[PATCH 1/1] Restore swap_space attr aviod krn panic

[<robin.kuo@mediatek.com>]

From: "robin.kuo" <robin.kuo@mediatek.com>

Restore swap_space attr avoid krn panic

Commit 8b47299a411a ('mm, swap: mark swap address space ro and add
context debug check') made the swap address space read-only.
It may lead to kernel panic if arch_prepare_to_swap returns a failure 
under heavy memory pressure as follows,

el1_abort+0x40/0x64
el1h_64_sync_handler+0x48/0xcc
el1h_64_sync+0x84/0x88
errseq_set+0x4c/0xb8 (P)
__filemap_set_wb_err+0x20/0xd0
shrink_folio_list+0xc20/0x11cc
evict_folios+0x1520/0x1be4
try_to_shrink_lruvec+0x27c/0x3dc
shrink_one+0x9c/0x228
shrink_node+0xb3c/0xeac
do_try_to_free_pages+0x170/0x4f0
try_to_free_pages+0x334/0x534
__alloc_pages_direct_reclaim+0x90/0x158
__alloc_pages_slowpath+0x334/0x588
__alloc_frozen_pages_noprof+0x224/0x2fc
__folio_alloc_noprof+0x14/0x64
vma_alloc_zeroed_movable_folio+0x34/0x44
do_pte_missing+0xad4/0x1040
handle_mm_fault+0x4a4/0x790
do_page_fault+0x288/0x5f8
do_translation_fault+0x38/0x54
do_mem_abort+0x54/0xa8

Restore swap address space as not ro to avoid the panic.

Signed-off-by: robin.kuo <robin.kuo@mediatek.com>
---
 mm/swap.h       | 2 +-
 mm/swap_state.c | 3 +--
 2 files changed, 2 insertions(+), 3 deletions(-)

diff --git a/mm/swap.h b/mm/swap.h
index d034c13d8dd2..1bd466da3039 100644
--- a/mm/swap.h
+++ b/mm/swap.h
@@ -198,7 +198,7 @@ int swap_writeout(struct folio *folio, struct swap_iocb **swap_plug);
 void __swap_writepage(struct folio *folio, struct swap_iocb **swap_plug);
 
 /* linux/mm/swap_state.c */
-extern struct address_space swap_space __ro_after_init;
+extern struct address_space swap_space __read_mostly;
 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 5f97c6ae70a2..44d228982521 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -37,8 +37,7 @@ static const struct address_space_operations swap_aops = {
 #endif
 };
 
-/* Set swap_space as read only as swap cache is handled by swap table */
-struct address_space swap_space __ro_after_init = {
+struct address_space swap_space __read_mostly = {
 	.a_ops = &swap_aops,
 };
 
-- 
2.45.2