---
id: "CVE-2026-98939"
kind: "vulnerability"
aliases: ["CVE-2026-98939"]
title: "Linux kernel x86 huge-vmap path can deadlock during memory reclaim"
severity: "unknown"
cvss: null
epss: null
exploited: false
status: "open"
category: "runtime"
products: []
ecosystems: []
tags: ["linux-kernel", "runtime", "self-hosted", "x86"]
not_affected_if: ["non-x86-architecture"]
published_at: "2026-10-23T13:17:14+00:00"
first_seen_at: "2026-10-23T21:57:33+00:00"
updated_at: "2026-10-25T04:58:03+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-98939", "first_seen_at": "2026-10-23T21:57:33+00:00"}]
---

# Linux kernel x86 huge-vmap path can deadlock during memory reclaim

On x86, the huge vmap code allocated memory with GFP_KERNEL while holding the init_mm mmap lock, which can deadlock when kswapd reclaims memory, as lockdep showed with a discrete GPU's TTM pool shrinker. The fix removes that allocation by freeing PTE tables straight from the detached PMD page.

_Summary, check and fix written by a model from the sources; labelled as such._

## How to check

- Run `uname -r` and compare against the fixed kernel builds listed in the NVD entry for CVE-2026-98939
- unknown

## Fix

- Update the Linux kernel to a build that includes the upstream commit for CVE-2026-98939 (check the NVD or distro advisory for the exact version)
- unknown

## Timeline

- 2026-10-23T21:57:33+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-98939

## From the sources

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

x86/mm: Drop unnecessary PMD page copy when freeing

On a box with a discrete GPU, lockdep reports a possible deadlock as
soon as kswapd shrinks the TTM page pool. The immediate cause is an
x86 commit that added an mmap_read_lock() to kernel page protection
munging code.

The huge vmap code holds the same lock over a GFP_KERNEL allocation,
which is a no-no now that reclaim can take it. That allocation is in a
page table *free* path and ends up being for dubious purposes[1].
Basically, it tries to avoid hardware setting Accessed=1 in page table
entries that are unreachable by the hardware, a non-issue.

Remove the PMD copy. Detach the original PMD page at the PUD, flush
the mid-level caches, and free the PTE tables straight from the
detached PMD page. With no allocation left, the locking issue is gone.

Lockdep splat/analysis:

  WARNING: possible circular locking dependency detected
  7.3.0-rc3-f6e7b42bf05b+ #183 Tainted: G     U
  ------------------------------------------------------
  kswapd0/269 is trying to acquire lock:
  ((init_mm).mmap_lock){++++}-{4:4}, at: change_page_attr_set_clr+0x29a/0x4a0
  but task is already holding lock:
  (pool_shrink_rwsem){.+.+}-{4:4}, at: ttm_pool_shrink+0xb2/0x330 [ttm]
  Chain exists of:
    (init_mm).mmap_lock --> fs_reclaim --> pool_shrink_rwsem

The cycle is built from three edges:

  1) pool_shrink_rwsem -> (init_mm).mmap_lock

     The TTM shrinker restores the caching attribute of every page it
     frees, while holding pool_shrink_rwsem:

       ttm_pool_shrink()
       -> ttm_pool_dispose_list()
       -> ttm_pool_free_page()
       -> set_pages_wb()
       -> change_page_attr_set_clr()   [ init_mm mmap read lock ]

  2) fs_reclaim -> pool_shrink_rwsem

     The same shrinker, called from reclaim.

  3) (init_mm).mmap_lock -> fs_reclaim

     ioremap() installing a huge PUD mapping over an existing PMD table:

       ioremap_page_range()
       -> vmap_range_noflush()
       -> vmap_try_huge_pud()          [ init_mm mmap read lock ]
       -> pud_free_pmd_page()
       -> __get_free_page(GFP_KERNEL)  [ enters reclaim ]

[ dhansen: Lots of changelog munging/trimming and merged comments from my
	   version of the fix. ]

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_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_