CVE-2026-93232: mm/hugetlb: fix boot panic with CONFIG_DEBUG_VM and HVO bootmem pages

In the Linux kernel, the following vulnerability has been resolved:

mm/hugetlb: fix boot panic with CONFIGDEBUGVM and HVO bootmem pages

Patch series "mm: Refactor bootmem gigantic hugepage allocation", v4.

This series is split out from the earlier larger series "mm: Generalize HVO for HugeTLB and device DAX" [1]. It collects the first 19 patches of that series as a standalone set of fixes and preparatory cleanups around bootmem HugeTLB handling, sparse initialization ordering, and related vmemmap setup.

The first patches fix a few bugs found while reviewing the existing code, including incorrect bootmem HVO handling, wrong vmemmap registration arguments, a powerpc compound-vmemmap tracking bug, and too-late initialization of gigantic bootmem HugeTLB struct pages.

The rest of the series reorders early memory initialization so the relevant zone state is available before sparse and HugeTLB boot-time setup runs, then simplifies the remaining bootmem gigantic hugepage allocation path and removes code made obsolete by that rework.

At a high level: - patches [1-4] fix boot-time and arch-specific bugs - patches [5-12] reorder and simplify sparse/mm/hugetlb early init - patches [13-19] refactor bootmem gigantic hugepage allocation and remove obsolete helpers and state

This patch (of 19):

Commit 622026e87c40 ("mm/hugetlb: remove fake head pages") switched HVO to reuse per-zone shared tail pages from zone->vmemmaptails[].

Those shared tail pages were initialized in hugetlbvmemmapinit(), but bootmem HugeTLB folios are prepared earlier from gatherbootmemprealloc(). With hugetlbfreevmemmap=on, prepandaddbootmemfolios() can access pageblock flags on bootmem HugeTLB pages whose mirrored tail struct pages already point to the shared tail page. On CONFIGDEBUGVM kernels, getpfnblockbitmapbitidx() then dereferences the still-uninitialized shared tail page and can panic during boot.

Initialize zone->vmemmaptails[] from gatherbootmemprealloc(), before bootmem HugeTLB folios are processed, and drop the later initialization from hugetlbvmemmapinit().

This bug only affects CONFIGDEBUGVM kernels, where the relevant assertion is evaluated.