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From: Mike Kravetz <mike.kravetz@oracle.com>
To: Matthew Wilcox <willy@infradead.org>,
	Muchun Song <songmuchun@bytedance.com>
Cc: 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 16:56:17 -0700	[thread overview]
Message-ID: <63b0f4e1-5d3e-477a-ae8c-ed7e7f47ffd1@oracle.com> (raw)
In-Reply-To: <YP8mKV4wTp5sPIZg@casper.infradead.org>

On 7/26/21 2:16 PM, Matthew Wilcox wrote:
> On Wed, Jul 14, 2021 at 05:17:58PM +0800, Muchun Song wrote:
>> +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 ;-)

Matthew, any suggestions for benchmarks/workloads to measure the
increased overhead?  Suspect you have some ideas based on folio work.

My concern is that we are introducing overhead for code paths not
associated with this feature.  The next patch even tries to minimize
this overhead a bit if hugetlb_free_vmemmap_enabled is not set.
-- 
Mike Kravetz

  reply	other threads:[~2021-07-26 23:57 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
2021-07-26 23:56     ` Mike Kravetz [this message]
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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