Re: [PATCH RFC 3/6] mm/zsmalloc: use a proper page type

[Mike Rapoport <rppt@kernel.org>]

On Thu, May 23, 2024 at 04:55:44PM +0200, 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);

Won't 

BUILD_BUG_ON(PAGE_SIZE > SZ_64K)

be clearer?
 
> -- 
> Cheers,
> 
> David / dhildenb
> 
> 

-- 
Sincerely yours,
Mike.