Re: [PATCH] mm/vmscan: don't scan adjust too much if current is not kswapd

[Matthew Wilcox]

On Thu, Sep 15, 2022 at 09:19:48AM +0800, Hongchen Zhang wrote:
> [ 3748.453561] INFO: task float_bessel:77920 blocked for more than 120
> seconds.
> [ 3748.460839]       Not tainted 5.15.0-46-generic #49-Ubuntu
> [ 3748.466490] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables
> this message.
> [ 3748.474618] task:float_bessel    state:D stack:    0 pid:77920 ppid:
> 77327 flags:0x00004002
> [ 3748.483358] Call Trace:
> [ 3748.485964]  <TASK>
> [ 3748.488150]  __schedule+0x23d/0x590
> [ 3748.491804]  schedule+0x4e/0xc0
> [ 3748.495038]  rwsem_down_read_slowpath+0x336/0x390
> [ 3748.499886]  ? copy_user_enhanced_fast_string+0xe/0x40
> [ 3748.505181]  down_read+0x43/0xa0
> [ 3748.508518]  do_user_addr_fault+0x41c/0x670
> [ 3748.512799]  exc_page_fault+0x77/0x170
> [ 3748.516673]  asm_exc_page_fault+0x26/0x30
> [ 3748.520824] RIP: 0010:copy_user_enhanced_fast_string+0xe/0x40
> [ 3748.526764] Code: 89 d1 c1 e9 03 83 e2 07 f3 48 a5 89 d1 f3 a4 31 c0 0f
> 01 ca c3 cc cc cc cc 0f 1f 00 0f 01 cb 83 fa 40 0f 82 70 ff ff ff 89 d1 <f3>
> a4 31 c0 0f 01 ca c3 cc cc cc cc 66 08
> [ 3748.546120] RSP: 0018:ffffaa9248fffb90 EFLAGS: 00050206
> [ 3748.551495] RAX: 00007f99faa1a010 RBX: ffffaa9248fffd88 RCX:
> 0000000000000010
> [ 3748.558828] RDX: 0000000000001000 RSI: ffff9db397ab8ff0 RDI:
> 00007f99faa1a000
> [ 3748.566160] RBP: ffffaa9248fffbf0 R08: ffffcc2fc2965d80 R09:
> 0000000000000014
> [ 3748.573492] R10: 0000000000000000 R11: 0000000000000014 R12:
> 0000000000001000
> [ 3748.580858] R13: 0000000000001000 R14: 0000000000000000 R15:
> ffffaa9248fffd98
> [ 3748.588196]  ? copy_page_to_iter+0x10e/0x400
> [ 3748.592614]  filemap_read+0x174/0x3e0

Interesting; it wasn't the process itself which triggered the page
fault; the process called read() and the kernel took the page fault to
satisfy the read() call.

> [ 3748.596354]  ? ima_file_check+0x6a/0xa0
> [ 3748.600301]  generic_file_read_iter+0xe5/0x150
> [ 3748.604884]  ext4_file_read_iter+0x5b/0x190
> [ 3748.609164]  ? aa_file_perm+0x102/0x250
> [ 3748.613125]  new_sync_read+0x10d/0x1a0
> [ 3748.617009]  vfs_read+0x103/0x1a0
> [ 3748.620423]  ksys_read+0x67/0xf0
> [ 3748.623743]  __x64_sys_read+0x19/0x20
> [ 3748.627511]  do_syscall_64+0x59/0xc0
> [ 3748.631203]  ? syscall_exit_to_user_mode+0x27/0x50
> [ 3748.636144]  ? do_syscall_64+0x69/0xc0
> [ 3748.639992]  ? exit_to_user_mode_prepare+0x96/0xb0
> [ 3748.644931]  ? irqentry_exit_to_user_mode+0x9/0x20
> [ 3748.649872]  ? irqentry_exit+0x1d/0x30
> [ 3748.653737]  ? exc_page_fault+0x89/0x170
> [ 3748.657795]  entry_SYSCALL_64_after_hwframe+0x61/0xcb
> [ 3748.663030] RIP: 0033:0x7f9a852989cc
> [ 3748.666713] RSP: 002b:00007f9a8497dc90 EFLAGS: 00000246 ORIG_RAX:
> 0000000000000000
> [ 3748.674487] RAX: ffffffffffffffda RBX: 00007f9a8497f5c0 RCX:
> 00007f9a852989cc
> [ 3748.681817] RDX: 0000000000027100 RSI: 00007f99faa18010 RDI:
> 0000000000000061
> [ 3748.689150] RBP: 00007f9a8497dd60 R08: 0000000000000000 R09:
> 00007f99faa18010
> [ 3748.696493] R10: 0000000000000000 R11: 0000000000000246 R12:
> 00007f99faa18010
> [ 3748.703841] R13: 00005605e11c406f R14: 0000000000000001 R15:
> 0000000000027100

ORIG_RAX is 0, which matches sys_read.
RDI is file descriptor 0x61
RSI is plausibly a userspace pointer, 0x7f99faa18010
RDX is the length, 0x27100 or 160kB.

That all seems reasonable.

What I really want to know is who is _holding_ the lock.  We stash
a pointer to the task_struct in 'owner', so we could clearly find this
out in the 'blocked for too long' report, and print their stack trace.

You must have done something like this already in order to deduce that
it was the direct reclaim path that was the problem?