[tip: x86/urgent] x86/mm: Drop unnecessary PMD page copy when freeing
From: tip-bot2 for Mikhail Gavrilov
Date: Mon Sep 28 2026 - 14:07:06 EST
The following commit has been merged into the x86/urgent branch of tip:
Commit-ID: e3ee38c1bc0b11a8d3e63dce7050759b61864286
Gitweb: https://git.kernel.org/tip/e3ee38c1bc0b11a8d3e63dce7050759b61864286
Author: Mikhail Gavrilov <mikhail.v.gavrilov@xxxxxxxxx>
AuthorDate: Thu, 24 Sep 2026 03:31:16 +05:00
Committer: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
CommitterDate: Mon, 28 Sep 2026 08:08:16 -07:00
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. ]
Fixes: d5d8b8662e6e ("x86/mm/pat: Acquire init_mm read lock on attribute changes to avoid UAF")
Suggested-by: Pedro Falcato <pfalcato@xxxxxxx>
Signed-off-by: Mikhail Gavrilov <mikhail.v.gavrilov@xxxxxxxxx>
Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Reviewed-by: Pedro Falcato <pfalcato@xxxxxxx>
Link: https://lore.kernel.org/20260916062222.27347-1-mikhail.v.gavrilov@xxxxxxxxx
Link: https://lore.kernel.org/all/e11449f0-d9ad-4d1b-ab21-2be7d71fe335@xxxxxxxxx/ [1]
Link: https://patch.msgid.link/20260923223116.20090-1-mikhail.v.gavrilov@xxxxxxxxx
Cc: stable@xxxxxxxxxxxxxxx
---
arch/x86/mm/pgtable.c | 38 ++++++++++++++------------------------
1 file changed, 14 insertions(+), 24 deletions(-)
diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c
index cb03f5a..4a105f2 100644
--- a/arch/x86/mm/pgtable.c
+++ b/arch/x86/mm/pgtable.c
@@ -705,47 +705,37 @@ int pmd_clear_huge(pmd_t *pmd)
}
#ifdef CONFIG_X86_64
-/**
- * pud_free_pmd_page - Clear PUD entry and free PMD page
- * @pud: Pointer to a PUD
- * @addr: Virtual address associated with PUD
- *
- * Context: The PUD range has been unmapped and TLB purged.
- * Return: 1 if clearing the entry succeeded. 0 otherwise.
- *
- * NOTE: Callers must allow a single page allocation.
+/*
+ * Given a PUD poitner, detach and free the pointed-to
+ * PMD page and any PTE page children. The entire range
+ * under the PUD must not have any valid translations
+ * and the TLB must have already been flushed.
*/
int pud_free_pmd_page(pud_t *pud, unsigned long addr)
{
- pmd_t *pmd, *pmd_sv;
struct ptdesc *pt;
+ pmd_t *pmd;
int i;
pmd = pud_pgtable(*pud);
- pmd_sv = (pmd_t *)__get_free_page(GFP_KERNEL);
- if (!pmd_sv)
- return 0;
-
- for (i = 0; i < PTRS_PER_PMD; i++) {
- pmd_sv[i] = pmd[i];
- if (!pmd_none(pmd[i]))
- pmd_clear(&pmd[i]);
- }
+ /* Detach the PMD page: */
pud_clear(pud);
- /* INVLPG to clear all paging-structure caches */
+ /*
+ * PMD and all its descendents are unreachable
+ * via normal page walks. Make them unreachable
+ * in cached mid-level walks too:
+ */
flush_tlb_kernel_range(addr, addr + PAGE_SIZE-1);
for (i = 0; i < PTRS_PER_PMD; i++) {
- if (!pmd_none(pmd_sv[i])) {
- pt = page_ptdesc(pmd_page(pmd_sv[i]));
+ if (!pmd_none(pmd[i])) {
+ pt = page_ptdesc(pmd_page(pmd[i]));
pagetable_dtor_free(pt);
}
}
- free_page((unsigned long)pmd_sv);
-
pmd_free(&init_mm, pmd);
return 1;