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From: Matthew Wilcox <willy@infradead.org>
To: Muchun Song <songmuchun@bytedance.com>
Cc: mike.kravetz@oracle.com, akpm@linux-foundation.org,
	osalvador@suse.de, mhocko@suse.com, song.bao.hua@hisilicon.com,
	david@redhat.com, chenhuang5@huawei.com, bodeddub@amazon.com,
	corbet@lwn.net, duanxiongchun@bytedance.com,
	fam.zheng@bytedance.com, linux-doc@vger.kernel.org,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	zhengqi.arch@bytedance.com
Subject: Re: [PATCH 3/5] mm: hugetlb: free the 2nd vmemmap page associated with each HugeTLB page
Date: Mon, 26 Jul 2021 22:16:25 +0100	[thread overview]
Message-ID: <YP8mKV4wTp5sPIZg@casper.infradead.org> (raw)
In-Reply-To: <20210714091800.42645-4-songmuchun@bytedance.com>

On Wed, Jul 14, 2021 at 05:17:58PM +0800, Muchun Song wrote:
> +#ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
> +extern bool hugetlb_free_vmemmap_enabled;
> +
> +/*
> + * If the feature of freeing some vmemmap pages associated with each HugeTLB
> + * page is enabled, the head vmemmap page frame is reused and all of the tail
> + * vmemmap addresses map to the head vmemmap page frame (furture details can
> + * refer to the figure at the head of the mm/hugetlb_vmemmap.c).  In other
> + * word, there are more than one page struct with PG_head associated with each
> + * HugeTLB page.  We __know__ that there is only one head page struct, the tail
> + * page structs with PG_head are fake head page structs.  We need an approach
> + * to distinguish between those two different types of page structs so that
> + * compound_head() can return the real head page struct when the parameter is
> + * the tail page struct but with PG_head. This is what page_head_if_fake()
> + * does.
> + *
> + * The page_head_if_fake() returns the real head page struct iff the @page may
> + * be fake, otherwise, returns NULL if the @page cannot be a fake page struct.
> + * The following figure describes how to distinguish between real and fake head
> + * page struct.
> + *
> + *	if (test_bit(PG_head, &page->flags)) {
> + *		unsigned long head = READ_ONCE(page[1].compound_head);
> + *
> + *		if (head & 1) {
> + *			if (head == (unsigned long)page + 1)
> + *				==> head page struct
> + *			else
> + *				==> tail page struct
> + *		} else
> + *			==> head page struct
> + *	} else
> + *		==> cannot be fake head page struct

I'm not sure we need the pseudocode when the code is right there ...

> + * We can safely access the field of the @page[1] with PG_head because it means
> + * that the @page is a compound page composed with at least two contiguous
> + * pages.
> + */
> +static __always_inline struct page *page_head_if_fake(const struct page *page)
> +{
> +	if (!hugetlb_free_vmemmap_enabled)
> +		return NULL;
> +
> +	/*
> +	 * Only addresses aligned with PAGE_SIZE of struct page may be fake head
> +	 * struct page. The alignment check aims to avoid access the fields (
> +	 * e.g. compound_head) of the @page[1]. It can avoid touch a (possibly)
> +	 * cold cacheline in some cases.
> +	 */
> +	if (IS_ALIGNED((unsigned long)page, PAGE_SIZE) &&
> +	    test_bit(PG_head, &page->flags)) {
> +		unsigned long head = READ_ONCE(page[1].compound_head);
> +
> +		if (likely(head & 1))
> +			return (struct page *)(head - 1);
> +	}
> +
> +	return NULL;
> +}

Why return 'NULL' instead of 'page'?

This is going to significantly increase the cost of calling
compound_page() (by whichever spelling it has).  That will make
the folio patchset more compelling ;-)


  reply	other threads:[~2021-07-26 21:18 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-14  9:17 [PATCH 0/5] Free " Muchun Song
2021-07-14  9:17 ` [PATCH 1/5] mm: introduce PAGEFLAGS_MASK to replace ((1UL << NR_PAGEFLAGS) - 1) Muchun Song
2021-07-26 21:04   ` Mike Kravetz
2021-07-27  6:27     ` Muchun Song
2021-07-29  6:00       ` Muchun Song
2021-07-14  9:17 ` [PATCH 2/5] mm: introduce save_page_flags to cooperate with show_page_flags Muchun Song
2021-07-26 23:18   ` Mike Kravetz
2021-07-27  7:06     ` Muchun Song
2021-07-14  9:17 ` [PATCH 3/5] mm: hugetlb: free the 2nd vmemmap page associated with each HugeTLB page Muchun Song
2021-07-26 21:16   ` Matthew Wilcox [this message]
2021-07-26 23:56     ` Mike Kravetz
2021-07-27  7:15     ` Muchun Song
2021-07-14  9:17 ` [PATCH 4/5] mm: hugetlb: replace hugetlb_free_vmemmap_enabled with a static_key Muchun Song
2021-07-14  9:18 ` [PATCH 5/5] mm: sparsemem: use page table lock to protect kernel pmd operations Muchun Song

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