linux-kernel - Re: [PATCH v2 14/14] arm64/mm: Add ptep_get_and_clear

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Date:   Tue, 28 Nov 2023 20:32:54 +1300
From:   Barry Song <21cnbao@...il.com>
To:     ryan.roberts@....com
Cc:     akpm@...ux-foundation.org, andreyknvl@...il.com,
        anshuman.khandual@....com, ardb@...nel.org,
        catalin.marinas@....com, david@...hat.com, dvyukov@...gle.com,
        glider@...gle.com, james.morse@....com, jhubbard@...dia.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, mark.rutland@....com, maz@...nel.org,
        oliver.upton@...ux.dev, ryabinin.a.a@...il.com,
        suzuki.poulose@....com, vincenzo.frascino@....com,
        wangkefeng.wang@...wei.com, will@...nel.org, willy@...radead.org,
        yuzenghui@...wei.com, yuzhao@...gle.com, ziy@...dia.com
Subject: Re: [PATCH v2 14/14] arm64/mm: Add ptep_get_and_clear_full() to optimize process teardown

> +#define __HAVE_ARCH_PTEP_GET_AND_CLEAR_FULL
> +static inline pte_t ptep_get_and_clear_full(struct mm_struct *mm,
> +				unsigned long addr, pte_t *ptep, int full)
> +{
> +	pte_t orig_pte = __ptep_get(ptep);
> +
> +	if (!pte_valid_cont(orig_pte) || !full) {
> +		contpte_try_unfold(mm, addr, ptep, orig_pte);
> +		return __ptep_get_and_clear(mm, addr, ptep);
> +	} else
> +		return contpte_ptep_get_and_clear_full(mm, addr, ptep);
> +}
> +

Hi Ryan,

I feel quite hard to understand the code. when !pte_valid_cont(orig_pte),
we will call contpte_try_unfold(mm, addr, ptep, orig_pte);

but in contpte_try_unfold(), we call unfold only if pte_valid_cont()
is true:
static inline void contpte_try_unfold(struct mm_struct *mm, unsigned long addr,
                                        pte_t *ptep, pte_t pte) 
{
        if (contpte_is_enabled(mm) && pte_valid_cont(pte))
                __contpte_try_unfold(mm, addr, ptep, pte);
}

so do you mean the below?

if (!pte_valid_cont(orig_pte))
	return __ptep_get_and_clear(mm, addr, ptep);

if (!full) {
	contpte_try_unfold(mm, addr, ptep, orig_pte);
	return __ptep_get_and_clear(mm, addr, ptep);	
} else {
	return contpte_ptep_get_and_clear_full(mm, addr, ptep);
}

Thanks
Barry