Alistair Popple wrote:
> 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@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.
>
> 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.
Nope, you're right. I crossed my eyes flipping between FUSE/XFS and EXT4.
>
> > 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.
Looks like smatch just caught it.
