[patch] zswap: fix zswap_frontswap_load() vs zsmalloc::map/unmap() might_sleep() splat

[Mike Galbraith <efault@gmx.de>]

Greetings,

Beating on zswap+zsmalloc leads to the splat below, possible patch
below that.

[  139.794413] BUG: sleeping function called from invalid context at kernel/locking/mutex.c:935
[  139.794585] in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 907, name: avahi-daemon
[  139.794608] 2 locks held by avahi-daemon/907:
[  139.794621]  #0: ffff8881010766d8 (&mm->mmap_lock#2){++++}-{3:3}, at: exc_page_fault+0x15b/0x6e0
[  139.794659]  #1: ffff8881b6b13110 (&zspage->lock){.+.+}-{2:2}, at: zs_map_object+0x89/0x350
[  139.794691] Preemption disabled at:
[  139.794693] [<ffffffff812763a8>] zs_map_object+0x38/0x350
[  139.794719] CPU: 0 PID: 907 Comm: avahi-daemon Kdump: loaded Tainted: G            E     5.10.0.g3644e2d-preempt #4
[  139.794746] Hardware name: MEDION MS-7848/MS-7848, BIOS M7848W08.20C 09/23/2013
[  139.794766] Call Trace:
[  139.794775]  dump_stack+0x77/0x97
[  139.794788]  ___might_sleep+0x17d/0x260
[  139.794806]  __mutex_lock+0x47/0x9d0
[  139.794823]  ? zswap_frontswap_load+0xcb/0x240
[  139.794841]  ? zs_map_object+0x248/0x350
[  139.794858]  ? zswap_frontswap_load+0xcb/0x240
[  139.794871]  zswap_frontswap_load+0xcb/0x240
[  139.794886]  ? lru_cache_add+0xe9/0x1b0
[  139.794904]  __frontswap_load+0x6e/0xd0
[  139.794921]  swap_readpage+0x70/0x240
[  139.794939]  swap_cluster_readahead+0x1d9/0x280
[  139.794964]  ? swapin_readahead+0x140/0x3e0
[  139.794979]  swapin_readahead+0x140/0x3e0
[  139.794997]  ? lookup_swap_cache+0x5c/0x190
[  139.795016]  ? do_swap_page+0x2f7/0x900
[  139.795030]  do_swap_page+0x2f7/0x900
[  139.795046]  handle_mm_fault+0xa06/0x13b0
[  139.795083]  exc_page_fault+0x2a3/0x6e0
[  139.795110]  ? asm_exc_page_fault+0x1e/0x30
[  139.795139]  ? asm_exc_page_fault+0x8/0x30
[  139.795166]  asm_exc_page_fault+0x1e/0x30
[  139.795191] RIP: 0033:0x560caa4074d0
[  139.795215] Code: 89 05 e4 74 21 00 0f 84 28 06 00 00 e8 d9 13 00 00 e8 b4 12 00 00 44 8b 25 b9 75 21 00 45 85 e4 0f 85 ac 04 00 00 41 83 cf ff <48> 8b 3d b1 74 21 00 44 89 fe e8 d1 f6 ff ff 85 c0 89 c3 0f 88 d9
[  139.795299] RSP: 002b:00007ffeb4ec5ce0 EFLAGS: 00010246
[  139.795316] RAX: 0000000000000000 RBX: 0000000000000000 RCX: 00007ff1616e4db4
[  139.795336] RDX: 0000000000000001 RSI: 00007ffeb4ec5c5f RDI: 0000000000000006
[  139.795356] RBP: 0000560cab63be60 R08: 0000560cab656220 R09: 0000560cab670750
[  139.795376] R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
[  139.795395] R13: 0000560cab63f550 R14: 0000560cab63c000 R15: 00000000ffffffff

(CC Sebastian because RT has the ~same problem at the same spot,
curable the same way, or by fixing the upstream buglet then take
backports+fix, and nuking the associated RT patch)

mm/zswap: fix zswap_frontswap_load() vs zsmalloc::map/unmap() might_sleep() splat

zsmalloc map/unmap methods use preemption disabling bit spinlocks.
Take the
mutex outside of pool map/unmap methods in zswap_frontswap_load() as is done
in zswap_frontswap_store().

Signed-off-by: Mike Galbraith <efault@gmx.de>
Fixes: 1ec3b5fe6eec "mm/zswap: move to use crypto_acomp API for hardware acceleration"
---
 mm/zswap.c |    6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -1258,20 +1258,20 @@ static int zswap_frontswap_load(unsigned

 	/* decompress */
 	dlen = PAGE_SIZE;
+	acomp_ctx = raw_cpu_ptr(entry->pool->acomp_ctx);
+	mutex_lock(acomp_ctx->mutex);
 	src = zpool_map_handle(entry->pool->zpool, entry->handle, ZPOOL_MM_RO);
 	if (zpool_evictable(entry->pool->zpool))
 		src += sizeof(struct zswap_header);

-	acomp_ctx = raw_cpu_ptr(entry->pool->acomp_ctx);
-	mutex_lock(acomp_ctx->mutex);
 	sg_init_one(&input, src, entry->length);
 	sg_init_table(&output, 1);
 	sg_set_page(&output, page, PAGE_SIZE, 0);
 	acomp_request_set_params(acomp_ctx->req, &input, &output, entry->length, dlen);
 	ret = crypto_wait_req(crypto_acomp_decompress(acomp_ctx->req), &acomp_ctx->wait);
-	mutex_unlock(acomp_ctx->mutex);

 	zpool_unmap_handle(entry->pool->zpool, entry->handle);
+	mutex_unlock(acomp_ctx->mutex);
 	BUG_ON(ret);

 freeentry: