linux-kernel - Re: [PATCH linux-next v3] swap_state: update shadow

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Message-ID: <Y8Wq3apsJh7keUVA@casper.infradead.org>
Date:   Mon, 16 Jan 2023 19:51:57 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     yang.yang29@....com.cn
Cc:     akpm@...ux-foundation.org, hannes@...xchg.org,
        bagasdotme@...il.com, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        iamjoonsoo.kim@....com, ran.xiaokai@....com.cn
Subject: Re: [PATCH linux-next v3] swap_state: update shadow_nodes for
 anonymous page

On Fri, Jan 13, 2023 at 05:36:45PM +0800, yang.yang29@....com.cn wrote:
> From: Yang Yang <yang.yang29@....com.cn>
> 
> Shadow_nodes is for shadow nodes reclaiming of workingset handling,
> it is updated when page cache add or delete since long time ago
> workingset only supported page cache. But when workingset supports
> anonymous page detection, we missied updating shadow nodes for
> it. This caused that shadow nodes of anonymous page will never be
> reclaimd by scan_shadow_nodes() even they use much memory and
> system memory is tense.
> 
> So update shadow_nodes of anonymous page when swap cache is
> add or delete by calling  xas_set_update(..workingset_update_node).

What testing did you do of this?  I have this crash in today's testing:

04304 BUG: kernel NULL pointer dereference, address: 0000000000000080
04304 #PF: supervisor read access in kernel mode
04304 #PF: error_code(0x0000) - not-present page
04304 PGD 0 P4D 0
04304 Oops: 0000 [#1] PREEMPT SMP NOPTI
04304 CPU: 4 PID: 3219629 Comm: sh Kdump: loaded Not tainted 6.2.0-rc4-next-20230116-00016-gd289d3de8ce5-dirty #69
04304 Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.0-debian-1.16.0-4 04/01/2014
04304 RIP: 0010:_raw_spin_trylock+0x12/0x50
04304 Code: e0 41 5c 5d c3 89 c6 48 89 df e8 89 06 00 00 4c 89 e0 5b 41 5c 5d c3 90 55 48 89 e5 53 48 89 fb bf 01 00 00 00 e8 be 5b 71 ff <8b> 03 85 c0 75 16 ba 01 00 00 00 f0 0f b1 13 b8 01 00 00 00 75 06
04304 RSP: 0018:ffff888059afbbb8 EFLAGS: 00010093
04304 RAX: 0000000000000003 RBX: 0000000000000080 RCX: 0000000000000000
04304 RDX: 0000000000000000 RSI: ffff8880033e24c8 RDI: 0000000000000001
04304 RBP: ffff888059afbbc0 R08: 0000000000000000 R09: ffff888059afbd68
04304 R10: ffff88807d9db868 R11: 0000000000000000 R12: ffff8880033e24c0
04304 R13: ffff88800a1d8008 R14: ffff8880033e24c8 R15: ffff8880033e24c0
04304 FS:  00007feeeabc6740(0000) GS:ffff88807d900000(0000) knlGS:0000000000000000
04304 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
04304 CR2: 0000000000000080 CR3: 0000000059830003 CR4: 0000000000770ea0
04304 PKRU: 55555554
04304 Call Trace:
04304  <TASK>
04304  shadow_lru_isolate+0x3a/0x120
04304  __list_lru_walk_one+0xa3/0x190
04304  ? memcg_list_lru_alloc+0x330/0x330
04304  ? memcg_list_lru_alloc+0x330/0x330
04304  list_lru_walk_one_irq+0x59/0x80
04304  scan_shadow_nodes+0x27/0x30
04304  do_shrink_slab+0x13b/0x2e0
04304  shrink_slab+0x92/0x250
04304  drop_slab+0x41/0x90
04304  drop_caches_sysctl_handler+0x70/0x80
04304  proc_sys_call_handler+0x162/0x210
04304  proc_sys_write+0xe/0x10
04304  vfs_write+0x1c7/0x3a0
04304  ksys_write+0x57/0xd0
04304  __x64_sys_write+0x14/0x20
04304  do_syscall_64+0x34/0x80
04304  entry_SYSCALL_64_after_hwframe+0x63/0xcd
04304 RIP: 0033:0x7feeeacc1190

Decoding it, shadow_lru_isolate+0x3a/0x120 maps back to this line:

        if (!spin_trylock(&mapping->host->i_lock)) {

i_lock is at offset 128 of struct inode, so that matches the dump.
I believe that swapper_spaces never have ->host set, so I don't
believe you've tested this patch since 51b8c1fe250d went in
back in 2021.