[PATCH 14/23] thp, mm: naive support of thp in generic_perform_write

["Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>]

From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>

For now we still write/read at most PAGE_CACHE_SIZE bytes a time.

This implementation doesn't cover address spaces with backing storage.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
 mm/filemap.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index b17ebb9..066bbff 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2382,6 +2382,7 @@ static ssize_t generic_perform_write(struct file *file,
 		unsigned long bytes;	/* Bytes to write to page */
 		size_t copied;		/* Bytes copied from user */
 		void *fsdata;
+		int subpage_nr = 0;
 
 		offset = (pos & (PAGE_CACHE_SIZE - 1));
 		bytes = min_t(unsigned long, PAGE_CACHE_SIZE - offset,
@@ -2411,8 +2412,14 @@ again:
 		if (mapping_writably_mapped(mapping))
 			flush_dcache_page(page);
 
+		if (PageTransHuge(page)) {
+			off_t huge_offset = pos & ~HPAGE_PMD_MASK;
+			subpage_nr = huge_offset >> PAGE_CACHE_SHIFT;
+		}
+
 		pagefault_disable();
-		copied = iov_iter_copy_from_user_atomic(page, i, offset, bytes);
+		copied = iov_iter_copy_from_user_atomic(page + subpage_nr, i,
+				offset, bytes);
 		pagefault_enable();
 		flush_dcache_page(page);
 
-- 
1.8.3.2