as it seems to be independent of
any other changes, am I right?

								Honza
> ---
>  fs/ext2/ext2.h  | 11 ++++++++
>  fs/ext2/file.c  | 84 ++++++++++++++++++++++++++++++++++++++++++++++++++++++---
>  fs/ext2/inode.c | 10 +++++++
>  fs/ext2/super.c |  3 +++
>  4 files changed, 104 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/ext2/ext2.h b/fs/ext2/ext2.h
> index 8d15feb..4c69c94 100644
> --- a/fs/ext2/ext2.h
> +++ b/fs/ext2/ext2.h
> @@ -684,6 +684,9 @@ struct ext2_inode_info {
>  	struct rw_semaphore xattr_sem;
>  #endif
>  	rwlock_t i_meta_lock;
> +#ifdef CONFIG_FS_DAX
> +	struct rw_semaphore dax_sem;
> +#endif
>  
>  	/*
>  	 * truncate_mutex is for serialising ext2_truncate() against
> @@ -699,6 +702,14 @@ struct ext2_inode_info {
>  #endif
>  };
>  
> +#ifdef CONFIG_FS_DAX
> +#define dax_sem_down_write(ext2_inode)	down_write(&(ext2_inode)->dax_sem)
> +#define dax_sem_up_write(ext2_inode)	up_write(&(ext2_inode)->dax_sem)
> +#else
> +#define dax_sem_down_write(ext2_inode)
> +#define dax_sem_up_write(ext2_inode)
> +#endif
> +
>  /*
>   * Inode dynamic state flags
>   */
> diff --git a/fs/ext2/file.c b/fs/ext2/file.c
> index 1982c3f..11a42c5 100644
> --- a/fs/ext2/file.c
> +++ b/fs/ext2/file.c
> @@ -27,27 +27,103 @@
>  #include "acl.h"
>  
>  #ifdef CONFIG_FS_DAX
> +/*
> + * The lock ordering for ext2 DAX fault paths is:
> + *
> + * mmap_sem (MM)
> + *   sb_start_pagefault (vfs, freeze)
> + *     ext2_inode_info->dax_sem
> + *       address_space->i_mmap_rwsem or page_lock (mutually exclusive in DAX)
> + *         ext2_inode_info->truncate_mutex
> + *
> + * The default page_lock and i_size verification done by non-DAX fault paths
> + * is sufficient because ext2 doesn't support hole punching.
> + */
>  static int ext2_dax_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
>  {
> -	return dax_fault(vma, vmf, ext2_get_block, NULL);
> +	struct inode *inode = file_inode(vma->vm_file);
> +	struct ext2_inode_info *ei = EXT2_I(inode);
> +	int ret;
> +
> +	if (vmf->flags & FAULT_FLAG_WRITE) {
> +		sb_start_pagefault(inode->i_sb);
> +		file_update_time(vma->vm_file);
> +	}
> +	down_read(&ei->dax_sem);
> +
> +	ret = __dax_fault(vma, vmf, ext2_get_block, NULL);
> +
> +	up_read(&ei->dax_sem);
> +	if (vmf->flags & FAULT_FLAG_WRITE)
> +		sb_end_pagefault(inode->i_sb);
> +	return ret;
>  }
>  
>  static int ext2_dax_pmd_fault(struct vm_area_struct *vma, unsigned long addr,
>  						pmd_t *pmd, unsigned int flags)
>  {
> -	return dax_pmd_fault(vma, addr, pmd, flags, ext2_get_block, NULL);
> +	struct inode *inode = file_inode(vma->vm_file);
> +	struct ext2_inode_info *ei = EXT2_I(inode);
> +	int ret;
> +
> +	if (flags & FAULT_FLAG_WRITE) {
> +		sb_start_pagefault(inode->i_sb);
> +		file_update_time(vma->vm_file);
> +	}
> +	down_read(&ei->dax_sem);
> +
> +	ret = __dax_pmd_fault(vma, addr, pmd, flags, ext2_get_block, NULL);
> +
> +	up_read(&ei->dax_sem);
> +	if (flags & FAULT_FLAG_WRITE)
> +		sb_end_pagefault(inode->i_sb);
> +	return ret;
>  }
>  
>  static int ext2_dax_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
>  {
> -	return dax_mkwrite(vma, vmf, ext2_get_block, NULL);
> +	struct inode *inode = file_inode(vma->vm_file);
> +	struct ext2_inode_info *ei = EXT2_I(inode);
> +	int ret;
> +
> +	sb_start_pagefault(inode->i_sb);
