Re: [PATCHv6 07/17] mm: Rework compound_head() for power-of-2 sizeof(struct page)

David Hildenbrand (Arm)
2026-02-05T14:09:25Z

On 2/2/26 16:56, 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 HugeTLB vmemmap optimization (HVO) where it makes
> a difference. The approach with mask would work in the wider set of
> conditions, 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@kernel.org>
> Reviewed-by: Muchun Song <muchun.song@linux.dev>
> Reviewed-by: Zi Yan <ziy@nvidia.com>

[...]

> struct folio *foliop;
> int loops = 5;
>
> @@ -1247,8 +1247,8 @@ 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)) {
> ps->idx = 0;
> foliop = (struct folio *)&ps->page_snapshot;
> if (!folio_test_large(foliop)) {
> @@ -1259,7 +1259,15 @@ 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);

IIUC, we don't care about clearing bit0 before the & as the page pointer
shouldn't have set it in the first page.

Pretty neat

Acked-by: David Hildenbrand (Arm) <david@kernel.org>

--
Cheers,

David