This reverts commit 1da0d94a3ec8c5f3793b7be8538b55e60ebeefe3.

If zswap initial failed or has not been initial, frontswap_ops will be
NULL. In such situation, swap device would enable failed with following
stack trace:

  Unable to handle kernel access to user memory outside uaccess routines at virtual address 0000000000000000
  Mem abort info:
    ESR = 0x0000000096000004
    EC = 0x25: DABT (current EL), IL = 32 bits
    SET = 0, FnV = 0
    EA = 0, S1PTW = 0
    FSC = 0x04: level 0 translation fault
  Data abort info:
    ISV = 0, ISS = 0x00000004
    CM = 0, WnR = 0
  user pgtable: 4k pages, 48-bit VAs, pgdp=00000020a4fab000
  [0000000000000000] pgd=0000000000000000, p4d=0000000000000000
  Internal error: Oops: 96000004 [#1] SMP
  Modules linked in: zram fsl_dpaa2_eth pcs_lynx phylink ahci_qoriq crct10dif_ce ghash_ce sbsa_gwdt fsl_mc_dpio nvme lm90 nvme_core at803x xhci_plat_hcd rtc_fsl_ftm_alarm xgmac_mdio ahci_platform i2c_imx ip6_tables ip_tables fuse
  Unloaded tainted modules: cppc_cpufreq():1
  CPU: 10 PID: 761 Comm: swapon Not tainted 6.0.0-rc2-00454-g22100432cf14 #1
  Hardware name: SolidRun Ltd. SolidRun CEX7 Platform, BIOS EDK II Jun 21 2022
  pstate: 00400005 (nzcv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
  pc : frontswap_init+0x38/0x60
  lr : __do_sys_swapon+0x8a8/0x9f4
  sp : ffff80000969bcf0
  x29: ffff80000969bcf0 x28: ffff37bee0d8fc00 x27: ffff80000a7f5000
  x26: fffffcdefb971e80 x25: ffffaba797453b90 x24: 0000000000000064
  x23: ffff37c1f209d1a8 x22: ffff37bee880e000 x21: ffffaba797748560
  x20: ffff37bee0d8fce4 x19: ffffaba797748488 x18: 0000000000000014
  x17: 0000000030ec029a x16: ffffaba795a479b0 x15: 0000000000000000
  x14: 0000000000000000 x13: 0000000000000030 x12: 0000000000000001
  x11: ffff37c63c0aba18 x10: 0000000000000000 x9 : ffffaba7956b8c88
  x8 : ffff80000969bcd0 x7 : 0000000000000000 x6 : 0000000000000000
  x5 : 0000000000000001 x4 : 0000000000000000 x3 : ffffaba79730f000
  x2 : ffff37bee0d8fc00 x1 : 0000000000000000 x0 : 0000000000000000
  Call trace:
  frontswap_init+0x38/0x60
  __do_sys_swapon+0x8a8/0x9f4
  __arm64_sys_swapon+0x28/0x3c
  invoke_syscall+0x78/0x100
  el0_svc_common.constprop.0+0xd4/0xf4
  do_el0_svc+0x38/0x4c
  el0_svc+0x34/0x10c
  el0t_64_sync_handler+0x11c/0x150
  el0t_64_sync+0x190/0x194
  Code: d000e283 910003fd f9006c41 f946d461 (f9400021)
  ---[ end trace 0000000000000000 ]---

Reported-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Liu Shixin <liushixin2@huawei.com>
---
 include/linux/frontswap.h |  3 ++-
 mm/frontswap.c            | 50 +++++++++++++++++++++++++++++----------
 mm/zswap.c                |  8 ++-----
 3 files changed, 42 insertions(+), 19 deletions(-)

diff --git a/include/linux/frontswap.h b/include/linux/frontswap.h
index a631bac12220..c5b2848d2240 100644
--- a/include/linux/frontswap.h
+++ b/include/linux/frontswap.h
@@ -13,9 +13,10 @@ struct frontswap_ops {
 	int (*load)(unsigned, pgoff_t, struct page *); /* load a page */
 	void (*invalidate_page)(unsigned, pgoff_t); /* page no longer needed */
 	void (*invalidate_area)(unsigned); /* swap type just swapoff'ed */
+	struct frontswap_ops *next; /* private pointer to next ops */
 };
 
