[PATCH 5.4 06/90] mm/filemap.c: dont bother dropping mmap_sem for zero size readahead

[Greg Kroah-Hartman]

From: Jan Kara <jack@xxxxxxx>

commit 5c72feee3e45b40a3c96c7145ec422899d0e8964 upstream.

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@xxxxxxxxxx>
Reported-by: Robert Stupp <snazy@xxxxxx>
Signed-off-by: Jan Kara <jack@xxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Reviewed-by: Josef Bacik <josef@xxxxxxxxxxxxxx>
Reviewed-by: Minchan Kim <minchan@xxxxxxxxxx>
Link: http://lkml.kernel.org/r/20200212101356.30759-1-jack@xxxxxxx
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: SeongJae Park <sjpark@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 mm/filemap.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2438,7 +2438,7 @@ static struct file *do_async_mmap_readah
 	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--;