mm: Add arch hooks for saving/restoring tags

Arm's Memory Tagging Extension (MTE) adds some metadata (tags) to
every physical page, when swapping pages out to disk it is necessary to
save these tags, and later restore them when reading the pages back.

Add some hooks along with dummy implementations to enable the
arch code to handle this.

Three new hooks are added to the swap code:
 * arch_prepare_to_swap() and
 * arch_swap_invalidate_page() / arch_swap_invalidate_area().
One new hook is added to shmem:
 * arch_swap_restore()

Signed-off-by: Steven Price <steven.price@arm.com>
[catalin.marinas@arm.com: add unlock_page() on the error path]
[catalin.marinas@arm.com: dropped the _tags suffix]
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/pgtable.h

@@ -633,6 +633,34 @@ static inline int arch_unmap_one(struct mm_struct *mm,
 }
 #endif
 
+/*
+ * Allow architectures to preserve additional metadata associated with
+ * swapped-out pages. The corresponding __HAVE_ARCH_SWAP_* macros and function
+ * prototypes must be defined in the arch-specific asm/pgtable.h file.
+ */
+#ifndef __HAVE_ARCH_PREPARE_TO_SWAP
+static inline int arch_prepare_to_swap(struct page *page)
+{
+	return 0;
+}
+#endif
+
+#ifndef __HAVE_ARCH_SWAP_INVALIDATE
+static inline void arch_swap_invalidate_page(int type, pgoff_t offset)
+{
+}
+
+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 page *page)
+{
+}
+#endif
+
 #ifndef __HAVE_ARCH_PGD_OFFSET_GATE
 #define pgd_offset_gate(mm, addr)	pgd_offset(mm, addr)
 #endif

mm/page_io.c

@@ -252,6 +252,16 @@ int swap_writepage(struct page *page, struct writeback_control *wbc)
 		unlock_page(page);
 		goto out;
 	}
+	/*
+	 * Arch code may have to preserve more data than just the page
+	 * contents, e.g. memory tags.
+	 */
+	ret = arch_prepare_to_swap(page);
+	if (ret) {
+		set_page_dirty(page);
+		unlock_page(page);
+		goto out;
+	}
 	if (frontswap_store(page) == 0) {
 		set_page_writeback(page);
 		unlock_page(page);

mm/shmem.c

@@ -1734,6 +1734,12 @@ static int shmem_swapin_page(struct inode *inode, pgoff_t index,
 	}
 	wait_on_page_writeback(page);
 
+	/*
+	 * Some architectures may have to restore extra metadata to the
+	 * physical page after reading from swap.
+	 */
+	arch_swap_restore(swap, page);
+
 	if (shmem_should_replace_page(page, gfp)) {
 		error = shmem_replace_page(&page, gfp, info, index);
 		if (error)

mm/swapfile.c

@@ -717,6 +717,7 @@ static void swap_range_free(struct swap_info_struct *si, unsigned long offset,
 	else
 		swap_slot_free_notify = NULL;
 	while (offset <= end) {
+		arch_swap_invalidate_page(si->type, offset);
 		frontswap_invalidate_page(si->type, offset);
 		if (swap_slot_free_notify)
 			swap_slot_free_notify(si->bdev, offset);
@@ -2682,6 +2683,7 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
 	frontswap_map = frontswap_map_get(p);
 	spin_unlock(&p->lock);
 	spin_unlock(&swap_lock);
+	arch_swap_invalidate_area(p->type);
 	frontswap_invalidate_area(p->type);
 	frontswap_map_set(p, NULL);
 	mutex_unlock(&swapon_mutex);