Commit 8b47299a authored Sep 17, 2025 by Kairui Song Committed by Andrew Morton Sep 21, 2025

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/20250916160100.31545-13-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>
Suggested-by: Chris Li <chrisl@kernel.org>
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: kernel test robot <oliver.sang@intel.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>
Cc: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

parent 8578e0c0

mm/swap.h

+11 −1

Original line number	Diff line number	Diff line
		@@ -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;

mm/swap_state.c

+2 −1

Original line number	Diff line number	Diff line
		@@ -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,
		};