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Date:   Tue, 9 Aug 2022 14:33:15 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     alexlzhu@fb.com
Cc:     linux-mm@kvack.org, kernel-team@fb.com,
        linux-kernel@vger.kernel.org, akpm@linux-foundation.org
Subject: Re: [PATCH v2] mm: add thp_utilization metrics to debugfs
Message-ID: <YvJiG3oviaRgPSVC@casper.infradead.org>
References: <20220809014950.3616464-1-alexlzhu@fb.com>
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On Mon, Aug 08, 2022 at 06:49:50PM -0700, alexlzhu@fb.com wrote:
> +int thp_number_utilized_pages(struct page *page)
> +{
> +	unsigned long page_index, page_offset, value;
> +	int thp_nr_utilized_pages = HPAGE_PMD_NR;
> +	int step_size = sizeof(unsigned long);
> +	bool is_all_zeroes;
> +	void *kaddr;
> +
> +	if (!page || !is_anon_transparent_hugepage(page))
> +		return -1;
> +
> +	kaddr = kmap_local_page(compound_head(page));
> +	for (page_index = 0; page_index < HPAGE_PMD_NR; page_index++) {
> +		is_all_zeroes = true;
> +		for (page_offset = 0; page_offset < PAGE_SIZE; page_offset += step_size) {
> +			value = *(unsigned long *)(kaddr + page_index * PAGE_SIZE + page_offset);
> +			if (value != 0) {
> +				is_all_zeroes = false;
> +				break;
> +			}
> +		}
> +		if (is_all_zeroes)
> +			thp_nr_utilized_pages--;
> +	}
> +	kunmap_local(kaddr);

You haven't tested this on 32-bit maachines with HIGHMEM, have you?
kmap_local_page() only maps PAGE_SIZE bytes, so you need to map it and
unmap each page.  Also, why are you calling compound_head()?  Surely
you're already assuming that page is a head page here?

(this is why I introduced the struct folio type, so you know whether
a pointer is already to a head page and you don't need to call
compound_head() again).