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@xxxxxxxxxx>
>
> ---
>
> 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 | 29 +++++------------------------
> fs/xfs/xfs_inode.c | 23 +++++------------------
> fs/xfs/xfs_inode.h | 2 +-
> include/linux/dax.h | 21 +++++++++++++++++++++
> mm/madvise.c | 8 ++++----
> 7 files changed, 70 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);
This implementations removes logic around @retry found in the XFS and
FUSE implementations, I think that is a mistake, and EXT4 has
apparently been broken in this regard.
wait_page_idle() returns after @page is idle, but that does not mean
@inode is DMA idle. After one found page from
dax_layout_busy_page_range() is waited upon a new call to
dax_break_mapping() needs to made to check if another DMA started, or if
there were originally more pages active.
> + } while (error == 0);
> +
> + return error;
Surprised that the compiler does not warn about an uninitialized
variable here?
