mm/swap_state: skip meaningless swap cache readahead when ra_info.win == 0

swap_ra_info() may leave ra_info untouched in non_swap_entry() case as
page table lock is not held.  In this case, we have ra_info.nr_pte == 0
and it is meaningless to continue with swap cache readahead.  Skip such
ops by init ra_info.win = 1.

[akpm@linux-foundation.org: clean up struct init]

Link: https://lkml.kernel.org/r/20201009133059.58407-1-linmiaohe@huawei.comSigned-off-by: Miaohe Lin <linmiaohe@huawei.com>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/swap_state.c

@@ -839,7 +839,9 @@ static struct page *swap_vma_readahead(swp_entry_t fentry, gfp_t gfp_mask,
 	swp_entry_t entry;
 	unsigned int i;
 	bool page_allocated;
-	struct vma_swap_readahead ra_info = {0,};
+	struct vma_swap_readahead ra_info = {
+		.win = 1,
+	};
 
 	swap_ra_info(vmf, &ra_info);
 	if (ra_info.win == 1)