Re: [RFC PATCH v2 06/19] fs/ext4: Teach dax_layout_busy_page() to operate on a sub-range

[Ira Weiny]

On Fri, Aug 23, 2019 at 11:18:26AM -0400, Vivek Goyal wrote:
> On Fri, Aug 09, 2019 at 03:58:20PM -0700, ira.weiny@xxxxxxxxx wrote:
> > From: Ira Weiny <ira.weiny@xxxxxxxxx>
> > 
> > Callers of dax_layout_busy_page() are only rarely operating on the
> > entire file of concern.
> > 
> > Teach dax_layout_busy_page() to operate on a sub-range of the
> > address_space provided.  Specifying 0 - ULONG_MAX however, will continue
> > to operate on the "entire file" and XFS is split out to a separate patch
> > by this method.
> > 
> > This could potentially speed up dax_layout_busy_page() as well.
> 
> I need this functionality as well for virtio_fs and posted a patch for
> this.
> 
> https://lkml.org/lkml/2019/8/21/825
> 
> Given this is an optimization which existing users can benefit from already,
> this patch could probably be pushed upstream independently.

I'm ok with that.

However, this patch does not apply cleanly to head as I had some other
additions to dax.h.

> 
> > 
> > Signed-off-by: Ira Weiny <ira.weiny@xxxxxxxxx>
> > 
> > ---
> > Changes from RFC v1
> > 	Fix 0-day build errors
> > 
> >  fs/dax.c            | 15 +++++++++++----
> >  fs/ext4/ext4.h      |  2 +-
> >  fs/ext4/extents.c   |  6 +++---
> >  fs/ext4/inode.c     | 19 ++++++++++++-------
> >  fs/xfs/xfs_file.c   |  3 ++-
> >  include/linux/dax.h |  6 ++++--
> >  6 files changed, 33 insertions(+), 18 deletions(-)
> > 
> > diff --git a/fs/dax.c b/fs/dax.c
> > index a14ec32255d8..3ad19c384454 100644
> > --- a/fs/dax.c
> > +++ b/fs/dax.c
> > @@ -573,8 +573,11 @@ bool dax_mapping_is_dax(struct address_space *mapping)
> >  EXPORT_SYMBOL_GPL(dax_mapping_is_dax);
> >  
> >  /**
> > - * dax_layout_busy_page - find first pinned page in @mapping
> > + * dax_layout_busy_page - find first pinned page in @mapping within
> > + *                        the range @off - @off + @len
> >   * @mapping: address space to scan for a page with ref count > 1
> > + * @off: offset to start at
> > + * @len: length to scan through
> >   *
> >   * DAX requires ZONE_DEVICE mapped pages. These pages are never
> >   * 'onlined' to the page allocator so they are considered idle when
> > @@ -587,9 +590,13 @@ EXPORT_SYMBOL_GPL(dax_mapping_is_dax);
> >   * to be able to run unmap_mapping_range() and subsequently not race
> >   * mapping_mapped() becoming true.
> >   */
> > -struct page *dax_layout_busy_page(struct address_space *mapping)
> > +struct page *dax_layout_busy_page(struct address_space *mapping,
> > +				  loff_t off, loff_t len)
> >  {
> > -	XA_STATE(xas, &mapping->i_pages, 0);
> > +	unsigned long start_idx = off >> PAGE_SHIFT;
> > +	unsigned long end_idx = (len == ULONG_MAX) ? ULONG_MAX
> > +				: start_idx + (len >> PAGE_SHIFT);
> > +	XA_STATE(xas, &mapping->i_pages, start_idx);
> >  	void *entry;
> >  	unsigned int scanned = 0;
> >  	struct page *page = NULL;
> > @@ -612,7 +619,7 @@ struct page *dax_layout_busy_page(struct address_space *mapping)
> >  	unmap_mapping_range(mapping, 0, 0, 1);
> 
> Should we unmap only those pages which fall in the range specified by caller.
> Unmapping whole file seems to be less efficient.

Seems reasonable to me.  I was focused on getting pages which were busy not
necessarily on what got unmapped.  So I did not consider this.  Thanks for the
suggestion.

However, I don't understand the math you do for length?  Is this comment/code
correct?

+  /* length is being calculated from lstart and not start.
+   * This is due to behavior of unmap_mapping_range(). If
+   * start is say 4094 and end is on 4093 then want to
+   * unamp two pages, idx 0 and 1. But unmap_mapping_range()
+   * will unmap only page at idx 0. If we calculate len
+   * from the rounded down start, this problem should not
+   * happen.
+   */
+  len = end - lstart + 1;


How can end (4093) be < start (4094)?  Is that valid?  And why would a start of
4094 unmap idx 0?

Ira