[PATCHv5 08/36] filemap: handle huge pages in do_generic_file_read()

Kirill A. Shutemov
2016-11-29T11:22:36Z

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@linux.intel.com>
---
mm/filemap.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index 74341f8b831e..6a2f9ea521fb 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1749,6 +1749,7 @@ 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,
@@ -1830,7 +1831,8 @@ 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;
@@ -2248,6 +2250,7 @@ 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) {