linux-kernel - Re: [PATCH v6 10/18] sh/tlb: Convert SH to generic mmu

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Date:   Thu, 5 Dec 2019 16:26:57 +0100
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Will Deacon <will.deacon@....com>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Nicholas Piggin <npiggin@...il.com>,
        Linux-Arch <linux-arch@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Russell King <linux@...linux.org.uk>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Rik van Riel <riel@...riel.com>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Rich Felker <dalias@...c.org>,
        Linux-sh list <linux-sh@...r.kernel.org>,
        Guenter Roeck <linux@...ck-us.net>
Subject: Re: [PATCH v6 10/18] sh/tlb: Convert SH to generic mmu_gather

Hoi Peter,

On Wed, Dec 4, 2019 at 5:42 PM Peter Zijlstra <peterz@...radead.org> wrote:
> On Wed, Dec 04, 2019 at 04:07:53PM +0100, Geert Uytterhoeven wrote:
> > On Wed, Dec 4, 2019 at 2:35 PM Peter Zijlstra <peterz@...radead.org> wrote:
> > > Does this fare better?
> >
> > Yes. Migo-R is happy again.
> > Tested-by: Geert Uytterhoeven <geert+renesas@...der.be>
> >
> > > --- a/arch/sh/include/asm/pgalloc.h
> > > +++ b/arch/sh/include/asm/pgalloc.h
> > > @@ -36,9 +36,7 @@ do {                                                  \
> > >  #if CONFIG_PGTABLE_LEVELS > 2
> > >  #define __pmd_free_tlb(tlb, pmdp, addr)                        \
> > >  do {                                                   \
> > > -       struct page *page = virt_to_page(pmdp);         \
> > > -       pgtable_pmd_page_dtor(page);                    \
> > > -       tlb_remove_page((tlb), page);                   \
> > > +       pmd_free((tlb)->mm, (pmdp));                    \
> > >  } while (0);
> > >  #endif
>
> OK, so I was going to write a Changelog to go with that, but then I
> realized that while this works and is similar to before the patch, I'm
> not sure this is in fact correct.
>
> With this on (and also before) we're freeing the PMD before we've done
> the TLB invalidate, that seems wrong!
>
> Looking at the size of that pmd_cache, that looks to be 30-(12+12-3)+3
> == 12, which is exactly 1 page, for PAGE_SIZE_4K, less for the larger
> pages.
>
> I'm thinking perhaps we should do something like the below instead?

Your advice is better when in close vicinity of an SH cross compiler,
though ;-)

> --- a/arch/sh/mm/pgtable.c
> +++ b/arch/sh/mm/pgtable.c
> @@ -5,9 +5,6 @@
>  #define PGALLOC_GFP GFP_KERNEL | __GFP_ZERO
>
>  static struct kmem_cache *pgd_cachep;
> -#if PAGETABLE_LEVELS > 2
> -static struct kmem_cache *pmd_cachep;
> -#endif
>
>  void pgd_ctor(void *x)
>  {
> @@ -23,11 +20,6 @@ void pgtable_cache_init(void)
>         pgd_cachep = kmem_cache_create("pgd_cache",
>                                        PTRS_PER_PGD * (1<<PTE_MAGNITUDE),
>                                        PAGE_SIZE, SLAB_PANIC, pgd_ctor);
> -#if PAGETABLE_LEVELS > 2
> -       pmd_cachep = kmem_cache_create("pmd_cache",
> -                                      PTRS_PER_PMD * (1<<PTE_MAGNITUDE),
> -                                      PAGE_SIZE, SLAB_PANIC, NULL);
> -#endif
>  }
>
>  pgd_t *pgd_alloc(struct mm_struct *mm)
> @@ -48,11 +40,7 @@ void pud_populate(struct mm_struct *mm, pud_t *pud, pmd_t *pmd)
>
>  pmd_t *pmd_alloc_one(struct mm_struct *mm, unsigned long address)
>  {
> -       return kmem_cache_alloc(pmd_cachep, PGALLOC_GFP);
> -}
> -
> -void pmd_free(struct mm_struct *mm, pmd_t *pmd)

mm/memory.o: In function `__pmd_alloc':
memory.c:(.text+0x1d74): undefined reference to `pmd_free'

> -{
> -       kmem_cache_free(pmd_cachep, pmd);
> +       BUILD_BUG_ON(PTRS_PER_PMD * (1<<PTE_MAGNITUDE) <= PAGE_SIZE);

... > PAGE_SIZE ?

Else it triggers all the time.

> +       return (pmd_t *)__get_free_page(PGALLOC_GFP);
>  }
>  #endif /* PAGETABLE_LEVELS > 2 */

BTW, I'm still running Willy's fix that never made it upstream
to kill an ugly boot warning, which also touches this code:
https://patchwork.kernel.org/patch/10549883/#22166333

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-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