Re: [PATCH v5 05/25] fs/dax: Create a common implementation to break DAX layouts

[Alistair Popple <apopple@nvidia.com>]

On Wed, Jan 08, 2025 at 04:14:20PM -0800, Dan Williams wrote:
> 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>
> > 
> > ---
> > 
> > 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.

I think both implementations are equivalent though, just that the XFS/FUSE ones are
spread across two functions with the retry happening in the outer function
whilst the EXT4 implementation is implemented in a single function with a do/
while loop.

Both exit early if dax_layout_busy_page() doesn't find a DMA-busy page, and
both call dax_layout_busy_page() a second time after waiting on a page to become
idle. So I don't think anything is broken here, unless I've missed something.

> 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?

So am I. Turns out this is built with -Wno-maybe-uninitialized and it's not
certain error is used uninitialized because we may bail early if this is not a
dax_mapping. So it's only maybe used uninitialized which isn't warned about.