[PATCH 2/3] mm: Call arch_swap_restore() from arch_do_swap_page() and deprecate the latter

[Peter Collingbourne]

The previous patch made it possible for MTE to restore tags before they
are freed by hooking arch_do_swap_page().

However, the arch_do_swap_page() hook API is incompatible with swap
restoration in circumstances where we do not have an mm or a vma,
such as swapoff with swapped out shmem, and I expect that ADI will
currently fail to restore tags in these circumstances. This implies that
arch-specific metadata stores ought to be indexed by swap index, as MTE
does, rather than by mm and vma, as ADI does, and we should discourage
hooking arch_do_swap_page(), preferring to hook arch_swap_restore()
instead, as MTE already does.

Therefore, instead of directly hooking arch_do_swap_page() for
MTE, deprecate that hook, change its default implementation to call
arch_swap_restore() and rely on the existing implementation of the latter
for MTE.

Fixes: c145e0b47c77 ("mm: streamline COW logic in do_swap_page()")
Link: https://linux-review.googlesource.com/id/Id2f1ad76eaf606ae210e1d2dd0b7fe287e5f7d87
Signed-off-by: Peter Collingbourne <pcc@xxxxxxxxxx>
Reported-by: Qun-wei Lin (林群崴) <Qun-wei.Lin@xxxxxxxxxxxx>
Link: https://lore.kernel.org/all/5050805753ac469e8d727c797c2218a9d780d434.camel@xxxxxxxxxxxx/
Cc: <stable@xxxxxxxxxxxxxxx> # 6.1
---
 include/linux/pgtable.h | 26 +++++++++++++-------------
 1 file changed, 13 insertions(+), 13 deletions(-)

diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index c63cd44777ec..fc0259cf60fb 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -740,6 +740,12 @@ static inline int pgd_same(pgd_t pgd_a, pgd_t pgd_b)
 	set_pgd(pgdp, pgd); \
 })
 
+#ifndef __HAVE_ARCH_SWAP_RESTORE
+static inline void arch_swap_restore(swp_entry_t entry, struct folio *folio)
+{
+}
+#endif
+
 #ifndef __HAVE_ARCH_DO_SWAP_PAGE
 /*
  * Some architectures support metadata associated with a page. When a
@@ -748,14 +754,14 @@ static inline int pgd_same(pgd_t pgd_a, pgd_t pgd_b)
  * processors support an ADI (Application Data Integrity) tag for the
  * page as metadata for the page. arch_do_swap_page() can restore this
  * metadata when a page is swapped back in.
+ *
+ * This hook is deprecated. Architectures should hook arch_swap_restore()
+ * instead, because this hook is not called on all code paths that can
+ * swap in a page, particularly those where mm and vma are not available
+ * (e.g. swapoff for shmem pages).
  */
-static inline void arch_do_swap_page(struct mm_struct *mm,
-				     struct vm_area_struct *vma,
-				     unsigned long addr,
-				     pte_t pte, pte_t oldpte)
-{
-
-}
+#define arch_do_swap_page(mm, vma, addr, pte, oldpte) \
+	arch_swap_restore(pte_to_swp_entry(oldpte), page_folio(pte_page(pte)))
 #endif
 
 #ifndef __HAVE_ARCH_UNMAP_ONE
@@ -798,12 +804,6 @@ static inline void arch_swap_invalidate_area(int type)
 }
 #endif
 
-#ifndef __HAVE_ARCH_SWAP_RESTORE
-static inline void arch_swap_restore(swp_entry_t entry, struct folio *folio)
-{
-}
-#endif
-
 #ifndef __HAVE_ARCH_PGD_OFFSET_GATE
 #define pgd_offset_gate(mm, addr)	pgd_offset(mm, addr)
 #endif
-- 
2.40.1.606.ga4b1b128d6-goog