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

[David Hildenbrand (Arm)]

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@xxxxxxxxxx>
> Reviewed-by: Muchun Song <muchun.song@xxxxxxxxx>
> Reviewed-by: Zi Yan <ziy@xxxxxxxxxx>

[...]

>   	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@xxxxxxxxxx>

--
Cheers,

David