CVE-2026-23100: mm/hugetlb: fix hugetlb_pmd_shared()

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

mm/hugetlb: fix hugetlbpmdshared()

Patch series "mm/hugetlb: fixes for PMD table sharing (incl. using mmugather)", v3.

One functional fix, one performance regression fix, and two related comment fixes.

I cleaned up my prototype I recently shared [1] for the performance fix, deferring most of the cleanups I had in the prototype to a later point. While doing that I identified the other things.

The goal of this patch set is to be backported to stable trees "fairly" easily. At least patch #1 and #4.

Patch #1 fixes hugetlbpmdshared() not detecting any sharing Patch #2 + #3 are simple comment fixes that patch #4 interacts with. Patch #4 is a fix for the reported performance regression due to excessive IPI broadcasts during fork()+exit().

The last patch is all about TLB flushes, IPIs and mmugather. Read: complicated

There are plenty of cleanups in the future to be had + one reasonable optimization on x86. But that's all out of scope for this series.

Runtime tested, with a focus on fixing the performance regression using the original reproducer [2] on x86.

This patch (of 4):

We switched from (wrongly) using the page count to an independent shared count. Now, shared page tables have a refcount of 1 (excluding speculative references) and instead use ptdesc->ptsharecount to identify sharing.

We didn't convert hugetlbpmdshared(), so right now, we would never detect a shared PMD table as such, because sharing/unsharing no longer touches the refcount of a PMD table.

Page migration, like mbind() or migratepages() would allow for migrating folios mapped into such shared PMD tables, even though the folios are not exclusive. In smaps we would account them as "private" although they are "shared", and we would be wrongly setting the PMMMAPEXCLUSIVE in the pagemap interface.

Fix it by properly using ptdescpmdisshared() in hugetlbpmdshared().