linux-kernel - Re: [PATCH] riscv: Fixup one-page wasting

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Message-ID: <CAAhSdy2_Vk5nPMUOni5JctV8Jd9NYh-WK=yZPXTN_yzaNdy-4g@mail.gmail.com>
Date:   Wed, 24 Nov 2021 13:26:13 +0530
From:   Anup Patel <anup@...infault.org>
To:     Guo Ren <guoren@...nel.org>
Cc:     "linux-kernel@...r.kernel.org List" <linux-kernel@...r.kernel.org>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        linux-csky@...r.kernel.org, Guo Ren <guoren@...ux.alibaba.com>,
        Alexandre Ghiti <alex@...ti.fr>,
        Atish Patra <Atish.Patra@....com>
Subject: Re: [PATCH] riscv: Fixup one-page wasting

On Wed, Nov 24, 2021 at 12:52 PM Guo Ren <guoren@...nel.org> wrote:
>
> On Wed, Nov 24, 2021 at 2:55 PM Anup Patel <anup@...infault.org> wrote:
> >
> > On Wed, Nov 24, 2021 at 11:35 AM <guoren@...nel.org> wrote:
> > >
> > > From: Guo Ren <guoren@...ux.alibaba.com>
> > >
> > > For small memory systems(Allwinner D1s/F133), one page size memory
> > > cannot be ignored.
> > >
> > > Signed-off-by: Guo Ren <guoren@...ux.alibaba.com>
> > > Cc: Alexandre Ghiti <alex@...ti.fr>
> > > Cc: Anup Patel <anup@...infault.org>
> > > Cc: Atish Patra <Atish.Patra@....com>
> >
> > This PATCH breaks the CPU hotplug functionality.
> >
> > When a CPU/HART is turned off and turned on at runtime, the
> > low-level relocate() will be called to enable MMU on the CPU
> > being brought-up which in-turn uses trampoline_pg_dir.
> Yeah, I forgot that.
>
> How about removing trampoline_pg_dir totally and using
> early_pg_dir/swapper_pg_dir for relocate directly?

The trampoline_pg_dir is to handle the case when RAM is
large enough such that RAM physical address range overlaps
kernel virtual address range (i.e. VA >= PAGE_OFFSET). This
is overlap of virtual address range and physical address range
can be problematic for low-level code which is trying to enable
MMU (such as the relocate() function).

Here's a old kernel thread which tries to summarize this:
https://lore.kernel.org/lkml/CAAhSdy3URWHVY_GPNb2yRBuctRELRtTTWPM2OpwUVSRFAyXyiA@mail.gmail.com/

Regards,
Anup

>
> >
> > Regards,
> > Anup
> >
> > > ---
> > >  arch/riscv/mm/init.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
> > > index 24b2b8044602..097bb3bc4020 100644
> > > --- a/arch/riscv/mm/init.c
> > > +++ b/arch/riscv/mm/init.c
> > > @@ -241,9 +241,9 @@ unsigned long riscv_pfn_base __ro_after_init;
> > >  EXPORT_SYMBOL(riscv_pfn_base);
> > >
> > >  pgd_t swapper_pg_dir[PTRS_PER_PGD] __page_aligned_bss;
> > > -pgd_t trampoline_pg_dir[PTRS_PER_PGD] __page_aligned_bss;
> > >  static pte_t fixmap_pte[PTRS_PER_PTE] __page_aligned_bss;
> > >
> > > +pgd_t trampoline_pg_dir[PTRS_PER_PGD] __initdata __aligned(PAGE_SIZE);
> > >  pgd_t early_pg_dir[PTRS_PER_PGD] __initdata __aligned(PAGE_SIZE);
> > >  static pmd_t __maybe_unused early_dtb_pmd[PTRS_PER_PMD] __initdata __aligned(PAGE_SIZE);
> > >
> > > --
> > > 2.25.1
> > >
>
>
>
> --
> Best Regards
>  Guo Ren
>
> ML: https://lore.kernel.org/linux-csky/