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Subject[PATCH 16/19] dax: Create a range version of dax_layout_busy_page()
Date
While reclaiming a dax range, we do not want to unamap whole file instead
want to make sure pages in a certain range do not have references taken
on them. Hence create a version of the function which allows to pass in
a range.

Cc: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
---
fs/dax.c | 66 ++++++++++++++++++++++++++++++++-------------
include/linux/dax.h | 6 +++++
2 files changed, 54 insertions(+), 18 deletions(-)

diff --git a/fs/dax.c b/fs/dax.c
index 60620a37030c..435f5b67e828 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -557,27 +557,20 @@ static void *grab_mapping_entry(struct xa_state *xas,
return xa_mk_internal(VM_FAULT_FALLBACK);
}

-/**
- * dax_layout_busy_page - find first pinned page in @mapping
- * @mapping: address space to scan for a page with ref count > 1
- *
- * DAX requires ZONE_DEVICE mapped pages. These pages are never
- * 'onlined' to the page allocator so they are considered idle when
- * page->count == 1. A filesystem uses this interface to determine if
- * any page in the mapping is busy, i.e. for DMA, or other
- * get_user_pages() usages.
- *
- * It is expected that the filesystem is holding locks to block the
- * establishment of new mappings in this address_space. I.e. it expects
- * to be able to run unmap_mapping_range() and subsequently not race
- * mapping_mapped() becoming true.
+/*
+ * Partial pages are included. If end is 0, pages in the range from start
+ * to end of the file are inluded.
*/
-struct page *dax_layout_busy_page(struct address_space *mapping)
+struct page *dax_layout_busy_page_range(struct address_space *mapping,
+ loff_t start, loff_t end)
{
- XA_STATE(xas, &mapping->i_pages, 0);
void *entry;
unsigned int scanned = 0;
struct page *page = NULL;
+ pgoff_t start_idx = start >> PAGE_SHIFT;
+ pgoff_t end_idx = end >> PAGE_SHIFT;
+ XA_STATE(xas, &mapping->i_pages, start_idx);
+ loff_t len, lstart = round_down(start, PAGE_SIZE);

/*
* In the 'limited' case get_user_pages() for dax is disabled.
@@ -588,6 +581,22 @@ struct page *dax_layout_busy_page(struct address_space *mapping)
if (!dax_mapping(mapping) || !mapping_mapped(mapping))
return NULL;

+ /* If end == 0, all pages from start to till end of file */
+ if (!end) {
+ end_idx = ULONG_MAX;
+ len = 0;
+ } else {
+ /* 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;
+ }
+
/*
* If we race get_user_pages_fast() here either we'll see the
* elevated page count in the iteration and wait, or
@@ -600,10 +609,10 @@ struct page *dax_layout_busy_page(struct address_space *mapping)
* guaranteed to either see new references or prevent new
* references from being established.
*/
- unmap_mapping_range(mapping, 0, 0, 0);
+ unmap_mapping_range(mapping, start, len, 0);

xas_lock_irq(&xas);
- xas_for_each(&xas, entry, ULONG_MAX) {
+ xas_for_each(&xas, entry, end_idx) {
if (WARN_ON_ONCE(!xa_is_value(entry)))
continue;
if (unlikely(dax_is_locked(entry)))
@@ -624,6 +633,27 @@ struct page *dax_layout_busy_page(struct address_space *mapping)
xas_unlock_irq(&xas);
return page;
}
+EXPORT_SYMBOL_GPL(dax_layout_busy_page_range);
+
+/**
+ * dax_layout_busy_page - find first pinned page in @mapping
+ * @mapping: address space to scan for a page with ref count > 1
+ *
+ * DAX requires ZONE_DEVICE mapped pages. These pages are never
+ * 'onlined' to the page allocator so they are considered idle when
+ * page->count == 1. A filesystem uses this interface to determine if
+ * any page in the mapping is busy, i.e. for DMA, or other
+ * get_user_pages() usages.
+ *
+ * It is expected that the filesystem is holding locks to block the
+ * establishment of new mappings in this address_space. I.e. it expects
+ * 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)
+{
+ return dax_layout_busy_page_range(mapping, 0, 0);
+}
EXPORT_SYMBOL_GPL(dax_layout_busy_page);

static int __dax_invalidate_entry(struct address_space *mapping,
diff --git a/include/linux/dax.h b/include/linux/dax.h
index e7f40108f2c9..3ef6686c080b 100644
--- a/include/linux/dax.h
+++ b/include/linux/dax.h
@@ -145,6 +145,7 @@ int dax_writeback_mapping_range(struct address_space *mapping,
struct writeback_control *wbc);

struct page *dax_layout_busy_page(struct address_space *mapping);
+struct page *dax_layout_busy_page_range(struct address_space *mapping, loff_t start, loff_t end);
dax_entry_t dax_lock_page(struct page *page);
void dax_unlock_page(struct page *page, dax_entry_t cookie);
#else
@@ -180,6 +181,11 @@ static inline struct page *dax_layout_busy_page(struct address_space *mapping)
return NULL;
}

+static inline struct page *dax_layout_busy_page_range(struct address_space *mapping, pgoff_t start, pgoff_t nr_pages)
+{
+ return NULL;
+}
+
static inline int dax_writeback_mapping_range(struct address_space *mapping,
struct block_device *bdev, struct dax_device *dax_dev,
struct writeback_control *wbc)
--
2.20.1
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