Re: [PATCH v6 05/26] fs/dax: Create a common implementation to break DAX layouts

["Darrick J. Wong" <djwong@kernel.org>]

On Fri, Jan 10, 2025 at 05:00:33PM +1100, Alistair Popple wrote:
> Prior to freeing a block file systems supporting FS DAX must check
> that the associated pages are both unmapped from user-space and not
> undergoing DMA or other access from eg. get_user_pages(). This is
> achieved by unmapping the file range and scanning the FS DAX
> page-cache to see if any pages within the mapping have an elevated
> refcount.
> 
> This is done using two functions - dax_layout_busy_page_range() which
> returns a page to wait for the refcount to become idle on. Rather than
> open-code this introduce a common implementation to both unmap and
> wait for the page to become idle.
> 
> Signed-off-by: Alistair Popple <apopple@nvidia.com>

So now that Dan Carpenter has complained, I guess I should look at
this...

> ---
> 
> Changes for v5:
> 
>  - Don't wait for idle pages on non-DAX mappings
> 
> Changes for v4:
> 
>  - Fixed some build breakage due to missing symbol exports reported by
>    John Hubbard (thanks!).
> ---
>  fs/dax.c            | 33 +++++++++++++++++++++++++++++++++
>  fs/ext4/inode.c     | 10 +---------
>  fs/fuse/dax.c       | 27 +++------------------------
>  fs/xfs/xfs_inode.c  | 23 +++++------------------
>  fs/xfs/xfs_inode.h  |  2 +-
>  include/linux/dax.h | 21 +++++++++++++++++++++
>  mm/madvise.c        |  8 ++++----
>  7 files changed, 68 insertions(+), 56 deletions(-)
> 
> diff --git a/fs/dax.c b/fs/dax.c
> index d010c10..9c3bd07 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -845,6 +845,39 @@ int dax_delete_mapping_entry(struct address_space *mapping, pgoff_t index)
>  	return ret;
>  }
>  
> +static int wait_page_idle(struct page *page,
> +			void (cb)(struct inode *),
> +			struct inode *inode)
> +{
> +	return ___wait_var_event(page, page_ref_count(page) == 1,
> +				TASK_INTERRUPTIBLE, 0, 0, cb(inode));
> +}
> +
> +/*
> + * Unmaps the inode and waits for any DMA to complete prior to deleting the
> + * DAX mapping entries for the range.
> + */
> +int dax_break_mapping(struct inode *inode, loff_t start, loff_t end,
> +		void (cb)(struct inode *))
> +{
> +	struct page *page;
> +	int error;
> +
> +	if (!dax_mapping(inode->i_mapping))
> +		return 0;
> +
> +	do {
> +		page = dax_layout_busy_page_range(inode->i_mapping, start, end);
> +		if (!page)
> +			break;
> +
> +		error = wait_page_idle(page, cb, inode);
> +	} while (error == 0);

You didn't initialize error to 0, so it could be any value.  What if
dax_layout_busy_page_range returns null the first time through the loop?

> +
> +	return error;
> +}
> +EXPORT_SYMBOL_GPL(dax_break_mapping);
> +
>  /*
>   * Invalidate DAX entry if it is clean.
>   */

<I'm no expert, skipping to xfs>

> diff --git a/fs/xfs/xfs_inode.c b/fs/xfs/xfs_inode.c
> index 42ea203..295730a 100644
> --- a/fs/xfs/xfs_inode.c
> +++ b/fs/xfs/xfs_inode.c
> @@ -2715,21 +2715,17 @@ xfs_mmaplock_two_inodes_and_break_dax_layout(
>  	struct xfs_inode	*ip2)
>  {
>  	int			error;
> -	bool			retry;
>  	struct page		*page;
>  
>  	if (ip1->i_ino > ip2->i_ino)
>  		swap(ip1, ip2);
>  
>  again:
> -	retry = false;
>  	/* Lock the first inode */
>  	xfs_ilock(ip1, XFS_MMAPLOCK_EXCL);
> -	error = xfs_break_dax_layouts(VFS_I(ip1), &retry);
> -	if (error || retry) {
> +	error = xfs_break_dax_layouts(VFS_I(ip1));
> +	if (error) {
>  		xfs_iunlock(ip1, XFS_MMAPLOCK_EXCL);
> -		if (error == 0 && retry)
> -			goto again;

Hmm, so the retry loop has moved into xfs_break_dax_layouts, which means
that we no longer cycle the MMAPLOCK.  Why was the lock cycling
unnecessary?

>  		return error;
>  	}
>  
> @@ -2988,19 +2984,11 @@ xfs_wait_dax_page(
>  
>  int
>  xfs_break_dax_layouts(
> -	struct inode		*inode,
> -	bool			*retry)
> +	struct inode		*inode)
>  {
> -	struct page		*page;
> -
>  	xfs_assert_ilocked(XFS_I(inode), XFS_MMAPLOCK_EXCL);
>  
> -	page = dax_layout_busy_page(inode->i_mapping);
> -	if (!page)
> -		return 0;
> -
> -	*retry = true;
> -	return dax_wait_page_idle(page, xfs_wait_dax_page, inode);
> +	return dax_break_mapping_inode(inode, xfs_wait_dax_page);
>  }
>  
>  int
> @@ -3018,8 +3006,7 @@ xfs_break_layouts(
>  		retry = false;
>  		switch (reason) {
>  		case BREAK_UNMAP:
> -			error = xfs_break_dax_layouts(inode, &retry);
> -			if (error || retry)
> +			if (xfs_break_dax_layouts(inode))

dax_break_mapping can return -ERESTARTSYS, right?  So doesn't this need
to be:
			error = xfs_break_dax_layouts(inode);
			if (error)
				break;

Hm?

