linux-kernel - Re: [PATCH v5 2/2] RISC-V: Setup initial page tables in two stages

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Date:   Fri, 16 Aug 2019 06:35:09 +0530
From:   Anup Patel <anup@...infault.org>
To:     Alistair Francis <Alistair.Francis@....com>
Cc:     "paul.walmsley@...ive.com" <paul.walmsley@...ive.com>,
        Anup Patel <Anup.Patel@....com>,
        "aou@...s.berkeley.edu" <aou@...s.berkeley.edu>,
        "palmer@...ive.com" <palmer@...ive.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "rppt@...ux.ibm.com" <rppt@...ux.ibm.com>,
        "hch@...radead.org" <hch@...radead.org>,
        Atish Patra <Atish.Patra@....com>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>
Subject: Re: [PATCH v5 2/2] RISC-V: Setup initial page tables in two stages

On Fri, Aug 16, 2019 at 12:27 AM Alistair Francis
<Alistair.Francis@....com> wrote:
>
> On Wed, 2019-07-10 at 17:05 -0700, Paul Walmsley wrote:
> > On Fri, 7 Jun 2019, Anup Patel wrote:
> >
> > > Currently, the setup_vm() does initial page table setup in one-shot
> > > very early before enabling MMU. Due to this, the setup_vm() has to
> > > map
> > > all possible kernel virtual addresses since it does not know size
> > > and
> > > location of RAM. This means we have kernel mappings for non-
> > > existent
> > > RAM and any buggy driver (or kernel) code doing out-of-bound access
> > > to RAM will not fault and cause underterministic behaviour.
> > >
> > > Further, the setup_vm() creates PMD mappings (i.e. 2M mappings) for
> > > RV64 systems. This means for PAGE_OFFSET=0xffffffe000000000 (i.e.
> > > MAXPHYSMEM_128GB=y), the setup_vm() will require 129 pages (i.e.
> > > 516 KB) of memory for initial page tables which is never freed. The
> > > memory required for initial page tables will further increase if
> > > we chose a lower value of PAGE_OFFSET (e.g. 0xffffff0000000000)
> > >
> > > This patch implements two-staged initial page table setup, as
> > > follows:
> > > 1. Early (i.e. setup_vm()): This stage maps kernel image and DTB in
> > > a early page table (i.e. early_pg_dir). The early_pg_dir will be
> > > used
> > > only by boot HART so it can be freed as-part of init memory free-
> > > up.
> > > 2. Final (i.e. setup_vm_final()): This stage maps all possible RAM
> > > banks in the final page table (i.e. swapper_pg_dir). The boot HART
> > > will start using swapper_pg_dir at the end of setup_vm_final(). All
> > > non-boot HARTs directly use the swapper_pg_dir created by boot
> > > HART.
> > >
> > > We have following advantages with this new approach:
> > > 1. Kernel mappings for non-existent RAM don't exists anymore.
> > > 2. Memory consumed by initial page tables is now indpendent of the
> > > chosen PAGE_OFFSET.
> > > 3. Memory consumed by initial page tables on RV64 system is 2 pages
> > > (i.e. 8 KB) which has significantly reduced and these pages will be
> > > freed as-part of the init memory free-up.
> > >
> > > The patch also provides a foundation for implementing strict kernel
> > > mappings where we protect kernel text and rodata using PTE
> > > permissions.
> > >
> > > Suggested-by: Mike Rapoport <rppt@...ux.ibm.com>
> > > Signed-off-by: Anup Patel <anup.patel@....com>
> >
> > Thanks, updated to apply and to fix a checkpatch warning, and
> > queued.
> >
> > This may not make it in for v5.3-rc1; if not, we'll submit it later.
>
> I'm seeing this failure on RV32 which I bisected to this patch:
>
> [    1.820461] systemd[1]: systemd 242-19-gdb2e367+ running in system
> mode. (-PAM -AUDIT -SELINUX +IMA -APPARMOR +SMACK +SYSVINIT +UTMP
> -LIBCRYPTSETUP -GCRYPT -GNUTLS +ACL +XZ -LZ4 -SECCOMP +BLKID -ELFUTILS
> +KMOD -IDN2 -IDN -PCRE2 default-hierarchy=hybrid)
> [    1.824320] Unable to handle kernel paging request at virtual
> address 9ff00c15
> [    1.824973] Oops [#1]
> [    1.825162] Modules linked in:
> [    1.825536] CPU: 0 PID: 1 Comm: systemd Not tainted 5.2.0-rc7 #1
> [    1.826039] sepc: c05c3c78 ra : c04b5a74 sp : df047ce0
> [    1.826514]  gp : c07a1038 tp : df04c000 t0 : 000000fc
> [    1.826919]  t1 : 00000002 t2 : 000003ef s0 : df047cf0
> [    1.827322]  s1 : df7090f8 a0 : 9ff00c15 a1 : c072166c
> [    1.827723]  a2 : 00000000 a3 : 00000001 a4 : 00000001
> [    1.828104]  a5 : df6f8138 a6 : 0000002f a7 : de62a000
> [    1.828534]  s2 : c072166c s3 : 00000000 s4 : 00000000
> [    1.828931]  s5 : c07a2000 s6 : 00400cc0 s7 : 00000400
> [    1.829319]  s8 : de491018 s9 : 00000000 s10: fffff000
> [    1.829702]  s11: de491030 t3 : de62b000 t4 : 00000000
> [    1.830090]  t5 : 00000000 t6 : 00000080
> [    1.830392] sstatus: 00000100 sbadaddr: 9ff00c15 scause: 0000000d
> [    1.831616] ---[ end trace 49a926a1a5300c00 ]---
> [    1.835776] Kernel panic - not syncing: Attempted to kill init!
> exitcode=0x0000000b
> [    1.836575] ---[ end Kernel panic - not syncing: Attempted to kill
> init! exitcode=0x0000000b ]---
>
> Does anyone else see this?
>
> A simple revert of this patch on 5.3-rc4 fixes the issue for me.

It looks like this patch is exposing some other bug of Linux RISC-V
32bit kernel.

We will be hiding the actual issue by reverting this patch because
previously we were mapping all possible kernel virtual addresses
even for non-existent RAM (after RAM ends).

Let me debug this more.

Regards,
Anup

>
> Alistair
>
> >
> >
> > - Paul
> >
> > _______________________________________________
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