[PATCH 4/8] mm: drop mmap_sem for swap read IO submission

[Josef Bacik]

From: Johannes Weiner <jweiner@xxxxxx>

We don't need to hold the mmap_sem while we're doing the IO, simply drop
it and retry appropriately.

Signed-off-by: Johannes Weiner <jweiner@xxxxxx>
Signed-off-by: Josef Bacik <josef@xxxxxxxxxxxxxx>
---
 mm/page_io.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/mm/page_io.c b/mm/page_io.c
index aafd19ec1db4..bf21b56a964e 100644
--- a/mm/page_io.c
+++ b/mm/page_io.c
@@ -365,6 +365,20 @@ int swap_readpage(struct page *page, bool synchronous)
 		goto out;
 	}
 
+	/*
+	 * XXX:
+	 *
+	 * Propagate mm->mmap_sem into this function. Then:
+	 *
+	 * get_file(sis->swap_file)
+	 * up_read(mm->mmap_sem)
+	 * submit io request
+	 * fput
+	 *
+	 * After mmap_sem is dropped, sis is no longer valid. Go
+	 * through swap_file->blah->bdev.
+	 */
+
 	if (sis->flags & SWP_FILE) {
 		struct file *swap_file = sis->swap_file;
 		struct address_space *mapping = swap_file->f_mapping;
-- 
2.14.3