Received-SPF: pass (google.com: domain of linux-kernel+bounces-187674-linux.lists.archive=gmail.com@vger.kernel.org designates 147.75.80.249 as permitted sender) client-ip=147.75.80.249;
Message-ID: <2563ea6b-7c65-46a0-adf3-552f2e863c94@redhat.com>
Date: Thu, 23 May 2024 16:55:44 +0200
Precedence: bulk
MIME-Version: 1.0
User-Agent: Mozilla Thunderbird
Subject: Re: [PATCH RFC 3/6] mm/zsmalloc: use a proper page type
To: linux-kernel@vger.kernel.org
Cc: linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
 "Matthew Wilcox (Oracle)" <willy@infradead.org>,
 Mike Rapoport <rppt@kernel.org>, Minchan Kim <minchan@kernel.org>,
 Sergey Senozhatsky <senozhatsky@chromium.org>,
 Hyeonggon Yoo <42.hyeyoo@gmail.com>
References: <20240522210341.1030552-1-david@redhat.com>
 <20240522210341.1030552-4-david@redhat.com>
From: David Hildenbrand <david@redhat.com>
Content-Language: en-US
Autocrypt: addr=david@redhat.com; keydata=
 xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ
 dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL
 QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp
 XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK
 Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9
 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt
 WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc
 UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv
 jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb
 B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk
 ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZgEEwEIAEICGwMGCwkIBwMCBhUIAgkKCwQW
 AgMBAh4BAheAAhkBFiEEG9nKrXNcTDpGDfzKTd4Q9wD/g1oFAl8Ox4kFCRKpKXgACgkQTd4Q
 9wD/g1oHcA//a6Tj7SBNjFNM1iNhWUo1lxAja0lpSodSnB2g4FCZ4R61SBR4l/psBL73xktp
 rDHrx4aSpwkRP6Epu6mLvhlfjmkRG4OynJ5HG1gfv7RJJfnUdUM1z5kdS8JBrOhMJS2c/gPf
 wv1TGRq2XdMPnfY2o0CxRqpcLkx4vBODvJGl2mQyJF/gPepdDfcT8/PY9BJ7FL6Hrq1gnAo4
 3Iv9qV0JiT2wmZciNyYQhmA1V6dyTRiQ4YAc31zOo2IM+xisPzeSHgw3ONY/XhYvfZ9r7W1l
 pNQdc2G+o4Di9NPFHQQhDw3YTRR1opJaTlRDzxYxzU6ZnUUBghxt9cwUWTpfCktkMZiPSDGd
 KgQBjnweV2jw9UOTxjb4LXqDjmSNkjDdQUOU69jGMUXgihvo4zhYcMX8F5gWdRtMR7DzW/YE
 BgVcyxNkMIXoY1aYj6npHYiNQesQlqjU6azjbH70/SXKM5tNRplgW8TNprMDuntdvV9wNkFs
 9TyM02V5aWxFfI42+aivc4KEw69SE9KXwC7FSf5wXzuTot97N9Phj/Z3+jx443jo2NR34XgF
 89cct7wJMjOF7bBefo0fPPZQuIma0Zym71cP61OP/i11ahNye6HGKfxGCOcs5wW9kRQEk8P9
 M/k2wt3mt/fCQnuP/mWutNPt95w9wSsUyATLmtNrwccz63XOwU0EVcufkQEQAOfX3n0g0fZz
 Bgm/S2zF/kxQKCEKP8ID+Vz8sy2GpDvveBq4H2Y34XWsT1zLJdvqPI4af4ZSMxuerWjXbVWb
 T6d4odQIG0fKx4F8NccDqbgHeZRNajXeeJ3R7gAzvWvQNLz4piHrO/B4tf8svmRBL0ZB5P5A
 2uhdwLU3NZuK22zpNn4is87BPWF8HhY0L5fafgDMOqnf4guJVJPYNPhUFzXUbPqOKOkL8ojk
 CXxkOFHAbjstSK5Ca3fKquY3rdX3DNo+EL7FvAiw1mUtS+5GeYE+RMnDCsVFm/C7kY8c2d0G
 NWkB9pJM5+mnIoFNxy7YBcldYATVeOHoY4LyaUWNnAvFYWp08dHWfZo9WCiJMuTfgtH9tc75
 7QanMVdPt6fDK8UUXIBLQ2TWr/sQKE9xtFuEmoQGlE1l6bGaDnnMLcYu+Asp3kDT0w4zYGsx
 5r6XQVRH4+5N6eHZiaeYtFOujp5n+pjBaQK7wUUjDilPQ5QMzIuCL4YjVoylWiBNknvQWBXS
 lQCWmavOT9sttGQXdPCC5ynI+1ymZC1ORZKANLnRAb0NH/UCzcsstw2TAkFnMEbo9Zu9w7Kv
 AxBQXWeXhJI9XQssfrf4Gusdqx8nPEpfOqCtbbwJMATbHyqLt7/oz/5deGuwxgb65pWIzufa
 N7eop7uh+6bezi+rugUI+w6DABEBAAHCwXwEGAEIACYCGwwWIQQb2cqtc1xMOkYN/MpN3hD3
 AP+DWgUCXw7HsgUJEqkpoQAKCRBN3hD3AP+DWrrpD/4qS3dyVRxDcDHIlmguXjC1Q5tZTwNB
 boaBTPHSy/Nksu0eY7x6HfQJ3xajVH32Ms6t1trDQmPx2iP5+7iDsb7OKAb5eOS8h+BEBDeq
 3ecsQDv0fFJOA9ag5O3LLNk+3x3q7e0uo06XMaY7UHS341ozXUUI7wC7iKfoUTv03iO9El5f
 XpNMx/YrIMduZ2+nd9Di7o5+KIwlb2mAB9sTNHdMrXesX8eBL6T9b+MZJk+mZuPxKNVfEQMQ
 a5SxUEADIPQTPNvBewdeI80yeOCrN+Zzwy/Mrx9EPeu59Y5vSJOx/z6OUImD/GhX7Xvkt3kq
 Er5KTrJz3++B6SH9pum9PuoE/k+nntJkNMmQpR4MCBaV/J9gIOPGodDKnjdng+mXliF3Ptu6
 3oxc2RCyGzTlxyMwuc2U5Q7KtUNTdDe8T0uE+9b8BLMVQDDfJjqY0VVqSUwImzTDLX9S4g/8
 kC4HRcclk8hpyhY2jKGluZO0awwTIMgVEzmTyBphDg/Gx7dZU1Xf8HFuE+UZ5UDHDTnwgv7E
 th6RC9+WrhDNspZ9fJjKWRbveQgUFCpe1sa77LAw+XFrKmBHXp9ZVIe90RMe2tRL06BGiRZr
 jPrnvUsUUsjRoRNJjKKA/REq+sAnhkNPPZ/NNMjaZ5b8Tovi8C0tmxiCHaQYqj7G2rgnT0kt
 WNyWQQ==
Organization: Red Hat
In-Reply-To: <20240522210341.1030552-4-david@redhat.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit

