Re: [PATCHv4 10/14] mm: Drop fake head checks

[Kiryl Shutsemau <kas@kernel.org>]

On Wed, Jan 21, 2026 at 01:16:23PM -0500, Zi Yan wrote:
> On 21 Jan 2026, at 11:22, Kiryl Shutsemau wrote:
> 
> > With fake head pages eliminated in the previous commit, remove the
> > supporting infrastructure:
> >
> >   - page_fixed_fake_head(): no longer needed to detect fake heads;
> >   - page_is_fake_head(): no longer needed;
> >   - page_count_writable(): no longer needed for RCU protection;
> >   - RCU read_lock in page_ref_add_unless(): no longer needed;
> >
> > This substantially simplifies compound_head() and page_ref_add_unless(),
> > removing both branches and RCU overhead from these hot paths.
> >
> > Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
> > Reviewed-by: Muchun Song <muchun.song@linux.dev>
> > ---
> >  include/linux/page-flags.h | 93 ++------------------------------------
> >  include/linux/page_ref.h   |  8 +---
> >  2 files changed, 4 insertions(+), 97 deletions(-)
> >
> > diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> > index e16a4bc82856..660f9154a211 100644
> > --- a/include/linux/page-flags.h
> > +++ b/include/linux/page-flags.h
> > @@ -221,102 +221,15 @@ static __always_inline bool compound_info_has_mask(void)
> >  	return is_power_of_2(sizeof(struct page));
> >  }
> >
> > -#ifdef CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP
> >  DECLARE_STATIC_KEY_FALSE(hugetlb_optimize_vmemmap_key);
> >
> > -/*
> > - * Return the real head page struct iff the @page is a fake head page, otherwise
> > - * return the @page itself. See Documentation/mm/vmemmap_dedup.rst.
> > - */
> > -static __always_inline const struct page *page_fixed_fake_head(const struct page *page)
> > -{
> > -	if (!static_branch_unlikely(&hugetlb_optimize_vmemmap_key))
> > -		return page;
> > -
> > -	/* Fake heads only exists if compound_info_has_mask() is true */
> > -	if (!compound_info_has_mask())
> > -		return page;
> > -
> > -	/*
> > -	 * 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_info) 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.f)) {
> > -		/*
> > -		 * We can safely access the field of the @page[1] with PG_head
> > -		 * because the @page is a compound page composed with at least
> > -		 * two contiguous pages.
> > -		 */
> > -		unsigned long info = READ_ONCE(page[1].compound_info);
> > -
> > -		/* See set_compound_head() */
> > -		if (likely(info & 1)) {
> > -			unsigned long p = (unsigned long)page;
> > -
> > -			return (const struct page *)(p & info);
> > -		}
> > -	}
> > -	return page;
> > -}
> > -
> 
> <snip>
> 
> >  static __always_inline unsigned long _compound_head(const struct page *page)
> >  {
> >  	unsigned long info = READ_ONCE(page->compound_info);
> >
> >  	/* Bit 0 encodes PageTail() */
> >  	if (!(info & 1))
> > -		return (unsigned long)page_fixed_fake_head(page);
> > +		return (unsigned long)page;
> 
> Is this right? Assuming 64B struct page and 4KB page size, thus 64 struct pages
> in a page, the 64th struct page (0-indexed) is mapped to the head page and
> has !(info & 1). But _compound_head() should return page & info here.
> Am I missing something? Thanks.

The point of removing fake heads is the we don't have head aliases
anymore. 64-th struct page will be a tail page that. No special
treatment is required.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov