Re: [PATCH] mm: avoid zeroing user movable page twice with init_on_alloc=1

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Zi,

On Wed, Dec 4, 2024 at 7:30 PM Zi Yan <ziy@nvidia.com> wrote:
> On 4 Dec 2024, at 12:33, Zi Yan wrote:
> > On 4 Dec 2024, at 11:29, Matthew Wilcox wrote:
> >> On Wed, Dec 04, 2024 at 11:16:51AM -0500, Zi Yan wrote:
> >>>> So maybe the clearing done as part of page allocator isn't enough here.
> >>>>
> >>> Basically, mips needs to flush data cache if kmap address is aliased to
> >>
> >> People use "aliased" in contronym ways.  Do you mean "has a
> >> non-congruent alias" or "has a congruent alias"?
> >>
> >>> userspace address. This means when mips has THP on, the patch below
> >>> is not enough to fix the issue.
> >>>
> >>> In post_alloc_hook(), it does not make sense to pass userspace address
> >>> in to determine whether to flush dcache or not.
> >>>
> >>> One way to fix it is to add something like arch_userpage_post_alloc()
> >>> to flush dcache if kmap address is aliased to userspace address.
> >>> But my questions are that
> >>> 1) if kmap address will always be the same for two separate kmap_local() calls,
> >>
> >> No.  It just takes the next address in the stack.
> >
> > Hmm, if kmap_local() gives different addresses, wouldn’t init_on_alloc be
> > causing issues before my patch? In the page allocator, the page is zeroed
> > from one kmap address without flush, then clear_user_highpage() clears
> > it again with another kmap address with flush. After returning to userspace,
> > the user application works on the page but when the cache line used by
> > init_on_alloc is written back (with 0s) at eviction, user data is corrupted.
> > Am I missing anything? Or all arch with cache aliasing never enables
> > init_on_alloc?
>
> Hi Geert,
>
> Regarding the above concern, have you ever had CONFIG_INIT_ON_ALLOC_DEFAULT_ON
> for your MIPS machine and encountered any issue? Or let me know if my reasoning
> above is flawed.
>
> To test it, I wonder if you can 1) revert my patch and 2) turn on
> CONFIG_INIT_ON_ALLOC_DEFAULT_ON for your MIPS machine and run some applications
> to see if any error happens.

That seems to work fine...

Kernel log confirms it's enabled:
-mem auto-init: stack:off, heap alloc:off, heap free:off
+mem auto-init: stack:off, heap alloc:on, heap free:off

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds