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    SubjectRe: [RFC PATCH v2 06/19] fs/ext4: Teach dax_layout_busy_page() to operate on a sub-range
    On Fri, Aug 09, 2019 at 03:58:20PM -0700, ira.weiny@intel.com wrote:
    > From: Ira Weiny <ira.weiny@intel.com>
    >
    > 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.

    >
    > Signed-off-by: Ira Weiny <ira.weiny@intel.com>
    >
    > ---
    > 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.

    Thanks
    Vivek

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