mm, swap: mark swap address space ro and add context debug check

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.

Link: https://lkml.kernel.org/r/20250910160833.3464-13-ryncsn@gmail.com
Signed-off-by: Kairui Song <kasong@tencent.com>
Acked-by: Chris Li <chrisl@kernel.org>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: Barry Song <baohua@kernel.org>
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: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Yosry Ahmed <yosryahmed@google.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/swap.h b/mm/swap.h
index 742db4d46d2356b82c1f3d7d2fb985f8beadb6b7..adcd85fa85381fb41a284cb77b6581400bdca40e 100644 (file)
--- 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);
@@ -192,7 +202,7 @@ 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 f4a579c23087be9338b6d637461eb98103e49732..97372539a575d1554cd3f70b1b564c8955bf21b0 100644 (file)
--- 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,
 };