linux-kernel - Re: [PATCH v1 03/16] arm64: hugetlb: Fix flush_hugetlb_tlb

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Message-ID: <49149fa7-4775-4d44-91f6-ce67c1b0ac2c@arm.com>
Date: Thu, 13 Feb 2025 10:27:40 +0530
From: Anshuman Khandual <anshuman.khandual@....com>
To: Ryan Roberts <ryan.roberts@....com>,
 Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 Muchun Song <muchun.song@...ux.dev>,
 Pasha Tatashin <pasha.tatashin@...een.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Uladzislau Rezki <urezki@...il.com>, Christoph Hellwig <hch@...radead.org>,
 Mark Rutland <mark.rutland@....com>, Ard Biesheuvel <ardb@...nel.org>,
 Dev Jain <dev.jain@....com>, Alexandre Ghiti <alexghiti@...osinc.com>,
 Steve Capper <steve.capper@...aro.org>, Kevin Brodsky <kevin.brodsky@....com>
Cc: linux-arm-kernel@...ts.infradead.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH v1 03/16] arm64: hugetlb: Fix flush_hugetlb_tlb_range()
 invalidation level



On 2/6/25 18:34, Ryan Roberts wrote:
> On 06/02/2025 06:46, Anshuman Khandual wrote:
>>
>>
>> On 2/5/25 20:39, Ryan Roberts wrote:
>>> commit c910f2b65518 ("arm64/mm: Update tlb invalidation routines for
>>> FEAT_LPA2") changed the "invalidation level unknown" hint from 0 to
>>> TLBI_TTL_UNKNOWN (INT_MAX). But the fallback "unknown level" path in
>>> flush_hugetlb_tlb_range() was not updated. So as it stands, when trying
>>> to invalidate CONT_PMD_SIZE or CONT_PTE_SIZE hugetlb mappings, we will
>>> spuriously try to invalidate at level 0 on LPA2-enabled systems.
>>>
>>> Fix this so that the fallback passes TLBI_TTL_UNKNOWN, and while we are
>>> at it, explicitly use the correct stride and level for CONT_PMD_SIZE and
>>> CONT_PTE_SIZE, which should provide a minor optimization.
>>>
>>> Cc: <stable@...r.kernel.org>
>>> Fixes: c910f2b65518 ("arm64/mm: Update tlb invalidation routines for FEAT_LPA2")
>>> Signed-off-by: Ryan Roberts <ryan.roberts@....com>
>>> ---
>>>  arch/arm64/include/asm/hugetlb.h | 20 ++++++++++++++------
>>>  1 file changed, 14 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/arch/arm64/include/asm/hugetlb.h b/arch/arm64/include/asm/hugetlb.h
>>> index 03db9cb21ace..8ab9542d2d22 100644
>>> --- a/arch/arm64/include/asm/hugetlb.h
>>> +++ b/arch/arm64/include/asm/hugetlb.h
>>> @@ -76,12 +76,20 @@ static inline void flush_hugetlb_tlb_range(struct vm_area_struct *vma,
>>>  {
>>>  	unsigned long stride = huge_page_size(hstate_vma(vma));
>>>  
>>> -	if (stride == PMD_SIZE)
>>> -		__flush_tlb_range(vma, start, end, stride, false, 2);
>>> -	else if (stride == PUD_SIZE)
>>> -		__flush_tlb_range(vma, start, end, stride, false, 1);
>>> -	else
>>> -		__flush_tlb_range(vma, start, end, PAGE_SIZE, false, 0);
>>> +	switch (stride) {
>>> +	case PUD_SIZE:
>>> +		__flush_tlb_range(vma, start, end, PUD_SIZE, false, 1);
>>> +		break;
>>
>> Just wondering - should not !__PAGETABLE_PMD_FOLDED and pud_sect_supported()
>> checks also be added here for this PUD_SIZE case ?
> 
> Yeah I guess so. TBH, it's never been entirely clear to me what the benefit is?
> Is it just to remove (a tiny amount of) dead code when we know we don't support
> blocks at the level? Or is there something more fundamental going on that I've
> missed?

There is a generic fallback for PUD_SIZE in include/asm-generic/pgtable-nopud.h when
it is not defined on arm64 platform and pud_sect_supported() might also get optimized
by the compiler.

static inline bool pud_sect_supported(void)
{
        return PAGE_SIZE == SZ_4K;
}

IIUC this just saves dead code from being compiled as you mentioned.

> 
> We seem to be quite inconsistent with the use of pud_sect_supported() in
> hugetlbpage.c.

PUD_SIZE switch cases in hugetlb_mask_last_page() and arch_make_huge_pte() ? Those
should be fixed.

> 
> Anyway, I'll add this in, I guess it's preferable to follow the established pattern.

Agreed.

> 
> Thanks,
> Ryan
> 
>>
>>> +	case CONT_PMD_SIZE:
>>> +	case PMD_SIZE:
>>> +		__flush_tlb_range(vma, start, end, PMD_SIZE, false, 2);
>>> +		break;
>>> +	case CONT_PTE_SIZE:
>>> +		__flush_tlb_range(vma, start, end, PAGE_SIZE, false, 3);
>>> +		break;
>>> +	default:
>>> +		__flush_tlb_range(vma, start, end, PAGE_SIZE, false, TLBI_TTL_UNKNOWN);
>>> +	}
>>>  }
>>>  
>>>  #endif /* __ASM_HUGETLB_H */
>