Re: [PATCHv4 05/14] mm: Rework compound_head() for power-of-2 sizeof(struct page)

[Zi Yan]

On 21 Jan 2026, at 11:22, Kiryl Shutsemau wrote:

> For tail pages, the kernel uses the 'compound_info' field to get to the
> head page. The bit 0 of the field indicates whether the page is a
> tail page, and if set, the remaining bits represent a pointer to the
> head page.
>
> For cases when size of struct page is power-of-2, change the encoding of
> compound_info to store a mask that can be applied to the virtual address
> of the tail page in order to access the head page. It is possible
> because struct page of the head page is naturally aligned with regards
> to order of the page.
>
> The significant impact of this modification is that all tail pages of
> the same order will now have identical 'compound_info', regardless of
> the compound page they are associated with. This paves the way for
> eliminating fake heads.
>
> The HugeTLB Vmemmap Optimization (HVO) creates fake heads and it is only
> applied when the sizeof(struct page) is power-of-2. Having identical
> tail pages allows the same page to be mapped into the vmemmap of all
> pages, maintaining memory savings without fake heads.
>
> If sizeof(struct page) is not power-of-2, there is no functional
> changes.
>
> Limit mask usage to SPARSEMEM_VMEMMAP where it makes a difference
> because HVO. The approach with mask would work for any memory model,
> but it requires validating that struct pages are naturally aligned for
> all orders up to the MAX_FOLIO order, which can be tricky.
>
> Signed-off-by: Kiryl Shutsemau <kas@xxxxxxxxxx>
> Reviewed-by: Muchun Song <muchun.song@xxxxxxxxx>
> ---
>  include/linux/page-flags.h | 81 ++++++++++++++++++++++++++++++++++----
>  mm/util.c                  | 16 ++++++--
>  2 files changed, 85 insertions(+), 12 deletions(-)
>

<snip>

> diff --git a/mm/util.c b/mm/util.c
> index cbf93cf3223a..f01a9655067f 100644
> --- a/mm/util.c
> +++ b/mm/util.c
> @@ -1234,7 +1234,7 @@ static void set_ps_flags(struct page_snapshot *ps, const struct folio *folio,
>   */
>  void snapshot_page(struct page_snapshot *ps, const struct page *page)
>  {
> -	unsigned long head, nr_pages = 1;
> +	unsigned long info, nr_pages = 1;
>  	struct folio *foliop;
>  	int loops = 5;
>
> @@ -1244,8 +1244,8 @@ void snapshot_page(struct page_snapshot *ps, const struct page *page)
>  again:
>  	memset(&ps->folio_snapshot, 0, sizeof(struct folio));
>  	memcpy(&ps->page_snapshot, page, sizeof(*page));
> -	head = ps->page_snapshot.compound_info;
> -	if ((head & 1) == 0) {
> +	info = ps->page_snapshot.compound_info;
> +	if ((info & 1) == 0) {

This could be “if (!(info & 1))” like _compound_head(), right?

>  		ps->idx = 0;
>  		foliop = (struct folio *)&ps->page_snapshot;
>  		if (!folio_test_large(foliop)) {
> @@ -1256,7 +1256,15 @@ void snapshot_page(struct page_snapshot *ps, const struct page *page)
>  		}
>  		foliop = (struct folio *)page;
>  	} else {
> -		foliop = (struct folio *)(head - 1);
> +		/* See compound_head() */
> +		if (compound_info_has_mask()) {
> +			unsigned long p = (unsigned long)page;
> +
> +			foliop = (struct folio *)(p & info);
> +		} else {
> +			foliop = (struct folio *)(info - 1);
> +		}
> +
>  		ps->idx = folio_page_idx(foliop, page);
>  	}

LGTM.

Reviewed-by: Zi Yan <ziy@xxxxxxxxxx>

Best Regards,
Yan, Zi