[PATCH 2/4] mm: Add arch hooks for saving/restoring tags

[Steven Price <steven.price@arm.com>]

Arm's Memory Tagging Extension (MTE) adds some metadata (taga) 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_tags()

Signed-off-by: Steven Price <steven.price@arm.com>
---
 include/asm-generic/pgtable.h | 23 +++++++++++++++++++++++
 mm/page_io.c                  |  6 ++++++
 mm/shmem.c                    |  6 ++++++
 mm/swapfile.c                 |  2 ++
 4 files changed, 37 insertions(+)

diff --git a/include/asm-generic/pgtable.h b/include/asm-generic/pgtable.h
index 329b8c8ca703..306cee75b9ec 100644
--- a/include/asm-generic/pgtable.h
+++ b/include/asm-generic/pgtable.h
@@ -475,6 +475,29 @@ static inline int arch_unmap_one(struct mm_struct *mm,
 }
 #endif
 
+#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_TAGS
+static inline void arch_swap_restore_tags(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
diff --git a/mm/page_io.c b/mm/page_io.c
index 76965be1d40e..7baee316ac99 100644
--- a/mm/page_io.c
+++ b/mm/page_io.c
@@ -253,6 +253,12 @@ 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)
+		goto out;
 	if (frontswap_store(page) == 0) {
 		set_page_writeback(page);
 		unlock_page(page);
diff --git a/mm/shmem.c b/mm/shmem.c
index 73754ed7af69..1010b91f267e 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1658,6 +1658,12 @@ 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_tags(swap, page);
+
 	if (shmem_should_replace_page(page, gfp)) {
 		error = shmem_replace_page(&page, gfp, info, index);
 		if (error)
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 5871a2aa86a5..b39c6520b0cf 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -722,6 +722,7 @@ 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);
@@ -2645,6 +2646,7 @@ 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);
-- 
2.20.1