> +	file_update_time(vma->vm_file);
> +	down_read(&ei->dax_sem);
> +
> +	ret = __dax_mkwrite(vma, vmf, ext2_get_block, NULL);
> +
> +	up_read(&ei->dax_sem);
> +	sb_end_pagefault(inode->i_sb);
> +	return ret;
> +}
> +
> +static int ext2_dax_pfn_mkwrite(struct vm_area_struct *vma,
> +		struct vm_fault *vmf)
> +{
> +	struct inode *inode = file_inode(vma->vm_file);
> +	struct ext2_inode_info *ei = EXT2_I(inode);
> +	int ret = VM_FAULT_NOPAGE;
> +	loff_t size;
> +
> +	sb_start_pagefault(inode->i_sb);
> +	file_update_time(vma->vm_file);
> +	down_read(&ei->dax_sem);
> +
> +	/* check that the faulting page hasn't raced with truncate */
> +	size = (i_size_read(inode) + PAGE_SIZE - 1) >> PAGE_SHIFT;
> +	if (vmf->pgoff >= size)
> +		ret = VM_FAULT_SIGBUS;
> +
> +	up_read(&ei->dax_sem);
> +	sb_end_pagefault(inode->i_sb);
> +	return ret;
>  }
>  
>  static const struct vm_operations_struct ext2_dax_vm_ops = {
>  	.fault		= ext2_dax_fault,
>  	.pmd_fault	= ext2_dax_pmd_fault,
>  	.page_mkwrite	= ext2_dax_mkwrite,
> -	.pfn_mkwrite	= dax_pfn_mkwrite,
> +	.pfn_mkwrite	= ext2_dax_pfn_mkwrite,
>  };
>  
>  static int ext2_file_mmap(struct file *file, struct vm_area_struct *vma)
> diff --git a/fs/ext2/inode.c b/fs/ext2/inode.c
> index c60a248..0aa9bf6 100644
> --- a/fs/ext2/inode.c
> +++ b/fs/ext2/inode.c
> @@ -1085,6 +1085,7 @@ static void ext2_free_branches(struct inode *inode, __le32 *p, __le32 *q, int de
>  		ext2_free_data(inode, p, q);
>  }
>  
> +/* dax_sem must be held when calling this function */
>  static void __ext2_truncate_blocks(struct inode *inode, loff_t offset)
>  {
>  	__le32 *i_data = EXT2_I(inode)->i_data;
> @@ -1100,6 +1101,10 @@ static void __ext2_truncate_blocks(struct inode *inode, loff_t offset)
>  	blocksize = inode->i_sb->s_blocksize;
>  	iblock = (offset + blocksize-1) >> EXT2_BLOCK_SIZE_BITS(inode->i_sb);
>  
> +#ifdef CONFIG_FS_DAX
> +	WARN_ON(!rwsem_is_locked(&ei->dax_sem));
> +#endif
> +
>  	n = ext2_block_to_path(inode, iblock, offsets, NULL);
>  	if (n == 0)
>  		return;
> @@ -1185,7 +1190,10 @@ static void ext2_truncate_blocks(struct inode *inode, loff_t offset)
>  		return;
>  	if (IS_APPEND(inode) || IS_IMMUTABLE(inode))
>  		return;
> +
> +	dax_sem_down_write(EXT2_I(inode));
>  	__ext2_truncate_blocks(inode, offset);
> +	dax_sem_up_write(EXT2_I(inode));
>  }
>  
>  static int ext2_setsize(struct inode *inode, loff_t newsize)
> @@ -1213,8 +1221,10 @@ static int ext2_setsize(struct inode *inode, loff_t newsize)
>  	if (error)
>  		return error;
>  
> +	dax_sem_down_write(EXT2_I(inode));
>  	truncate_setsize(inode, newsize);
>  	__ext2_truncate_blocks(inode, newsize);
> +	dax_sem_up_write(EXT2_I(inode));
>  
>  	inode->i_mtime = inode->i_ctime = CURRENT_TIME_SEC;
>  	if (inode_needs_sync(inode)) {
> diff --git a/fs/ext2/super.c b/fs/ext2/super.c
> index 900e19c..3a71cea 100644
> --- a/fs/ext2/super.c
> +++ b/fs/ext2/super.c
> @@ -192,6 +192,9 @@ static void init_once(void *foo)
>  	init_rwsem(&ei->xattr_sem);
>  #endif
>  	mutex_init(&ei->truncate_mutex);
> +#ifdef CONFIG_FS_DAX
> +	init_rwsem(&ei->dax_sem);
> +#endif
>  	inode_init_once(&ei->vfs_inode);
>  }
>  
> -- 
> 2.1.0
> 
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR
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