-int frontswap_register_ops(const struct frontswap_ops *ops);
+extern void frontswap_register_ops(struct frontswap_ops *ops);
 
 extern void frontswap_init(unsigned type, unsigned long *map);
 extern int __frontswap_store(struct page *page);
diff --git a/mm/frontswap.c b/mm/frontswap.c
index 1a97610308cb..8d644f56a1d2 100644
--- a/mm/frontswap.c
+++ b/mm/frontswap.c
@@ -27,7 +27,10 @@ DEFINE_STATIC_KEY_FALSE(frontswap_enabled_key);
  * may be registered, but implementations can never deregister.  This
  * is a simple singly-linked list of all registered implementations.
  */
-static const struct frontswap_ops *frontswap_ops __read_mostly;
+static struct frontswap_ops *frontswap_ops __read_mostly;
+
+#define for_each_frontswap_ops(ops)		\
+	for ((ops) = frontswap_ops; (ops); (ops) = (ops)->next)
 
 #ifdef CONFIG_DEBUG_FS
 /*
@@ -94,14 +97,18 @@ static inline void inc_frontswap_invalidates(void) { }
 /*
  * Register operations for frontswap
  */
-int frontswap_register_ops(const struct frontswap_ops *ops)
+void frontswap_register_ops(struct frontswap_ops *ops)
 {
-	if (frontswap_ops)
-		return -EINVAL;
+	/*
+	 * Setting frontswap_ops must happen after the ops->init() calls
+	 * above; cmpxchg implies smp_mb() which will ensure the init is
+	 * complete at this point.
+	 */
+	do {
+		ops->next = frontswap_ops;
+	} while (cmpxchg(&frontswap_ops, ops->next, ops) != ops->next);
 
-	frontswap_ops = ops;
 	static_branch_inc(&frontswap_enabled_key);
-	return 0;
 }
 
 /*
@@ -110,6 +117,7 @@ int frontswap_register_ops(const struct frontswap_ops *ops)
 void frontswap_init(unsigned type, unsigned long *map)
 {
 	struct swap_info_struct *sis = swap_info[type];
+	struct frontswap_ops *ops;
 
 	VM_BUG_ON(sis == NULL);
 
@@ -125,7 +133,9 @@ void frontswap_init(unsigned type, unsigned long *map)
 	 * p->frontswap set to something valid to work properly.
 	 */
 	frontswap_map_set(sis, map);
-	frontswap_ops->init(type);
+
+	for_each_frontswap_ops(ops)
+		ops->init(type);
 }
 
 static bool __frontswap_test(struct swap_info_struct *sis,
@@ -164,6 +174,7 @@ int __frontswap_store(struct page *page)
 	int type = swp_type(entry);
 	struct swap_info_struct *sis = swap_info[type];
 	pgoff_t offset = swp_offset(entry);
+	struct frontswap_ops *ops;
 
 	VM_BUG_ON(!frontswap_ops);
 	VM_BUG_ON(!PageLocked(page));
@@ -177,10 +188,16 @@ int __frontswap_store(struct page *page)
 	 */
 	if (__frontswap_test(sis, offset)) {
 		__frontswap_clear(sis, offset);
-		frontswap_ops->invalidate_page(type, offset);
+		for_each_frontswap_ops(ops)
+			ops->invalidate_page(type, offset);
 	}
 
-	ret = frontswap_ops->store(type, offset, page);
+	/* Try to store in each implementation, until one succeeds. */
+	for_each_frontswap_ops(ops) {
+		ret = ops->store(type, offset, page);
+		if (!ret) /* successful store */
+			break;
+	}
 	if (ret == 0) {
 		__frontswap_set(sis, offset);
 		inc_frontswap_succ_stores();
@@ -203,6 +220,7 @@ int __frontswap_load(struct page *page)
 	int type = swp_type(entry);
 	struct swap_info_struct *sis = swap_info[type];
 	pgoff_t offset = swp_offset(entry);
+	struct frontswap_ops *ops;
 
