[PATCH 01/15] dax: export a low-level __dax_zero_page_range helper

[Christoph Hellwig <hch@xxxxxx>]

This allows XFS to perform zeroing using the iomap infrastructure and
avoid buffer heads.

Signed-off-by: Christoph Hellwig <hch@xxxxxx>
---
 fs/dax.c            | 35 ++++++++++++++++++++---------------
 include/linux/dax.h |  7 +++++++
 2 files changed, 27 insertions(+), 15 deletions(-)

diff --git a/fs/dax.c b/fs/dax.c
index 90322eb..6d5d744 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -1082,6 +1082,23 @@ int dax_pfn_mkwrite(struct vm_area_struct *vma, struct 
vm_fault *vmf)
 }
 EXPORT_SYMBOL_GPL(dax_pfn_mkwrite);
 
+int __dax_zero_page_range(struct block_device *bdev, sector_t sector,
+               unsigned int offset, unsigned int length)
+{
+       struct blk_dax_ctl dax = {
+               .sector         = sector,
+               .size           = PAGE_CACHE_SIZE,
+       };
+
+       if (dax_map_atomic(bdev, &dax) < 0)
+               return PTR_ERR(dax.addr);
+       clear_pmem(dax.addr + offset, length);
+       wmb_pmem();
+       dax_unmap_atomic(bdev, &dax);
+       return 0;
+}
+EXPORT_SYMBOL_GPL(__dax_zero_page_range);
+
 /**
  * dax_zero_page_range - zero a range within a page of a DAX file
  * @inode: The file being truncated
@@ -1117,23 +1134,11 @@ int dax_zero_page_range(struct inode *inode, loff_t 
from, unsigned length,
        bh.b_bdev = inode->i_sb->s_bdev;
        bh.b_size = PAGE_CACHE_SIZE;
        err = get_block(inode, index, &bh, 0);
-       if (err < 0)
+       if (err < 0 || !buffer_written(&bh))
                return err;
-       if (buffer_written(&bh)) {
-               struct block_device *bdev = bh.b_bdev;
-               struct blk_dax_ctl dax = {
-                       .sector = to_sector(&bh, inode),
-                       .size = PAGE_CACHE_SIZE,
-               };
 
-               if (dax_map_atomic(bdev, &dax) < 0)
-                       return PTR_ERR(dax.addr);
-               clear_pmem(dax.addr + offset, length);
-               wmb_pmem();
-               dax_unmap_atomic(bdev, &dax);
-       }
-
-       return 0;
+       return __dax_zero_page_range(bh.b_bdev, to_sector(&bh, inode),
+                       offset, length);
 }
 EXPORT_SYMBOL_GPL(dax_zero_page_range);
 
diff --git a/include/linux/dax.h b/include/linux/dax.h
index 636dd59..8155b81 100644
--- a/include/linux/dax.h
+++ b/include/linux/dax.h
@@ -17,12 +17,19 @@ int __dax_fault(struct vm_area_struct *, struct vm_fault *, 
get_block_t,
 
 #ifdef CONFIG_FS_DAX
 struct page *read_dax_sector(struct block_device *bdev, sector_t n);
+int __dax_zero_page_range(struct block_device *bdev, sector_t sector,
+               unsigned int offset, unsigned int length);
 #else
 static inline struct page *read_dax_sector(struct block_device *bdev,
                sector_t n)
 {
        return ERR_PTR(-ENXIO);
 }
+static inline int __dax_zero_page_range(struct block_device *bdev,
+               sector_t sector, unsigned int offset, unsigned int length)
+{
+       return -ENXIO;
+}
 #endif
 
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
-- 
2.1.4