linux-kernel - Re: [PATCH v2 12/13] arm64: mm: Wrap flush_tlb_page() around ___flush_tlb

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Message-ID: <8184baab-8774-4a73-8cee-d8d3e22553c0@arm.com>
Date: Tue, 27 Jan 2026 14:03:43 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: Jonathan Cameron <jonathan.cameron@...wei.com>
Cc: Will Deacon <will@...nel.org>, Ard Biesheuvel <ardb@...nel.org>,
 Catalin Marinas <catalin.marinas@....com>,
 Mark Rutland <mark.rutland@....com>,
 Linus Torvalds <torvalds@...ux-foundation.org>,
 Oliver Upton <oliver.upton@...ux.dev>, Marc Zyngier <maz@...nel.org>,
 Dev Jain <dev.jain@....com>, Linu Cherian <Linu.Cherian@....com>,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 12/13] arm64: mm: Wrap flush_tlb_page() around
 ___flush_tlb_range()

On 27/01/2026 12:59, Jonathan Cameron wrote:
> On Mon, 19 Jan 2026 17:21:59 +0000
> Ryan Roberts <ryan.roberts@....com> wrote:
> 
>> Flushing a page from the tlb is just a special case of flushing a range.
>> So let's rework flush_tlb_page() so that it simply wraps
>> ___flush_tlb_range(). While at it, let's also update the API to take the
>> same flags that we use when flushing a range. This allows us to delete
>> all the ugly "_nosync", "_local" and "_nonotify" variants.
>>
>> Thanks to constant folding, all of the complex looping and tlbi-by-range
>> options get eliminated so that the generated code for flush_tlb_page()
>> looks very similar to the previous version.
>>
>> Reviewed-by: Linu Cherian <linu.cherian@....com>
>> Signed-off-by: Ryan Roberts <ryan.roberts@....com>
> 
> So this does include the use of the
> 
> Case TLBF_NOBROADCAST from previous patch, but only whilst (I think)
> slightly changing behavior.
> 
> Gah.  I'm regretting looking at this series. The original code is really hard to
> read :)  Rather you than me to fix it!
> 
>>  static inline void flush_tlb_kernel_range(unsigned long start, unsigned long end)
>>  {
>>  	const unsigned long stride = PAGE_SIZE;
>> diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
>> index be9dab2c7d6a..f91aa686f142 100644
>> --- a/arch/arm64/mm/fault.c
>> +++ b/arch/arm64/mm/fault.c
>> @@ -239,7 +239,7 @@ int __ptep_set_access_flags(struct vm_area_struct *vma,
>>  	 * flush_tlb_fix_spurious_fault().
>>  	 */
>>  	if (dirty)
>> -		local_flush_tlb_page(vma, address);
>> +		__flush_tlb_page(vma, address, TLBF_NOBROADCAST);
> 
> Ultimately I think this previously did __tlbi(vale1) and now does __tlbi(vae1)
> Original call was to __local_flush_tlb_page_notify_nosync()

No not quite; the new code is still doing __tlbi(vale1).

The trick is that the __flush_tlb_page() wrapper unconditionally adds
TLBF_NOWALKCACHE to the flags. Since this API is operating on a *page* it is
implicit that we should only be evicting a leaf entry (as per the old
implementation).

You'll see I've also updated the documentation to make that clear in tlbflush.h.

Now that you have raised it, I can see how it might be confusing though, since
__flush_tlb_page() does not explicitly have TLBF_NOWALKCACHE. We could require
all __flush_tlb_page() callers to explicitly pass TLBF_NOWALKCACHE if you think
that helps? It would still be implicit for flush_tlb_page() (the generic kernel
API) though.

> 
> I'd like to see that sort of change called out and explained in the patch description.
> It's a broader scoped flush so not a bug, but still a functional change.

As I say, the emitted code is the same. It's my new API that's the problem here...

Thanks,
Ryan

> 
>>  	return 1;
>>  }
>>  
>