On 22.05.24 23:03, David Hildenbrand wrote:
> Let's clean it up: use a proper page type and store our data (offset
> into a page) in the lower 16 bit as documented.
> 
> We'll have to restrict ourselves to <= 64KB base page size (so the offset
> fits into 16 bit), which sounds reasonable. Unfortunately, we don't have
> any space to store it elsewhere for now.
> 
> Based on this, we should do a proper "struct zsdesc" conversion, as
> proposed in [1].
> 
> This removes the last _mapcount/page_type offender.
> 
> [1] https://lore.kernel.org/all/20231130101242.2590384-1-42.hyeyoo@gmail.com/
> 
> Cc: Hyeonggon Yoo <42.hyeyoo@gmail.com>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
>   include/linux/page-flags.h |  3 +++
>   mm/Kconfig                 |  1 +
>   mm/zsmalloc.c              | 23 +++++++++++++++++++----
>   3 files changed, 23 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index ed9ac4b5233d..ccaf16656de9 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -959,6 +959,7 @@ PAGEFLAG_FALSE(HasHWPoisoned, has_hwpoisoned)
>   #define PG_guard	0x00080000
>   #define PG_hugetlb	0x00100800
>   #define PG_slab		0x00200000
> +#define PG_zsmalloc	0x00400000
>   
>   #define PageType(page, flag)						\
>   	((page->page_type & (PAGE_TYPE_BASE | flag)) == PAGE_TYPE_BASE)
> @@ -1073,6 +1074,8 @@ FOLIO_TYPE_OPS(hugetlb, hugetlb)
>   FOLIO_TEST_FLAG_FALSE(hugetlb)
>   #endif
>   
> +PAGE_TYPE_OPS(Zsmalloc, zsmalloc, zsmalloc)
> +
>   /**
>    * PageHuge - Determine if the page belongs to hugetlbfs
>    * @page: The page to test.
> diff --git a/mm/Kconfig b/mm/Kconfig
> index b4cb45255a54..0371d79b1b75 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -190,6 +190,7 @@ config ZSMALLOC
>   	tristate
>   	prompt "N:1 compression allocator (zsmalloc)" if ZSWAP
>   	depends on MMU
> +	depends on PAGE_SIZE_LESS_THAN_256KB # we want <= 64KB
>   	help
>   	  zsmalloc is a slab-based memory allocator designed to store
>   	  pages of various compression levels efficiently. It achieves
> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> index b42d3545ca85..6f0032e06242 100644
> --- a/mm/zsmalloc.c
> +++ b/mm/zsmalloc.c
> @@ -20,7 +20,8 @@
>    *	page->index: links together all component pages of a zspage
>    *		For the huge page, this is always 0, so we use this field
>    *		to store handle.
> - *	page->page_type: first object offset in a subpage of zspage
> + *	page->page_type: PG_zsmalloc, lower 16 bit locate the first object
> + *		offset in a subpage of a zspage
>    *
>    * Usage of struct page flags:
>    *	PG_private: identifies the first component page
> @@ -450,14 +451,22 @@ static inline struct page *get_first_page(struct zspage *zspage)
>   	return first_page;
>   }
>   
> +static inline void reset_first_obj_offset(struct page *page)
> +{
> +	page->page_type |= 0xffff;
> +}
> +
>   static inline unsigned int get_first_obj_offset(struct page *page)
>   {
> -	return page->page_type;
> +	return page->page_type & 0xffff;
>   }
>   
>   static inline void set_first_obj_offset(struct page *page, unsigned int offset)
>   {
> -	page->page_type = offset;
> +	BUILD_BUG_ON(PAGE_SIZE & ~0xffff);

Buildbot is correctly complaining with PAGE_SIZE=64KiB.

We must check BUILD_BUG_ON((PAGE_SIZE -1) & ~0xffff);

-- 
Cheers,

David / dhildenb