[PATCHv4 14/43] filemap: handle huge pages in do_generic_file_read()

[Kirill A. Shutemov]

Most of work happans on head page. Only when we need to do copy data to
userspace we find relevant subpage.

We are still limited by PAGE_SIZE per iteration. Lifting this limitation
would require some more work.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
---
 mm/filemap.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index f8387488636f..ca4536f2035e 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1906,6 +1906,7 @@ static ssize_t do_generic_file_read(struct file *filp, loff_t *ppos,
 			if (unlikely(page == NULL))
 				goto no_cached_page;
 		}
+		page = compound_head(page);
 		if (PageReadahead(page)) {
 			page_cache_async_readahead(mapping,
 					ra, filp, page,
@@ -1984,7 +1985,8 @@ static ssize_t do_generic_file_read(struct file *filp, loff_t *ppos,
 		 * now we can copy it to user space...
 		 */
 
-		ret = copy_page_to_iter(page, offset, nr, iter);
+		ret = copy_page_to_iter(page + index - page->index, offset,
+				nr, iter);
 		offset += ret;
 		index += offset >> PAGE_SHIFT;
 		offset &= ~PAGE_MASK;
@@ -2402,6 +2404,7 @@ int filemap_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
 	 * because there really aren't any performance issues here
 	 * and we need to check for errors.
 	 */
+	page = compound_head(page);
 	ClearPageError(page);
 	error = mapping->a_ops->readpage(file, page);
 	if (!error) {
-- 
2.9.3