linux-kernel - Re: [GIT PULL] x86/mm changes for v5.9

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Message-ID: <20200806195726.GI4520@mtj.thefacebook.com>
Date:   Thu, 6 Aug 2020 15:57:26 -0400
From:   Tejun Heo <tj@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Joerg Roedel <jroedel@...e.de>, Ingo Molnar <mingo@...nel.org>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [GIT PULL] x86/mm changes for v5.9

On Thu, Aug 06, 2020 at 12:42:23PM -0700, Linus Torvalds wrote:
> that admittedly odd sequence is get_work_pwq(work)
> 
> And then the faulting instruction is:
> 
> >   2a:* 49 8b 46 08          mov    0x8(%r14),%rax <-- trapping instruction
> 
> and this is the "->wq" dereference.
> 
> So it's the pwq->wq that traps, with 'pwq' being the trapping base
> pointer, and clearly being in the vmalloc space.
> 
> I think pwq may a percpu allocation, so not _directly_ vmalloc().
> Adding Tejun to the cc in case he can clarify ("No, silly Linus, it's
> allocated here..").

Hey, silly Linus, yeap, they're per-cpu allocations and will be in vmalloc
address space for per-cpu workqueues. For unbound workqueues, they're
regular kmallocs. The per-cpu allocation happens in alloc_and_link_pwqs():

  static int alloc_and_link_pwqs(struct workqueue_struct *wq)
  {
          bool highpri = wq->flags & WQ_HIGHPRI;
          int cpu, ret;

          if (!(wq->flags & WQ_UNBOUND)) {
                  wq->cpu_pwqs = alloc_percpu(struct pool_workqueue);
                  if (!wq->cpu_pwqs)
                          return -ENOMEM;

Thanks.

-- 
tejun