Re: [PATCH v1 1/5] mm: pagewalk: Fix walk for hugepage tables

From: Daniel Axtens <dja@axtens.net>
To: Christophe Leroy <christophe.leroy@csgroup.eu>,
	Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Paul Mackerras <paulus@samba.org>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Steven Price <steven.price@arm.com>,
	akpm@linux-foundation.org
Cc: linux-arch@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
	linux-kernel@vger.kernel.org, linux-s390@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	linux-riscv@lists.infradead.org, x86@kernel.org,
	linux-mm@kvack.org
Subject: Re: [PATCH v1 1/5] mm: pagewalk: Fix walk for hugepage tables
Date: Fri, 16 Apr 2021 08:43:38 +1000	[thread overview]
Message-ID: <877dl3184l.fsf@dja-thinkpad.axtens.net> (raw)
In-Reply-To: <733408f48b1ed191f53518123ee6fc6d42287cc6.1618506910.git.christophe.leroy@csgroup.eu>

Hi Christophe,

> 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@csgroup.eu>
> ---
>  mm/pagewalk.c | 54 +++++++++++++++++++++++++++++++++++++++++++++------
>  1 file changed, 48 insertions(+), 6 deletions(-)
>
> diff --git a/mm/pagewalk.c b/mm/pagewalk.c
> index e81640d9f177..410a9d8f7572 100644
> --- a/mm/pagewalk.c
> +++ b/mm/pagewalk.c
> @@ -58,6 +58,32 @@ static int walk_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
>  	return err;
>  }
>  
> +static int walk_hugepd_range(hugepd_t *phpd, unsigned long addr,
> +			     unsigned long end, struct mm_walk *walk, int pdshift)
> +{
> +	int err = 0;
> +#ifdef CONFIG_ARCH_HAS_HUGEPD
> +	const struct mm_walk_ops *ops = walk->ops;
> +	int shift = hugepd_shift(*phpd);
> +	int page_size = 1 << shift;
> +
> +	if (addr & (page_size - 1))
> +		return 0;
> +
> +	for (;;) {
> +		pte_t *pte = hugepte_offset(*phpd, addr, pdshift);
> +
> +		err = ops->pte_entry(pte, addr, addr + page_size, walk);
> +		if (err)
> +			break;
> +		if (addr >= end - page_size)
> +			break;
> +		addr += page_size;
> +	}

Initially I thought this was a somewhat unintuitive way to structure
this loop, but I see it parallels the structure of walk_pte_range_inner,
so I think the consistency is worth it.

I notice the pte walking code potentially takes some locks: does this
code need to do that?

arch/powerpc/mm/hugetlbpage.c says that hugepds are protected by the
mm->page_table_lock, but I don't think we're taking it in this code.

> +#endif
> +	return err;
> +}
> +
>  static int walk_pmd_range(pud_t *pud, unsigned long addr, unsigned long end,
>  			  struct mm_walk *walk)
>  {
> @@ -108,7 +134,10 @@ 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 +186,10 @@ 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);

I'm a bit worried you might end up calling into walk_hugepd_range with
ops->pte_entry == NULL, and then jumping to 0.

static int walk_pud_range(p4d_t *p4d, unsigned long addr, unsigned long end,
			  struct mm_walk *walk)
{
...
        pud = pud_offset(p4d, addr);
	do {
                ...
                if ((!walk->vma && (pud_leaf(*pud) || !pud_present(*pud))) ||
		    walk->action == ACTION_CONTINUE ||
		    !(ops->pmd_entry || ops->pte_entry)) <<< THIS CHECK
			continue;
                ...
		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);

walk_pud_range will proceed if there is _either_ an ops->pmd_entry _or_
an ops->pte_entry, but walk_hugepd_range will call ops->pte_entry
unconditionally.

The same issue applies to walk_{p4d,pgd}_range...

Kind regards,
Daniel