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

[Rafael Aquini <aquini@redhat.com>]

On Wed, Jun 23, 2021 at 06:43:50PM +1000, Nicholas Piggin wrote:
> Excerpts from Rafael Aquini's message of June 23, 2021 8:50 am:
> > 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
> 
> This is vfreeing()ing 2TB of memory? Maybe not too realistic but
> I guess it doesn't hurt to add.
>

Andrew has recently addressed https://bugzilla.kernel.org/show_bug.cgi?id=210023
via 34fe653716b0d ("mm/vmalloc.c:__vmalloc_area_node(): avoid 32-bit overflow")

The person that filed that bug also filed a case requesting the issue
to be addressed in RHEL. So the case for large vmaps such as that one
in the test case is real, albeit not being a frequent use case perhaps.

Realistically, though, that vunmap loop can still burn considerable amount
of CPU time, even when vfreeing smaller areas, thus as you well noted, 
it's better to play nice and back off a bit and let others having a 
chance to run as well. 

> Acked-by: Nicholas Piggin <npiggin@gmail.com>

Thanks!

-- Rafael


> Thanks,
> Nick
> 
> > [  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.
> > 
> > Signed-off-by: Rafael Aquini <aquini@redhat.com>
> > ---
> >  mm/vmalloc.c | 1 +
> >  1 file changed, 1 insertion(+)
> > 
> > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > index a13ac524f6ff..cd4b23d65748 100644
> > --- a/mm/vmalloc.c
> > +++ b/mm/vmalloc.c
> > @@ -2564,6 +2564,7 @@ static void __vunmap(const void *addr, int deallocate_pages)
> >  
> >  			BUG_ON(!page);
> >  			__free_pages(page, page_order);
> > +			cond_resched();
> >  		}
> >  		atomic_long_sub(area->nr_pages, &nr_vmalloc_pages);
> >  
> > -- 
> > 2.26.3
> > 
> > 
> > 
>