Re: [PATCH] mm/ptdump: always stabilise against page table freeing using init_mm

[Andrew Morton]

On Fri, 10 Jul 2026 14:29:21 +0100 Lorenzo Stoakes <ljs@xxxxxxxxxx> wrote:

> x86 and arm64 invokes ptdump_walk_pgd() with non-init_mm mm whilst still
> walking kernel page table ranges.
> 
> For x86 this is done in ptdump_curknl_show() and ptdump_efi_show(), the
> first passing current->mm, and the second passing efi_mm (we reach kernel
> mappings that init_mm protects for current->mm due to x86 cloning shared
> kernel page tables for arbitrary mm's).
> 
> arm64 does so via ptdump_debugfs_register(), configured by efi_ptdump_info
> for efi ranges against efi_mm.
> 
> The init_mm mmap lock is used to stabilise page table freeing against
> ptdump, so take a nested lock on init_mm to ensure that we are correctly
> stabilised.
> 
> We take this after mmap write locking the non-init_mm mm. Nothing acquires
> the init_mm lock first before locking an arbitrary mm, so no deadlock is
> possible.
> 
> Other fixes have been sent which update the two cases which can cause races
> with ptdump in init_mm ranges to acquire the init_mm mmap write lock - vmap
> and x86 CPA huge page promotion.
> 
> For arm64, commit fa93b45fd397 ("arm64: Enable vmalloc-huge with ptdump")
> already provides exclusion against init_mm for the vmap case, which this
> patch also pairs with.
> 
> The first point at which ptdump can race kernel page table freeing is
> commit b6bdb7517c3d ("mm/vmalloc: add interfaces to free unmapped page
> table"), so we target this in the Fixes tag.

Does this have any known userspace impact?

> Fixes: b6bdb7517c3d ("mm/vmalloc: add interfaces to free unmapped page table")

Since 2018, so I won't make this a hotfix, OK?