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

[Mike Rapoport <rppt@kernel.org>]

On Fri, Jul 10, 2026 at 02:29:21PM +0100, Lorenzo Stoakes wrote:
> x86 and arm64 invokes ptdump_walk_pgd() with non-init_mm mm whilst still

Nit:            ^ invoke

> 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.
> 
> Fixes: b6bdb7517c3d ("mm/vmalloc: add interfaces to free unmapped page table")
> Cc: stable@vger.kernel.org
> Signed-off-by: Lorenzo Stoakes <ljs@kernel.org>
>
> ---
>  mm/ptdump.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/mm/ptdump.c b/mm/ptdump.c
> index 973020000096..6bef47b1a073 100644
> --- a/mm/ptdump.c
> +++ b/mm/ptdump.c
> @@ -178,11 +178,18 @@ void ptdump_walk_pgd(struct ptdump_state *st, struct mm_struct *mm, pgd_t *pgd)
>  
>  	get_online_mems();
>  	mmap_write_lock(mm);
> +	/* To stabilise page tables we must hold the init_mm lock too. */

Maybe

	/* To stabilise kernel page tables we must hold the init_mm lock too. */

Other than these nits

Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

-- 
Sincerely yours,
Mike.