 	VM_BUG_ON(!frontswap_ops);
 	VM_BUG_ON(!PageLocked(page));
@@ -212,7 +230,11 @@ int __frontswap_load(struct page *page)
 		return -1;
 
 	/* Try loading from each implementation, until one succeeds. */
-	ret = frontswap_ops->load(type, offset, page);
+	for_each_frontswap_ops(ops) {
+		ret = ops->load(type, offset, page);
+		if (!ret) /* successful load */
+			break;
+	}
 	if (ret == 0)
 		inc_frontswap_loads();
 	return ret;
@@ -225,6 +247,7 @@ int __frontswap_load(struct page *page)
 void __frontswap_invalidate_page(unsigned type, pgoff_t offset)
 {
 	struct swap_info_struct *sis = swap_info[type];
+	struct frontswap_ops *ops;
 
 	VM_BUG_ON(!frontswap_ops);
 	VM_BUG_ON(sis == NULL);
@@ -232,7 +255,8 @@ void __frontswap_invalidate_page(unsigned type, pgoff_t offset)
 	if (!__frontswap_test(sis, offset))
 		return;
 
-	frontswap_ops->invalidate_page(type, offset);
+	for_each_frontswap_ops(ops)
+		ops->invalidate_page(type, offset);
 	__frontswap_clear(sis, offset);
 	inc_frontswap_invalidates();
 }
@@ -244,6 +268,7 @@ void __frontswap_invalidate_page(unsigned type, pgoff_t offset)
 void __frontswap_invalidate_area(unsigned type)
 {
 	struct swap_info_struct *sis = swap_info[type];
+	struct frontswap_ops *ops;
 
 	VM_BUG_ON(!frontswap_ops);
 	VM_BUG_ON(sis == NULL);
@@ -251,7 +276,8 @@ void __frontswap_invalidate_area(unsigned type)
 	if (sis->frontswap_map == NULL)
 		return;
 
-	frontswap_ops->invalidate_area(type);
+	for_each_frontswap_ops(ops)
+		ops->invalidate_area(type);
 	atomic_set(&sis->frontswap_pages, 0);
 	bitmap_zero(sis->frontswap_map, sis->max);
 }
diff --git a/mm/zswap.c b/mm/zswap.c
index 2d48fd59cc7a..c22208df292a 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -1415,7 +1415,7 @@ static void zswap_frontswap_init(unsigned type)
 	zswap_trees[type] = tree;
 }
 
-static const struct frontswap_ops zswap_frontswap_ops = {
+static struct frontswap_ops zswap_frontswap_ops = {
 	.store = zswap_frontswap_store,
 	.load = zswap_frontswap_load,
 	.invalidate_page = zswap_frontswap_invalidate_page,
@@ -1512,15 +1512,11 @@ static int __init init_zswap(void)
 	if (!shrink_wq)
 		goto fallback_fail;
 
-	ret = frontswap_register_ops(&zswap_frontswap_ops);
-	if (ret)
-		goto destroy_wq;
+	frontswap_register_ops(&zswap_frontswap_ops);
 	if (zswap_debugfs_init())
 		pr_warn("debugfs initialization failed\n");
 	return 0;
 
-destroy_wq:
-	destroy_workqueue(shrink_wq);
 fallback_fail:
 	if (pool)
 		zswap_pool_destroy(pool);
-- 
2.25.1

On Mon, Aug 29, 2022 at 09:22:57PM +0800, Liu Shixin wrote:
> This reverts commit 1da0d94a3ec8c5f3793b7be8538b55e60ebeefe3.

Err, no, this is a way to big hammer and going in the wrong way.
If we can't have a sane way to fix it, the fix is just to remove
the now rather pointless fontswap abstraction entirely.

I'm on vacation this week, but I'll look into it ASAP once back.