Re: [PATCH] mm: vmalloc: add cond_resched() in __vunmap()

[Uladzislau Rezki]

On Thu, Jun 24, 2021 at 02:21:21PM +0200, Michal Hocko wrote:
> On Tue 22-06-21 18:50:30, Rafael Aquini wrote:
> > On non-preemptible kernel builds the watchdog can complain
> > about soft lockups when vfree() is called against large
> > vmalloc areas:
> > 
> > [  210.851798] kvmalloc-test: vmalloc(2199023255552) succeeded
> > [  238.654842] watchdog: BUG: soft lockup - CPU#181 stuck for 26s! [rmmod:5203]
> > [  238.662716] Modules linked in: kvmalloc_test(OE-) ...
> > [  238.772671] CPU: 181 PID: 5203 Comm: rmmod Tainted: G S         OE     5.13.0-rc7+ #1
> > [  238.781413] Hardware name: Intel Corporation PURLEY/PURLEY, BIOS PLYXCRB1.86B.0553.D01.1809190614 09/19/2018
> > [  238.792383] RIP: 0010:free_unref_page+0x52/0x60
> > [  238.797447] Code: 48 c1 fd 06 48 89 ee e8 9c d0 ff ff 84 c0 74 19 9c 41 5c fa 48 89 ee 48 89 df e8 b9 ea ff ff 41 f7 c4 00 02 00 00 74 01 fb 5b <5d> 41 5c c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 f0 29 77
> > [  238.818406] RSP: 0018:ffffb4d87868fe98 EFLAGS: 00000206
> > [  238.824236] RAX: 0000000000000000 RBX: 000000001da0c945 RCX: ffffb4d87868fe40
> > [  238.832200] RDX: ffffd79d3beed108 RSI: ffffd7998501dc08 RDI: ffff9c6fbffd7010
> > [  238.840166] RBP: 000000000d518cbd R08: ffffd7998501dc08 R09: 0000000000000001
> > [  238.848131] R10: 0000000000000000 R11: ffffd79d3beee088 R12: 0000000000000202
> > [  238.856095] R13: ffff9e5be3eceec0 R14: 0000000000000000 R15: 0000000000000000
> > [  238.864059] FS:  00007fe082c2d740(0000) GS:ffff9f4c69b40000(0000) knlGS:0000000000000000
> > [  238.873089] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > [  238.879503] CR2: 000055a000611128 CR3: 000000f6094f6006 CR4: 00000000007706e0
> > [  238.887467] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > [  238.895433] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> > [  238.903397] PKRU: 55555554
> > [  238.906417] Call Trace:
> > [  238.909149]  __vunmap+0x17c/0x220
> > [  238.912851]  __x64_sys_delete_module+0x13a/0x250
> > [  238.918008]  ? syscall_trace_enter.isra.20+0x13c/0x1b0
> > [  238.923746]  do_syscall_64+0x39/0x80
> > [  238.927740]  entry_SYSCALL_64_after_hwframe+0x44/0xae
> > 
> > Like in other range zapping routines that iterate over
> > a large list, lets just add cond_resched() within __vunmap()'s
> > page-releasing loop in order to avoid the watchdog splats.
> 
> cond_resched makes a lot of sense. We do not want vmalloc to be visible
> the userspace (e.g. by stalling it) so all time consuming operations
> should yield regularly whenever possible. I would expect that any
> susbsystem which needs huge vmalloc areas would have it for the whole
> boot life time so such large vfrees should be really rare.
> 
There is at least one more place with potentially similar issue. I see that
the bulk allocator disables irqs during obtaining pages and filling an array.

So i suspect if we request a huge size to allocate over vmalloc same soft
lockup should occur. For example 10G alloactions simultaneously on different
CPUs.

I will try to reproduce it on !CONFIG_PREEMPT kernel and post feedback here.

--
Vlad Rezki