[PATCH] mm: Don't bother dropping mmap_sem for zero size readahead

[Jan Kara <jack@suse.cz>]

When handling a page fault, we drop mmap_sem to start async readahead so
that we don't block on IO submission with mmap_sem held.  However
there's no point to drop mmap_sem in case readahead is disabled. Handle
that case to avoid pointless dropping of mmap_sem and retrying the
fault. This was actually reported to block mlockall(MCL_CURRENT)
indefinitely.

Fixes: 6b4c9f446981 ("filemap: drop the mmap_sem for all blocking operations")
Reported-by: Minchan Kim <minchan@kernel.org>
Reported-by: Robert Stupp <snazy@gmx.de>
Signed-off-by: Jan Kara <jack@suse.cz>
---
 mm/filemap.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Andrew, could you please pick up this patch? Minchan also tripped over this
bug...

diff --git a/mm/filemap.c b/mm/filemap.c
index 1146fcfa3215..3d39c437b07e 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2458,7 +2458,7 @@ static struct file *do_async_mmap_readahead(struct vm_fault *vmf,
 	pgoff_t offset = vmf->pgoff;
 
 	/* If we don't want any read-ahead, don't bother */
-	if (vmf->vma->vm_flags & VM_RAND_READ)
+	if (vmf->vma->vm_flags & VM_RAND_READ || !ra->ra_pages)
 		return fpin;
 	if (ra->mmap_miss > 0)
 		ra->mmap_miss--;
-- 
2.16.4