On 4/25/26 08:20, Muchun Song wrote: > > >> On Apr 25, 2026, at 13:48, David Hildenbrand (Arm) <[email protected]> wrote: >> >> >>> >>> >>> Hi David, >>> >>> Sorry, I missed the 1GB hugepage scenario earlier. Given that >>> sparse_add_section() >>> operates on a scale between PAGES_PER_SUBSECTION and PAGES_PER_SECTION, the >>> pfn and >>> nr_pages parameters wouldn't be aligned with the hugepage size >>> (pages_per_compound), >>> but rather with the PAGES_PER_SECTION boundary. Do you think this >>> explanation makes >>> it clearer? In the interest of code clarity, do you think the modification >>> below >>> makes it easier to follow? >>> >>> diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c >>> index 2e642c5ff3f2..ce675c5fb94d 100644 >>> --- a/mm/sparse-vmemmap.c >>> +++ b/mm/sparse-vmemmap.c >>> @@ -658,15 +658,18 @@ static int __meminit >>> section_nr_vmemmap_pages(unsigned long pfn, unsigned long n >>> const unsigned int order = pgmap ? pgmap->vmemmap_shift : 0; >>> const unsigned long pages_per_compound = 1UL << order; >>> >>> - VM_WARN_ON_ONCE(!IS_ALIGNED(pfn | nr_pages, >>> - min(pages_per_compound, >>> PAGES_PER_SECTION))); >>> + VM_WARN_ON_ONCE(!IS_ALIGNED(pfn | nr_pages, PAGES_PER_SUBSECTION)); >> >> That here makes sense. We can only add/remove in multiples of >> PAGES_PER_SECTION. >> I think what we are saying is that we want that check in addition to the >> existing min() check. > > Right. > >> >>> VM_WARN_ON_ONCE(pfn_to_section_nr(pfn) != pfn_to_section_nr(pfn + >>> nr_pages - 1)); >>> >>> if (!vmemmap_can_optimize(altmap, pgmap)) >>> return DIV_ROUND_UP(nr_pages * sizeof(struct page), >>> PAGE_SIZE); >>> >>> - if (order < PFN_SECTION_SHIFT) >>> + if (order < PFN_SECTION_SHIFT) { >>> + VM_WARN_ON_ONCE(!IS_ALIGNED(pfn | nr_pages, >>> pages_per_compound)); >>> return VMEMMAP_RESERVE_NR * nr_pages / pages_per_compound; >> >> That makes sense as well, within a section, we expect that we always >> add/remove >> entire "compound"-managed chunks. >> >>> + } >>> + >>> + VM_WARN_ON_ONCE(!IS_ALIGNED(pfn | nr_pages, PAGES_PER_SECTION)); >> >> And this is then for the case where a 1G page spans multiple sections, where >> we >> expect to add/remove an entire section. >> >> So here, indeed the "min" makes sense. I guess we also assume: >> >> VM_WARN_ON_ONCE(nr_pages > PAGES_PER_SECTION); > > Yes. But this one we do not need to explicit it to > assert it since at the front of this function we have > > VM_WARN_ON_ONCE(pfn_to_section_nr(pfn) != pfn_to_section_nr(pfn + nr_pages - > 1));
Ah, yes. The alignment checks + VM_WARN_ON_ONCE(nr_pages > PAGES_PER_SECTION); however imply that. So you could simplify by using that check instead of the pfn_to_section_nr() check. But it's still early here ... so whatever you prefer :) -- Cheers, David
