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/20221219185840.25441-5-ryncsn@gmail.com
Signed-off-by: Kairui Song <kasong@tencent.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: "Huang, Ying" <ying.huang@intel.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 2 insertions, 6 deletions

diff --git a/mm/swap_state.c b/mm/swap_state.c
index d8d171195a3a..cb9aaa00951d 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -321,19 +321,15 @@ static inline bool swap_use_vma_readahead(void)
  * 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;