[PATCH RT] Revert "ARM: Initialize split page table locks for vector page"
From: Sebastian Andrzej Siewior
Date: Mon Oct 14 2019 - 12:02:47 EST
I'm dropping this patch, with its original description:
|ARM: Initialize split page table locks for vector page
|Without this patch, ARM can not use SPLIT_PTLOCK_CPUS if
|PREEMPT_RT_FULL=y because vectors_user_mapping() creates a
|VM_ALWAYSDUMP mapping of the vector page (address 0xffff0000), but no
|ptl->lock has been allocated for the page. An attempt to coredump
|that page will result in a kernel NULL pointer dereference when
|follow_page() attempts to lock the page.
|The call tree to the NULL pointer dereference is:
| pte_offset_map_lock() <----- a #define
|The underlying problem is exposed by mm-shrink-the-page-frame-to-rt-size.patch.
The patch named mm-shrink-the-page-frame-to-rt-size.patch was dropped
from the RT queue once the SPLIT_PTLOCK_CPUS feature (in a slightly
different shape) went upstream (somewhere between v3.12 and v3.14).
I can see that the patch still allocates a lock which wasn't there
before. However I can't trigger a kernel oops like described in the
patch by triggering a coredump.
arch/arm/kernel/process.c | 24 ------------------------
1 file changed, 24 deletions(-)
diff --git a/arch/arm/kernel/process.c b/arch/arm/kernel/process.c
index 1041300022177..f934a6739fc05 100644
@@ -325,30 +325,6 @@ unsigned long arch_randomize_brk(struct mm_struct *mm)
- * CONFIG_SPLIT_PTLOCK_CPUS results in a page->ptl lock. If the lock is not
- * initialized by pgtable_page_ctor() then a coredump of the vector page will
- * fail.
-static int __init vectors_user_mapping_init_page(void)
- struct page *page;
- unsigned long addr = 0xffff0000;
- pgd_t *pgd;
- pud_t *pud;
- pmd_t *pmd;
- pgd = pgd_offset_k(addr);
- pud = pud_offset(pgd, addr);
- pmd = pmd_offset(pud, addr);
- page = pmd_page(*(pmd));
- return 0;
* The vectors page is always readable from user space for the