linux-kernel - Re: [PATCH v2 1/3] mm/page_table_check: Do WARN_ON instead of BUG

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Date:   Sun, 23 Jul 2023 02:56:51 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Pasha Tatashin <pasha.tatashin@...een.com>
Cc:     akpm@...ux-foundation.org, corbet@....net, linux-mm@...ck.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        rick.p.edgecombe@...el.com
Subject: Re: [PATCH v2 1/3] mm/page_table_check: Do WARN_ON instead of BUG_ON

On Sat, Jul 22, 2023 at 11:15:06PM +0000, Pasha Tatashin wrote:
>  static struct page_table_check *get_page_table_check(struct page_ext *page_ext)
>  {
> -	BUG_ON(!page_ext);
> +	PAGE_TABLE_CHECK_WARN(!page_ext);
> +
>  	return (void *)(page_ext) + page_table_check_ops.offset;
>  }

[...]

> @@ -137,15 +144,15 @@ void __page_table_check_zero(struct page *page, unsigned int order)
>  	struct page_ext *page_ext;
>  	unsigned long i;
>  
> -	BUG_ON(PageSlab(page));
> +	PAGE_TABLE_CHECK_WARN(PageSlab(page));
>  
>  	page_ext = page_ext_get(page);
> -	BUG_ON(!page_ext);
> +	PAGE_TABLE_CHECK_WARN(!page_ext);
>  	for (i = 0; i < (1ul << order); i++) {
>  		struct page_table_check *ptc = get_page_table_check(page_ext);

Seems like we're going to warn about !page_ext twice?  Or more than
twice -- once per tail page?

But then we'll crash because page_ext was NULL and offset was small?