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

[Lorenzo Stoakes <ljs@kernel.org>]

On Sat, Jul 11, 2026 at 01:08:46PM +0300, Mike Rapoport wrote:
> 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

Ack on nits will fixup on respin.

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

Thanks!

>
> --
> Sincerely yours,
> Mike.

Cheers, Lorenzo