linux-kernel - Re: [GIT PULL] MIPS fixes for v6.18

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Message-ID: <alpine.DEB.2.21.2511250512500.36486@angie.orcam.me.uk>
Date: Tue, 25 Nov 2025 07:32:16 +0000 (GMT)
From: "Maciej W. Rozycki" <macro@...am.me.uk>
To: Thomas Bogendoerfer <tsbogend@...ha.franken.de>
cc: Gregory CLEMENT <gregory.clement@...tlin.com>, 
    torvalds@...ux-foundation.org, linux-mips@...r.kernel.org, 
    linux-kernel@...r.kernel.org
Subject: Re: [GIT PULL] MIPS fixes for v6.18

On Mon, 24 Nov 2025, Thomas Bogendoerfer wrote:

> > > Maciej W. Rozycki (2):
> > >       MIPS: Malta: Fix !EVA SOC-it PCI MMIO
> > >       MIPS: mm: Prevent a TLB shutdown on initial uniquification
> > 
> > Today, the kernel v6.18-rc7 no longer boots on EyeQ5 and EyeQ6H (MIPS
> > I6500)-based boards. After a git bisect between v6.18-rc6 and v6.18-rc7,
> > we found that the culprit is the commit "MIPS: mm: Prevent a TLB
> > shutdown on initial uniquification".
> > 
> > Here is the log from a vanilla v6.18-rc7:
> 
> [..]
> 
> I guess your cores have more than 64 TLB entries. The Octeon CPU has
> 256 entries... Patch below fixes the issue there.
> 
> Thomas.
> 
> >From b74abcb21103519ae48726c715d39a6aa3f57462 Mon Sep 17 00:00:00 2001
> From: Thomas Bogendoerfer <tsbogend@...ha.franken.de>
> Date: Mon, 24 Nov 2025 22:46:43 +0100
> Subject: [PATCH] MIPS: mm: kmalloc tlb_vpn array to avoid stack overflow
> 
> 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.

 Thank you for chasing this up.

 Indeed, in the absence of a cross-reference from Config1.MMUSize in the 
ISA manual I missed the somewhat recent addition of Config4.MMUSizeExt and 
VTLB/FTLB MMU features I haven't dealt with before.  I've looked through 
the relevant documents and ISTM there's nothing else needed here so let's 
hope your fix covers it all.

 For the record, the I6500 has a documented TLB size of 16 VTLB + 512 FTLB 
entries and the array needs to hold them all.  Though for VTLB/FTLB we 
necessarily rely on the EntryHi.EHINV feature, which means we could skip 
the call to `r4k_tlb_uniquify' altogether.  Something for a possible later 
improvement, I suppose.

> diff --git a/arch/mips/mm/tlb-r4k.c b/arch/mips/mm/tlb-r4k.c
> index 3facf7cc6c7d..577055b50c41 100644
> --- a/arch/mips/mm/tlb-r4k.c
> +++ b/arch/mips/mm/tlb-r4k.c
> @@ -524,15 +524,19 @@ static int r4k_vpn_cmp(const void *a, const void *b)
>   */
>  static void r4k_tlb_uniquify(void)
>  {
> -	unsigned long tlb_vpns[1 << MIPS_CONF1_TLBS_SIZE];
>  	int tlbsize = current_cpu_data.tlbsize;
>  	int start = num_wired_entries();
> +	unsigned long *tlb_vpns;
>  	unsigned long vpn_mask;
>  	int cnt, ent, idx, i;
>  
>  	vpn_mask = GENMASK(cpu_vmbits - 1, 13);
>  	vpn_mask |= IS_ENABLED(CONFIG_64BIT) ? 3ULL << 62 : 1 << 31;
>  
> +	tlb_vpns = kmalloc_array(tlbsize, sizeof(unsigned long), GFP_KERNEL);
> +	if (!tlb_vpns)
> +		return; /* pray local_flush_tlb_all() is good enough */

 I can't say I'm particularly happy with this bail-out hack, but then I 
have nothing better in my mind right now, so OK, but can you please make 
the comment a proper sentence (starting with a capital letter and ending 
with a full stop)?

  Maciej