linux-kernel - Re: [patch V3 13/37] mips/mm/highmem: Switch to generic kmap atomic

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Message-ID: <20210111091646.hkugbtlcced3vmno@linutronix.de>
Date:   Mon, 11 Jan 2021 10:16:46 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Thomas Bogendoerfer <tsbogend@...ha.franken.de>
Cc:     Paul Cercueil <paul@...pouillou.net>, tglx@...utronix.de,
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Subject: Re: [patch V3 13/37] mips/mm/highmem: Switch to generic kmap atomic

On 2021-01-09 01:33:52 [+0100], Thomas Bogendoerfer wrote:
> On Sat, Jan 09, 2021 at 12:58:05AM +0100, Thomas Bogendoerfer wrote:
> > On Fri, Jan 08, 2021 at 08:20:43PM +0000, Paul Cercueil wrote:
> > > Hi Thomas,
> > > 
> > > 5.11 does not boot anymore on Ingenic SoCs, I bisected it to this commit.
> > > 
> > > Any idea what could be happening?
> > 
> > not yet, kernel crash log of a Malta QEMU is below.
> 
> update:
> 
> This dirty hack lets the Malta QEMU boot again:
> 
> diff --git a/mm/highmem.c b/mm/highmem.c
> index c3a9ea7875ef..190cdda1149d 100644
> --- a/mm/highmem.c
> +++ b/mm/highmem.c
> @@ -515,7 +515,7 @@ void *__kmap_local_pfn_prot(unsigned long pfn, pgprot_t prot)
>  	vaddr = __fix_to_virt(FIX_KMAP_BEGIN + idx);
>  	BUG_ON(!pte_none(*(kmap_pte - idx)));
>  	pteval = pfn_pte(pfn, prot);
> -	set_pte_at(&init_mm, vaddr, kmap_pte - idx, pteval);
> +	set_pte(kmap_pte - idx, pteval);
>  	arch_kmap_local_post_map(vaddr, pteval);
>  	current->kmap_ctrl.pteval[kmap_local_idx()] = pteval;
>  	preempt_enable();
> 
> set_pte_at() tries to update cache and could do an kmap_atomic() there.
So the old implementation used set_pte() while the new one uses
set_pte_at().

> Not sure, if this is allowed at this point.
The problem is the recursion
  kmap_atomic() -> __update_cache() -> kmap_atomic()

and kmap_local_idx_push() runs out if index space before stack space.

I'm not sure if the __update_cache() worked for highmem. It has been
added for that in commit
   f4281bba81810 ("MIPS: Handle highmem pages in __update_cache")

but it assumes that the address returned by kmap_atomic() is the same or
related enough for flush_data_cache_page() to work.

> Thomas.
> 

Sebastian