--D

>  				break;
>  			fallthrough;
>  		case BREAK_WRITE:
> diff --git a/fs/xfs/xfs_inode.h b/fs/xfs/xfs_inode.h
> index 1648dc5..c4f03f6 100644
> --- a/fs/xfs/xfs_inode.h
> +++ b/fs/xfs/xfs_inode.h
> @@ -593,7 +593,7 @@ xfs_itruncate_extents(
>  	return xfs_itruncate_extents_flags(tpp, ip, whichfork, new_size, 0);
>  }
>  
> -int	xfs_break_dax_layouts(struct inode *inode, bool *retry);
> +int	xfs_break_dax_layouts(struct inode *inode);
>  int	xfs_break_layouts(struct inode *inode, uint *iolock,
>  		enum layout_break_reason reason);
>  
> diff --git a/include/linux/dax.h b/include/linux/dax.h
> index 9b1ce98..f6583d3 100644
> --- a/include/linux/dax.h
> +++ b/include/linux/dax.h
> @@ -228,6 +228,20 @@ static inline void dax_read_unlock(int id)
>  {
>  }
>  #endif /* CONFIG_DAX */
> +
> +#if !IS_ENABLED(CONFIG_FS_DAX)
> +static inline int __must_check dax_break_mapping(struct inode *inode,
> +			    loff_t start, loff_t end, void (cb)(struct inode *))
> +{
> +	return 0;
> +}
> +
> +static inline void dax_break_mapping_uninterruptible(struct inode *inode,
> +						void (cb)(struct inode *))
> +{
> +}
> +#endif
> +
>  bool dax_alive(struct dax_device *dax_dev);
>  void *dax_get_private(struct dax_device *dax_dev);
>  long dax_direct_access(struct dax_device *dax_dev, pgoff_t pgoff, long nr_pages,
> @@ -251,6 +265,13 @@ vm_fault_t dax_finish_sync_fault(struct vm_fault *vmf,
>  int dax_delete_mapping_entry(struct address_space *mapping, pgoff_t index);
>  int dax_invalidate_mapping_entry_sync(struct address_space *mapping,
>  				      pgoff_t index);
> +int __must_check dax_break_mapping(struct inode *inode, loff_t start,
> +				loff_t end, void (cb)(struct inode *));
> +static inline int __must_check dax_break_mapping_inode(struct inode *inode,
> +						void (cb)(struct inode *))
> +{
> +	return dax_break_mapping(inode, 0, LLONG_MAX, cb);
> +}
>  int dax_dedupe_file_range_compare(struct inode *src, loff_t srcoff,
>  				  struct inode *dest, loff_t destoff,
>  				  loff_t len, bool *is_same,
> diff --git a/mm/madvise.c b/mm/madvise.c
> index 49f3a75..1f4c99e 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -1063,7 +1063,7 @@ static int guard_install_pud_entry(pud_t *pud, unsigned long addr,
>  	pud_t pudval = pudp_get(pud);
>  
>  	/* If huge return >0 so we abort the operation + zap. */
> -	return pud_trans_huge(pudval) || pud_devmap(pudval);
> +	return pud_trans_huge(pudval);
>  }
>  
>  static int guard_install_pmd_entry(pmd_t *pmd, unsigned long addr,
> @@ -1072,7 +1072,7 @@ static int guard_install_pmd_entry(pmd_t *pmd, unsigned long addr,
>  	pmd_t pmdval = pmdp_get(pmd);
>  
>  	/* If huge return >0 so we abort the operation + zap. */
> -	return pmd_trans_huge(pmdval) || pmd_devmap(pmdval);
> +	return pmd_trans_huge(pmdval);
>  }
>  
>  static int guard_install_pte_entry(pte_t *pte, unsigned long addr,
> @@ -1183,7 +1183,7 @@ static int guard_remove_pud_entry(pud_t *pud, unsigned long addr,
>  	pud_t pudval = pudp_get(pud);
>  
>  	/* If huge, cannot have guard pages present, so no-op - skip. */
> -	if (pud_trans_huge(pudval) || pud_devmap(pudval))
> +	if (pud_trans_huge(pudval))
>  		walk->action = ACTION_CONTINUE;
>  
>  	return 0;
> @@ -1195,7 +1195,7 @@ static int guard_remove_pmd_entry(pmd_t *pmd, unsigned long addr,
>  	pmd_t pmdval = pmdp_get(pmd);
>  
>  	/* If huge, cannot have guard pages present, so no-op - skip. */
> -	if (pmd_trans_huge(pmdval) || pmd_devmap(pmdval))
> +	if (pmd_trans_huge(pmdval))
>  		walk->action = ACTION_CONTINUE;
>  
>  	return 0;
> -- 
> git-series 0.9.1
>