linux-kernel - Re: [PATCH v2 1/4] mm: pagewalk: Fix walk for hugepage tables

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Message-ID: <1fdb0abe-b4b5-937c-0d9b-859a5cbb5726@arm.com>
Date:   Mon, 19 Apr 2021 15:07:47 +0100
From:   Steven Price <steven.price@....com>
To:     Christophe Leroy <christophe.leroy@...roup.eu>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        akpm@...ux-foundation.org, dja@...ens.net
Cc:     Oliver O'Halloran <oohall@...il.com>, linux-arch@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH v2 1/4] mm: pagewalk: Fix walk for hugepage tables

On 19/04/2021 11:47, Christophe Leroy wrote:
> Pagewalk ignores hugepd entries and walk down the tables
> as if it was traditionnal entries, leading to crazy result.
> 
> Add walk_hugepd_range() and use it to walk hugepage tables.
> 
> Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>

Looks correct to me, sadly I don't have a suitable system to test it.

Reviewed-by: Steven Price <steven.price@....com>

> ---
> v2:
> - Add a guard for NULL ops->pte_entry
> - Take mm->page_table_lock when walking hugepage table, as suggested by follow_huge_pd()
> ---
>   mm/pagewalk.c | 58 ++++++++++++++++++++++++++++++++++++++++++++++-----
>   1 file changed, 53 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/pagewalk.c b/mm/pagewalk.c
> index e81640d9f177..9b3db11a4d1d 100644
> --- a/mm/pagewalk.c
> +++ b/mm/pagewalk.c
> @@ -58,6 +58,45 @@ static int walk_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
>   	return err;
>   }
>   
> +#ifdef CONFIG_ARCH_HAS_HUGEPD
> +static int walk_hugepd_range(hugepd_t *phpd, unsigned long addr,
> +			     unsigned long end, struct mm_walk *walk, int pdshift)
> +{
> +	int err = 0;
> +	const struct mm_walk_ops *ops = walk->ops;
> +	int shift = hugepd_shift(*phpd);
> +	int page_size = 1 << shift;
> +
> +	if (!ops->pte_entry)
> +		return 0;
> +
> +	if (addr & (page_size - 1))
> +		return 0;
> +
> +	for (;;) {
> +		pte_t *pte;
> +
> +		spin_lock(&walk->mm->page_table_lock);
> +		pte = hugepte_offset(*phpd, addr, pdshift);
> +		err = ops->pte_entry(pte, addr, addr + page_size, walk);
> +		spin_unlock(&walk->mm->page_table_lock);
> +
> +		if (err)
> +			break;
> +		if (addr >= end - page_size)
> +			break;
> +		addr += page_size;
> +	}
> +	return err;
> +}
> +#else
> +static int walk_hugepd_range(hugepd_t *phpd, unsigned long addr,
> +			     unsigned long end, struct mm_walk *walk, int pdshift)
> +{
> +	return 0;
> +}
> +#endif
> +
>   static int walk_pmd_range(pud_t *pud, unsigned long addr, unsigned long end,
>   			  struct mm_walk *walk)
>   {
> @@ -108,7 +147,10 @@ static int walk_pmd_range(pud_t *pud, unsigned long addr, unsigned long end,
>   				goto again;
>   		}
>   
> -		err = walk_pte_range(pmd, addr, next, walk);
> +		if (is_hugepd(__hugepd(pmd_val(*pmd))))
> +			err = walk_hugepd_range((hugepd_t *)pmd, addr, next, walk, PMD_SHIFT);
> +		else
> +			err = walk_pte_range(pmd, addr, next, walk);
>   		if (err)
>   			break;
>   	} while (pmd++, addr = next, addr != end);
> @@ -157,7 +199,10 @@ static int walk_pud_range(p4d_t *p4d, unsigned long addr, unsigned long end,
>   		if (pud_none(*pud))
>   			goto again;
>   
> -		err = walk_pmd_range(pud, addr, next, walk);
> +		if (is_hugepd(__hugepd(pud_val(*pud))))
> +			err = walk_hugepd_range((hugepd_t *)pud, addr, next, walk, PUD_SHIFT);
> +		else
> +			err = walk_pmd_range(pud, addr, next, walk);
>   		if (err)
>   			break;
>   	} while (pud++, addr = next, addr != end);
> @@ -189,7 +234,9 @@ static int walk_p4d_range(pgd_t *pgd, unsigned long addr, unsigned long end,
>   			if (err)
>   				break;
>   		}
> -		if (ops->pud_entry || ops->pmd_entry || ops->pte_entry)
> +		if (is_hugepd(__hugepd(p4d_val(*p4d))))
> +			err = walk_hugepd_range((hugepd_t *)p4d, addr, next, walk, P4D_SHIFT);
> +		else if (ops->pud_entry || ops->pmd_entry || ops->pte_entry)
>   			err = walk_pud_range(p4d, addr, next, walk);
>   		if (err)
>   			break;
> @@ -224,8 +271,9 @@ static int walk_pgd_range(unsigned long addr, unsigned long end,
>   			if (err)
>   				break;
>   		}
> -		if (ops->p4d_entry || ops->pud_entry || ops->pmd_entry ||
> -		    ops->pte_entry)
> +		if (is_hugepd(__hugepd(pgd_val(*pgd))))
> +			err = walk_hugepd_range((hugepd_t *)pgd, addr, next, walk, PGDIR_SHIFT);
> +		else if (ops->p4d_entry || ops->pud_entry || ops->pmd_entry || ops->pte_entry)
>   			err = walk_p4d_range(pgd, addr, next, walk);
>   		if (err)
>   			break;
>