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

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Message-ID: <6780c4862b417_2aff42943b@dwillia2-xfh.jf.intel.com.notmuch>
Date: Thu, 9 Jan 2025 22:56:06 -0800
From: Dan Williams <dan.j.williams@...el.com>
To: Alistair Popple <apopple@...dia.com>, Dan Williams
	<dan.j.williams@...el.com>
CC: <akpm@...ux-foundation.org>, <linux-mm@...ck.org>, <lina@...hilina.net>,
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Subject: Re: [PATCH v5 05/25] fs/dax: Create a common implementation to break
 DAX layouts

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@...dia.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.

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.