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Date: Thu, 23 May 2024 22:38:41 +0200
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Subject: Re: [PATCH RFC 3/6] mm/zsmalloc: use a proper page type
From: David Hildenbrand <david@redhat.com>
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>
 <2563ea6b-7c65-46a0-adf3-552f2e863c94@redhat.com>
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In-Reply-To: <2563ea6b-7c65-46a0-adf3-552f2e863c94@redhat.com>
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On 23.05.24 16:55, David Hildenbrand wrote:
> 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);

.. and I'll have to resolve the "select ZSMALLOC" situation. :)

-- 
Cheers,

David / dhildenb