mm, swap: use unified helper for swap cache look up

The swap cache lookup helper swap_cache_get_folio currently does readahead
updates as well, so callers that are not doing swapin from any VMA or
mapping are forced to reuse filemap helpers instead, and have to access
the swap cache space directly.

So decouple readahead update with swap cache lookup.  Move the readahead
update part into a standalone helper.  Let the caller call the readahead
update helper if they do readahead.  And convert all swap cache lookups to
use swap_cache_get_folio.

After this commit, there are only three special cases for accessing swap
cache space now: huge memory splitting, migration, and shmem replacing,
because they need to lock the XArray.  The following commits will wrap
their accesses to the swap cache too, with special helpers.

And worth noting, currently dropbehind is not supported for anon folio,
and we will never see a dropbehind folio in swap cache.  The unified
helper can be updated later to handle that.

While at it, add proper kernedoc for touched helpers.

No functional change.

Link: https://lkml.kernel.org/r/20250916160100.31545-3-ryncsn@gmail.com
Signed-off-by: Kairui Song <kasong@tencent.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Reviewed-by: Barry Song <baohua@kernel.org>
Acked-by: David Hildenbrand <david@redhat.com>
Acked-by: Chris Li <chrisl@kernel.org>
Acked-by: Nhat Pham <nphamcs@gmail.com>
Suggested-by: Chris Li <chrisl@kernel.org>
Cc: Baoquan He <bhe@redhat.com>
Cc: "Huang, Ying" <ying.huang@linux.alibaba.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: kernel test robot <oliver.sang@intel.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Yosry Ahmed <yosryahmed@google.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 9 insertions, 4 deletions

diff --git a/mm/swap.h b/mm/swap.h
index 1ae44d4193b1..efb6d7ff9f30 100644
--- a/mm/swap.h
+++ b/mm/swap.h
@@ -62,8 +62,7 @@ void delete_from_swap_cache(struct folio *folio);
 void clear_shadow_from_swap_cache(int type, unsigned long begin,
 				  unsigned long end);
 void swapcache_clear(struct swap_info_struct *si, swp_entry_t entry, int nr);
-struct folio *swap_cache_get_folio(swp_entry_t entry,
-		struct vm_area_struct *vma, unsigned long addr);
+struct folio *swap_cache_get_folio(swp_entry_t entry);
 struct folio *read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
 		struct vm_area_struct *vma, unsigned long addr,
 		struct swap_iocb **plug);
@@ -74,6 +73,8 @@ struct folio *swap_cluster_readahead(swp_entry_t entry, gfp_t flag,
 		struct mempolicy *mpol, pgoff_t ilx);
 struct folio *swapin_readahead(swp_entry_t entry, gfp_t flag,
 		struct vm_fault *vmf);
+void swap_update_readahead(struct folio *folio, struct vm_area_struct *vma,
+			   unsigned long addr);
 
 static inline unsigned int folio_swap_flags(struct folio *folio)
 {
@@ -159,6 +160,11 @@ static inline struct folio *swapin_readahead(swp_entry_t swp, gfp_t gfp_mask,
 	return NULL;
 }
 
+static inline void swap_update_readahead(struct folio *folio,
+		struct vm_area_struct *vma, unsigned long addr)
+{
+}
+
 static inline int swap_writeout(struct folio *folio,
 		struct swap_iocb **swap_plug)
 {
@@ -169,8 +175,7 @@ static inline void swapcache_clear(struct swap_info_struct *si, swp_entry_t entr
 {
 }
 
-static inline struct folio *swap_cache_get_folio(swp_entry_t entry,
-		struct vm_area_struct *vma, unsigned long addr)
+static inline struct folio *swap_cache_get_folio(swp_entry_t entry)
 {
 	return NULL;
 }