Re: [PATCH v2] tools/mm: rewrite show_page_info.py with modular design and new features

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] tools/mm: _compound_fields() queries compound order on tail page instead of head
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commit 559aca0f2b19f1194e73dadf4d12451f4bc08a7d
Author: Ye Liu <liuye@kylinos.cn>

tools/mm: rewrite show_page_info.py with modular design and new features

This commit rewrites tools/mm/show_page_info.py into a modular
field-collector design, adds new input modes and page debugging fields, and
fixes field access issues for slab, memcg, and compound pages.

> diff --git a/tools/mm/show_page_info.py b/tools/mm/show_page_info.py
> index c46d8ea283d7d..638fc669b694a 100644
> --- a/tools/mm/show_page_info.py
> +++ b/tools/mm/show_page_info.py

[ ... ]

> +def _compound_fields(page, head, fields):
> +    """Collect compound page fields."""
> +    if not PageCompound(page):
> +        return
> +    fields["Compound Order"] = compound_order(page).value_()
> +    offset = page_to_pfn(page).value_() - page_to_pfn(head).value_()
> +    fields["Folio Page Index"] = f"{offset}/{compound_nr(page).value_()}"
> +    if head.value_() != page.value_():
> +        fields["Head Page"] = hex(head.value_())

[Severity: Medium]
When show_page_state() calls _compound_fields() in
tools/mm/show_page_info.py for a tail page of a compound page, should
compound_order() and compound_nr() be called on head instead of page?

Both the kernel's compound_order() in include/linux/mm.h and drgn's
compound_order() check whether PG_head is set on the page passed to them
and return 0 for tail pages:

include/linux/mm.h:compound_order() {
	const struct folio *folio = (struct folio *)page;

	if (!test_bit(PG_head, &folio->flags.f))
		return 0;
	return folio_large_order(folio);
}

Because compound_nr() computes 1 << compound_order(), passing a tail page
causes _compound_fields() to report Compound Order as 0 and Folio Page
Index as <offset>/1 instead of using the compound head's order and page
count.

[ ... ]

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Sashiko AI review · https://sashiko.dev/#/patchset/20261021-show_page_info_refactor-v2-1-c28d1173b336@linux.dev?part=1