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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>

authored by

Kairui Song and committed by
Andrew Morton 8b47299a 8578e0c0

+13 -2
+11 -1
mm/swap.h
··· 99 99 { 100 100 struct swap_cluster_info *ci = __swap_offset_to_cluster(si, offset); 101 101 102 + /* 103 + * Nothing modifies swap cache in an IRQ context. All access to 104 + * swap cache is wrapped by swap_cache_* helpers, and swap cache 105 + * writeback is handled outside of IRQs. Swapin or swapout never 106 + * occurs in IRQ, and neither does in-place split or replace. 107 + * 108 + * Besides, modifying swap cache requires synchronization with 109 + * swap_map, which was never IRQ safe. 110 + */ 111 + VM_WARN_ON_ONCE(!in_task()); 102 112 VM_WARN_ON_ONCE(percpu_ref_is_zero(&si->users)); /* race with swapoff */ 103 113 if (irq) 104 114 spin_lock_irq(&ci->lock); ··· 202 192 #define SWAP_ADDRESS_SPACE_SHIFT 14 203 193 #define SWAP_ADDRESS_SPACE_PAGES (1 << SWAP_ADDRESS_SPACE_SHIFT) 204 194 #define SWAP_ADDRESS_SPACE_MASK (SWAP_ADDRESS_SPACE_PAGES - 1) 205 - extern struct address_space swap_space; 195 + extern struct address_space swap_space __ro_after_init; 206 196 static inline struct address_space *swap_address_space(swp_entry_t entry) 207 197 { 208 198 return &swap_space;
+2 -1
mm/swap_state.c
··· 37 37 #endif 38 38 }; 39 39 40 - struct address_space swap_space __read_mostly = { 40 + /* Set swap_space as read only as swap cache is handled by swap table */ 41 + struct address_space swap_space __ro_after_init = { 41 42 .a_ops = &swap_aops, 42 43 }; 43 44