linux-kernel - Re: [PATCH v2] MIPS: mm: kmalloc tlb

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Message-ID: <alpine.DEB.2.21.2511262043590.36486@angie.orcam.me.uk>
Date: Wed, 26 Nov 2025 20:45:15 +0000 (GMT)
From: "Maciej W. Rozycki" <macro@...am.me.uk>
To: Thomas Bogendoerfer <tsbogend@...ha.franken.de>
cc: Jiaxun Yang <jiaxun.yang@...goat.com>, linux-mips@...r.kernel.org, 
    linux-kernel@...r.kernel.org, 
    Gregory CLEMENT <gregory.clement@...tlin.com>
Subject: Re: [PATCH v2] MIPS: mm: kmalloc tlb_vpn array to avoid stack
 overflow

On Wed, 26 Nov 2025, Thomas Bogendoerfer wrote:

> > > Latest MIPS cores could have much more than 64 TLB entries, therefore
> > > allocate array for unification instead of placing a too small array
> > > on stack.
> > 
> >  Hmm, I get:
> > 
> > ------------[ cut here ]------------
> > WARNING: CPU: 0 PID: 0 at arch/mips/mm/tlb-r4k.c:540 tlb_init+0x2a0/0x4bc
> > Modules linked in:
> > CPU: 0 UID: 0 PID: 0 Comm: swapper Not tainted 6.18.0-rc1-dirty #60 NONE
> > Hardware name: mti,malta
> > Stack : 00000000 00000004 00000000 0000001d 809d1d2c 00000000 00000100 80944048
> >         809d91bc 80944048 80a16a73 00000000 80b634a4 00000001 809d1ce0 809f7bc8
> >         00000000 00000000 80944048 0000001f 00000001 809d1c14 00000000 653a206d
> >         00000000 80b656d4 80b6570b 00000000 00000000 00000000 80944048 00000000
> >         00000000 0000021c 80a40000 00000000 00000000 00000020 00000000 800472a4
> >         ...
> > Call Trace:
> > [<80112bd8>] show_stack+0x28/0xf0
> > [<8010a69c>] dump_stack_lvl+0x48/0x7c
> > [<8012fedc>] __warn+0x9c/0x118
> > [<801015e8>] warn_slowpath_fmt+0x58/0xa4
> > [<8012ba84>] tlb_init+0x2a0/0x4bc
> > [<80114738>] per_cpu_trap_init+0x17c/0x27c
> > [<80a1d0f8>] trap_init+0xf0/0x794
> > [<80a19ae4>] start_kernel+0x3c4/0x598
> > 
> > ---[ end trace 0000000000000000 ]---
> > 
> > exactly here:
> > 
> > > +	tlb_vpns = kmalloc_array(tlbsize, sizeof(unsigned long), GFP_KERNEL);
> > > +	if (WARN_ON(!tlb_vpns))
> > > +		return; /* Pray local_flush_tlb_all() is good enough. */
> > 
> > I'll try to find out more, but right now this doesn't appear to work.
> 
> kmalloc() doesn't work at that point :-( It only fixed the problem, because
> we skip unification... d'oh

 Correct, I have a fix in the works.

  Maciej