Received-SPF: pass (google.com: domain of linux-kernel+bounces-100547-linux.lists.archive=gmail.com@vger.kernel.org designates 147.75.80.249 as permitted sender) client-ip=147.75.80.249;
Precedence: bulk
MIME-Version: 1.0
References: <20240311164638.2015063-1-pasha.tatashin@soleen.com>
 <20240311164638.2015063-7-pasha.tatashin@soleen.com> <f5105c5d-59fb-4bca-af70-353e480f2b12@suse.com>
In-Reply-To: <f5105c5d-59fb-4bca-af70-353e480f2b12@suse.com>
From: Pasha Tatashin <pasha.tatashin@soleen.com>
Date: Tue, 12 Mar 2024 12:53:16 -0400
Message-ID: <CA+CK2bD0ShasU0U5gzp_m99V=NAg1o8wAKH3_+1ghZHmsWhL9w@mail.gmail.com>
Subject: Re: [RFC 06/14] fork: zero vmap stack using clear_page() instead of memset()
To: Nikolay Borisov <nik.borisov@suse.com>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, 
	akpm@linux-foundation.org, x86@kernel.org, bp@alien8.de, brauner@kernel.org, 
	bristot@redhat.com, bsegall@google.com, dave.hansen@linux.intel.com, 
	dianders@chromium.org, dietmar.eggemann@arm.com, eric.devolder@oracle.com, 
	hca@linux.ibm.com, hch@infradead.org, hpa@zytor.com, 
	jacob.jun.pan@linux.intel.com, jgg@ziepe.ca, jpoimboe@kernel.org, 
	jroedel@suse.de, juri.lelli@redhat.com, kent.overstreet@linux.dev, 
	kinseyho@google.com, kirill.shutemov@linux.intel.com, lstoakes@gmail.com, 
	luto@kernel.org, mgorman@suse.de, mic@digikod.net, 
	michael.christie@oracle.com, mingo@redhat.com, mjguzik@gmail.com, 
	mst@redhat.com, npiggin@gmail.com, peterz@infradead.org, pmladek@suse.com, 
	rick.p.edgecombe@intel.com, rostedt@goodmis.org, surenb@google.com, 
	tglx@linutronix.de, urezki@gmail.com, vincent.guittot@linaro.org, 
	vschneid@redhat.com
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable

On Tue, Mar 12, 2024 at 3:16=E2=80=AFAM Nikolay Borisov <nik.borisov@suse.c=
om> wrote:
>
>
>
> On 11.03.24 =D0=B3. 18:46 =D1=87., Pasha Tatashin wrote:
> > In preporation for dynamic kernel stacks do not zero the whole span of
> > the stack, but instead only the pages that are part of the vm_area.
> >
> > This is because with dynamic stacks we might have only partially
> > populated stacks.
> >
> > Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
> > ---
> >   kernel/fork.c | 6 ++++--
> >   1 file changed, 4 insertions(+), 2 deletions(-)
> >
> > diff --git a/kernel/fork.c b/kernel/fork.c
> > index 6a2f2c85e09f..41e0baee79d2 100644
> > --- a/kernel/fork.c
> > +++ b/kernel/fork.c
> > @@ -263,8 +263,8 @@ static int memcg_charge_kernel_stack(struct vm_stru=
ct *vm)
> >   static int alloc_thread_stack_node(struct task_struct *tsk, int node)
> >   {
> >       struct vm_struct *vm_area;
> > +     int i, j, nr_pages;
> >       void *stack;
> > -     int i;
> >
> >       for (i =3D 0; i < NR_CACHED_STACKS; i++) {
> >               vm_area =3D this_cpu_xchg(cached_stacks[i], NULL);
> > @@ -282,7 +282,9 @@ static int alloc_thread_stack_node(struct task_stru=
ct *tsk, int node)
> >               stack =3D kasan_reset_tag(vm_area->addr);
> >
> >               /* Clear stale pointers from reused stack. */
> > -             memset(stack, 0, THREAD_SIZE);
> > +             nr_pages =3D vm_area->nr_pages;
> > +             for (j =3D 0; j < nr_pages; j++)
> > +                     clear_page(page_address(vm_area->pages[j]));
>
> Can't this be memset(stack, 0, nr_pages*PAGE_SIZE) ?

No, we can't, because the pages can be physically discontiguous.

Pasha