linux-kernel - Re: [PATCH 2/4] mm: page_table

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Message-ID: <fdef74f3-ada8-40c7-afea-9a0105f5c05f@redhat.com>
Date: Thu, 20 Feb 2025 12:05:23 +0100
From: David Hildenbrand <david@...hat.com>
To: Luiz Capitulino <luizcap@...hat.com>, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, yuzhao@...gle.com, pasha.tatashin@...een.com
Cc: akpm@...ux-foundation.org, hannes@...xchg.org, muchun.song@...ux.dev
Subject: Re: [PATCH 2/4] mm: page_table_check: use new iteration API

On 19.02.25 03:17, Luiz Capitulino wrote:
> The page_ext_next() function assumes that page extension objects for a
> page order allocation always reside in the same memory section, which
> may not be true and could lead to crashes. Use the new page_ext
> iteration API instead.
> 
> Fixes: cf54f310d0d3 ("mm/hugetlb: use __GFP_COMP for gigantic folios")
> Signed-off-by: Luiz Capitulino <luizcap@...hat.com>
> ---
>   mm/page_table_check.c | 39 ++++++++++++---------------------------
>   1 file changed, 12 insertions(+), 27 deletions(-)
> 
> diff --git a/mm/page_table_check.c b/mm/page_table_check.c
> index 509c6ef8de400..b52e04d31c809 100644
> --- a/mm/page_table_check.c
> +++ b/mm/page_table_check.c
> @@ -62,24 +62,20 @@ static struct page_table_check *get_page_table_check(struct page_ext *page_ext)
>    */
>   static void page_table_check_clear(unsigned long pfn, unsigned long pgcnt)
>   {
> +	struct page_ext_iter iter;
>   	struct page_ext *page_ext;
>   	struct page *page;
> -	unsigned long i;
>   	bool anon;
>   
>   	if (!pfn_valid(pfn))
>   		return;
>   
>   	page = pfn_to_page(pfn);
> -	page_ext = page_ext_get(page);
> -
> -	if (!page_ext)
> -		return;
> -
>   	BUG_ON(PageSlab(page));
>   	anon = PageAnon(page);
>   
> -	for (i = 0; i < pgcnt; i++) {
> +	rcu_read_lock();
> +	for_each_page_ext(page, pgcnt, page_ext, iter) {
>   		struct page_table_check *ptc = get_page_table_check(page_ext);
>   
>   		if (anon) {
> @@ -89,9 +85,8 @@ static void page_table_check_clear(unsigned long pfn, unsigned long pgcnt)
>   			BUG_ON(atomic_read(&ptc->anon_map_count));
>   			BUG_ON(atomic_dec_return(&ptc->file_map_count) < 0);
>   		}
> -		page_ext = page_ext_next(page_ext);
>   	}
> -	page_ext_put(page_ext);
> +	rcu_read_unlock();
>   }

[...]

>   
>   /*
> @@ -140,24 +130,19 @@ static void page_table_check_set(unsigned long pfn, unsigned long pgcnt,
>    */
>   void __page_table_check_zero(struct page *page, unsigned int order)
>   {
> +	struct page_ext_iter iter;
>   	struct page_ext *page_ext;
> -	unsigned long i;
>   
>   	BUG_ON(PageSlab(page));
>   
> -	page_ext = page_ext_get(page);
> -
> -	if (!page_ext)
> -		return;
> -
> -	for (i = 0; i < (1ul << order); i++) {
> +	rcu_read_lock();
> +	for_each_page_ext_order(page, order, page_ext, iter) {

I would avoid introducing for_each_page_ext_order() and just pass "1 << 
order" as the page count.

>   		struct page_table_check *ptc = get_page_table_check(page_ext);
>   
>   		BUG_ON(atomic_read(&ptc->anon_map_count));
>   		BUG_ON(atomic_read(&ptc->file_map_count));
> -		page_ext = page_ext_next(page_ext);
>   	}
> -	page_ext_put(page_ext);
> +	rcu_read_unlock();
>   }
>   
>   void __page_table_check_pte_clear(struct mm_struct *mm, pte_t pte)

Apart from that, this looks very nice to me

Acked-by: David Hildenbrand <david@...hat.com>

-- 
Cheers,

David / dhildenb