Re: [Bug report] hash_name() may cross page boundary and trigger sleep in RCU context

[Linus Torvalds <torvalds@linux-foundation.org>]

On Thu, 27 Nov 2025 at 02:58, Russell King (Oracle)
<linux@armlinux.org.uk> wrote:
>
> Ha!
>
> As said elsewhere, it looks like 32-bit ARM has been missing updates to
> the fault handler since pre-git history - this was modelled in the dim
> and distant i386 handling, and it just hasn't kept up.

I actually have this dim memory of having seen something along these
lines before, and I just had never realized how it could happen,
because that call to do_page_fault() in do_translation_fault()
visually *looks* like the only call-site, and so that

        if (addr < TASK_SIZE)
                return do_page_fault(addr, fsr, regs);

looks like it does everything correctly. That "do_page_fault()"
function is static to the arch/arm/mm/fault.c file, and that's the
only place that appears to call it.

The operative word being "appears".

Becuse I had never before realized that that fault.c then also does that

  #include "fsr-2level.c"

and then that do_page_fault() function is exposed through those
fsr_info[] operation arrays.

Anyway, I don't think that the ARM fault handling is all *that* bad.
Sure, it might be worth double-checking, but it *has* been converted
to the generic accounting helpers a few years ago and to the stack
growing fixes.

I think the fix here may be as simple as this trivial patch:

  diff --git a/arch/arm/mm/fault.c b/arch/arm/mm/fault.c
  index 2bc828a1940c..27024ec2d46d 100644
  --- a/arch/arm/mm/fault.c
  +++ b/arch/arm/mm/fault.c
  @@ -277,6 +277,10 @@ do_page_fault(unsigned long addr, ...
        if (interrupts_enabled(regs))
                local_irq_enable();

  +     /* non-user address faults never have context */
  +     if (addr >= TASK_SIZE)
  +             goto no_context;
  +
        /*
         * If we're in an interrupt or have no user
         * context, we must not take the fault..

but I really haven't thought much about it.

> I'm debating whether an entire rewrite would be appropriate

I don't think it's necessarily all that big of a deal. Yeah, this is
old code, and yeah, it could probably be cleaned up a bit, but at the
same time, "old and crusty" also means "fairly well tested". This
whole fault on a kernel address is a fairly unusual case, and as
mentioned, I *think* the above fix is sufficient.

Zizhi Wo - can you confirm that that patch (whitespace-damaged, but
simple enough to just do manually) fixes things for your test-case?

           Linus