mm/filemap.c: don't bother dropping mmap_sem for zero size readahead

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: 6b4c9f4 ("filemap: drop the mmap_sem for all blocking operations")
Reported-by: Minchan Kim <[email protected]>
Reported-by: Robert Stupp <[email protected]>
Signed-off-by: Jan Kara <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Reviewed-by: Josef Bacik <[email protected]>
Reviewed-by: Minchan Kim <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>

mm/filemap.c

@@ -2416,7 +2416,7 @@ 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--;