Re: [PATCH v2] mm: introduce arch_has_hw_nonleaf_pmd_young()

[Yu Zhao <yuzhao@google.com>]

On Thu, Nov 24, 2022 at 7:30 AM Juergen Gross <jgross@suse.com> wrote:
>
> Hi,
>
> On 24.11.22 15:08, Geert Uytterhoeven wrote:
> > Hi Jürgen,
> >
> > On Wed, Nov 23, 2022 at 7:53 AM Juergen Gross <jgross@suse.com> wrote:
> >> When running as a Xen PV guests commit eed9a328aa1a ("mm: x86: add
> >> CONFIG_ARCH_HAS_NONLEAF_PMD_YOUNG") can cause a protection violation
> >> in pmdp_test_and_clear_young():
> >>
> >>   BUG: unable to handle page fault for address: ffff8880083374d0
> >>   #PF: supervisor write access in kernel mode
> >>   #PF: error_code(0x0003) - permissions violation
> >>   PGD 3026067 P4D 3026067 PUD 3027067 PMD 7fee5067 PTE 8010000008337065
> >>   Oops: 0003 [#1] PREEMPT SMP NOPTI
> >>   CPU: 7 PID: 158 Comm: kswapd0 Not tainted 6.1.0-rc5-20221118-doflr+ #1
> >>   RIP: e030:pmdp_test_and_clear_young+0x25/0x40
> >>
> >> This happens because the Xen hypervisor can't emulate direct writes to
> >> page table entries other than PTEs.
> >>
> >> This can easily be fixed by introducing arch_has_hw_nonleaf_pmd_young()
> >> similar to arch_has_hw_pte_young() and test that instead of
> >> CONFIG_ARCH_HAS_NONLEAF_PMD_YOUNG.
> >>
> >> Fixes: eed9a328aa1a ("mm: x86: add CONFIG_ARCH_HAS_NONLEAF_PMD_YOUNG")
> >> Reported-by: Sander Eikelenboom <linux@eikelenboom.it>
> >> Signed-off-by: Juergen Gross <jgross@suse.com>
> >> Acked-by: Yu Zhao <yuzhao@google.com>
> >> Tested-by: Sander Eikelenboom <linux@eikelenboom.it>
> >> ---
> >> V2:
> >> - correct function name in commit message to match patch
> >
> > Thanks for your patch, which is now commit 3f85e711d5af4fb4 ("mm:
> > introduce arch_has_hw_nonleaf_pmd_young()") in next-20221124.
> >
> > noreply@ellerman.id.au reported a build failure for m68k/allmodconfig,
> > which I have bisected to this commit.
> >
> >> --- a/mm/vmscan.c
> >> +++ b/mm/vmscan.c
> >
> >> @@ -4073,14 +4073,14 @@ static void walk_pmd_range(pud_t *pud, unsigned long start, unsigned long end,
> >>   #endif
> >>                  walk->mm_stats[MM_NONLEAF_TOTAL]++;
> >>
> >> -#ifdef CONFIG_ARCH_HAS_NONLEAF_PMD_YOUNG
> >> -               if (get_cap(LRU_GEN_NONLEAF_YOUNG)) {
> >> +               if (arch_has_hw_nonleaf_pmd_young() &&
> >> +                   get_cap(LRU_GEN_NONLEAF_YOUNG)) {
> >>                          if (!pmd_young(val))
> >
> > mm/vmscan.c:4102:30: error: implicit declaration of function
> > 'pmd_young'; did you mean 'pte_young'?
> > [-Werror=implicit-function-declaration]
> >
> > pmd_young() seems to be defined only on a handful of architectures.
>
> What would be the preferred fix for that?
>
> I could offer:
>
> - use V1 of the patch
> - add the #ifdefs again to this patch (which would be kind of weird)
> - use the attached patch

Your patch looks good to me:

Acked-by: Yu Zhao <yuzhao@google.com>

Thanks.