swap: avoid holding swap reference in swap_cache_get_folio

All its callers either already hold a reference to, or lock the swap
device while calling this function. There is only one exception in
shmem_swapin_folio, just make this caller also hold a reference of the
swap device, so this helper can be simplified and saves a few cycles.

This also provides finer control of error handling in shmem_swapin_folio,
on race (with swap off), it can just try again. For invalid swap entry,
it can fail with a proper error code.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Kairui Song <[e-mail]>
Cc: David Hildenbrand <[e-mail]>
Cc: "Huang, Ying" <[e-mail]>
Cc: Hugh Dickins <[e-mail]>
Cc: Matthew Wilcox (Oracle) <[e-mail]>
Cc: Miaohe Lin <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

mm/shmem.c

···
 	struct address_space *mapping = inode->i_mapping;
 	struct shmem_inode_info *info = SHMEM_I(inode);
 	struct mm_struct *charge_mm = vma ? vma->vm_mm : NULL;
+	struct swap_info_struct *si;
 	struct folio *folio = NULL;
 	swp_entry_t swap;
 	int error;
···
 
 	if (is_swapin_error_entry(swap))
 		return -EIO;
+
+	si = get_swap_device(swap);
+	if (!si) {
+		if (!shmem_confirm_swap(mapping, index, swap))
+			return -EEXIST;
+		else
+			return -EINVAL;
+	}
 
 	/* Look it up and read it in.. */
 	folio = swap_cache_get_folio(swap, NULL, 0);
···
 	delete_from_swap_cache(folio);
 	folio_mark_dirty(folio);
 	swap_free(swap);
+	put_swap_device(si);
 
 	*foliop = folio;
 	return 0;
···
 		folio_unlock(folio);
 		folio_put(folio);
 	}
+	put_swap_device(si);
 
 	return error;
 }

mm/swap_state.c

···
  * unlocked and with its refcount incremented - we rely on the kernel
  * lock getting page table operations atomic even if we drop the folio
  * lock before returning.
+ *
+ * Caller must lock the swap device or hold a reference to keep it valid.
  */
 struct folio *swap_cache_get_folio(swp_entry_t entry,
 		struct vm_area_struct *vma, unsigned long addr)
 {
 	struct folio *folio;
-	struct swap_info_struct *si;
 
-	si = get_swap_device(entry);
-	if (!si)
-		return NULL;
 	folio = filemap_get_folio(swap_address_space(entry), swp_offset(entry));
-	put_swap_device(si);
-
 	if (folio) {
 		bool vma_ra = swap_use_vma_readahead();
 		bool readahead;