[PATCH v2 12/15] mm, swap: mark swap address space ro and add context debug check

[Kairui Song <ryncsn@gmail.com>]

From: Kairui Song <kasong@tencent.com>

Swap cache is now backed by swap table, and the address space is not
holding any mutable data anymore. And swap cache is now protected by
the swap cluster lock, instead of the XArray lock. All access to swap
cache are wrapped by swap cache helpers. Locking is mostly handled
internally by swap cache helpers, only a few __swap_cache_* helpers
require the caller to lock the cluster by themselves.

Worth noting that, unlike XArray, the cluster lock is not IRQ safe.
The swap cache was very different compared to filemap, and now it's
completely separated from filemap. Nothing wants to mark or change
anything or do a writeback callback in IRQ.

So explicitly document this and add a debug check to avoid further
potential misuse. And mark the swap cache space as read-only to avoid
any user wrongly mixing unexpected filemap helpers with swap cache.

Signed-off-by: Kairui Song <kasong@tencent.com>
---
 mm/swap.h       | 12 +++++++++++-
 mm/swap_state.c |  3 ++-
 2 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/mm/swap.h b/mm/swap.h
index bf4e54f1f6b6..e48431a26f89 100644
--- a/mm/swap.h
+++ b/mm/swap.h
@@ -99,6 +99,16 @@ static __always_inline struct swap_cluster_info *__swap_cluster_lock(
 {
 	struct swap_cluster_info *ci = __swap_offset_to_cluster(si, offset);
 
+	/*
+	 * Nothing modifies swap cache in an IRQ context. All access to
+	 * swap cache is wrapped by swap_cache_* helpers, and swap cache
+	 * writeback is handled outside of IRQs. Swapin or swapout never
+	 * occurs in IRQ, and neither does in-place split or replace.
+	 *
+	 * Besides, modifying swap cache requires synchronization with
+	 * swap_map, which was never IRQ safe.
+	 */
+	VM_WARN_ON_ONCE(!in_task());
 	VM_WARN_ON_ONCE(percpu_ref_is_zero(&si->users)); /* race with swapoff */
 	if (irq)
 		spin_lock_irq(&ci->lock);
@@ -191,7 +201,7 @@ void __swap_writepage(struct folio *folio, struct swap_iocb **swap_plug);
 #define SWAP_ADDRESS_SPACE_SHIFT	14
 #define SWAP_ADDRESS_SPACE_PAGES	(1 << SWAP_ADDRESS_SPACE_SHIFT)
 #define SWAP_ADDRESS_SPACE_MASK		(SWAP_ADDRESS_SPACE_PAGES - 1)
-extern struct address_space swap_space;
+extern struct address_space swap_space __ro_after_init;
 static inline struct address_space *swap_address_space(swp_entry_t entry)
 {
 	return &swap_space;
diff --git a/mm/swap_state.c b/mm/swap_state.c
index 7147b390745f..209d5e9e8d90 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -37,7 +37,8 @@ static const struct address_space_operations swap_aops = {
 #endif
 };
 
-struct address_space swap_space __read_mostly = {
+/* Set swap_space as read only as swap cache is handled by swap table */
+struct address_space swap_space __ro_after_init = {
 	.a_ops = &swap_aops,
 };
 
-- 
2.51.0