[PATCHv3, RFC 18/34] thp, mm: naive support of thp in generic read/write routines

["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 store.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
 mm/filemap.c |   18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index bcb679c..3296f5c 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1161,6 +1161,16 @@ find_page:
 			if (unlikely(page == NULL))
 				goto no_cached_page;
 		}
+		if (PageTransCompound(page)) {
+			struct page *head = compound_trans_head(page);
+			/*
+			 * We don't yet support huge pages in page cache
+			 * for filesystems with backing device, so pages
+			 * should always be up-to-date.
+			 */
+			BUG_ON(PageReadahead(head) || !PageUptodate(head));
+			goto page_ok;
+		}
 		if (PageReadahead(page)) {
 			page_cache_async_readahead(mapping,
 					ra, filp, page,
@@ -2439,8 +2449,13 @@ again:
 		if (mapping_writably_mapped(mapping))
 			flush_dcache_page(page);
 
+		if (PageTransHuge(page))
+			offset = pos & ~HPAGE_PMD_MASK;
+
 		pagefault_disable();
-		copied = iov_iter_copy_from_user_atomic(page, i, offset, bytes);
+		copied = iov_iter_copy_from_user_atomic(
+				page + (offset >> PAGE_CACHE_SHIFT),
+				i, offset & ~PAGE_CACHE_MASK, bytes);
 		pagefault_enable();
 		flush_dcache_page(page);
 
@@ -2463,6 +2478,7 @@ again:
 			 * because not all segments in the iov can be copied at
 			 * once without a pagefault.
 			 */
+			offset = pos & ~PAGE_CACHE_MASK;
 			bytes = min_t(unsigned long, PAGE_CACHE_SIZE - offset,
 						iov_iter_single_seg_count(i));
 			goto again;
-- 
1.7.10.4