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From:   Alexandre Ghiti <alexghiti@rivosinc.com>
Date:   Wed, 28 Jun 2023 13:39:48 +0200
Message-ID: <CAHVXubhV62aT3=NFAoKU==C7MXrugeeEyphQqJGYBK+yZV=2qw@mail.gmail.com>
Subject: Re: [PATCH V1 0/3] Revert huge-paged linear mapping and its related fixups
To:     Song Shuai <suagrfillet@gmail.com>
Cc:     paul.walmsley@sifive.com, palmer@dabbelt.com,
        aou@eecs.berkeley.edu, robh+dt@kernel.org, frowand.list@gmail.com,
        ajones@ventanamicro.com, mpe@ellerman.id.au, arnd@arndb.de,
        rppt@kernel.org, samuel@sholland.org, panqinglin2020@iscas.ac.cn,
        conor.dooley@microchip.com, anup@brainfault.org,
        xianting.tian@linux.alibaba.com, anshuman.khandual@arm.com,
        heiko@sntech.de, linux-riscv@lists.infradead.org,
        linux-kernel@vger.kernel.org, devicetree@vger.kernel.org
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On Tue, Jun 27, 2023 at 5:13=E2=80=AFPM Song Shuai <suagrfillet@gmail.com> =
wrote:
>
> Hi Alex,
>
> =E5=9C=A8 2023/6/27 19:47, Alexandre Ghiti =E5=86=99=E9=81=93:
> > On Sun, Jun 25, 2023 at 10:36=E2=80=AFPM Alexandre Ghiti <alexghiti@riv=
osinc.com> wrote:
> >>
> >> Hi Song,
> >>
> >> On Sun, Jun 25, 2023 at 4:10=E2=80=AFPM Song Shuai <songshuaishuai@tin=
ylab.org> wrote:
> >>>
> >>> We have encountered these two issues about huge-paged linear mapping =
since v6.4-rc1:
> >>>
> >>> 1. Bug report: kernel paniced when system hibernates[1]
> >>>
> >>> OpenSbi [v0.8,v1.3) set the PMP regions as !no-map, and the commit 33=
35068f8721
> >>> ("riscv: Use PUD/P4D/PGD pages for the linear mapping") mapped them i=
n linear mapping.
> >>> The hibernation process attempted to save/restore these mapped region=
s resulting in access fault.
> >>>
> >>> This issue was temporarily fixed by commit ed309ce52218 ("RISC-V: mar=
k hibernation as nonportable").
> >>> But as Alex's RFC and Rob's response stats in another thread [2] ,
> >>> "Hibernation is only one case. Speculative accesses could also occur.=
"
> >>> So this fixing commit seems not the perfect answer to this issue.
> >>>
> >>>
> >>> 2. Bug report: kernel paniced while booting (with UEFI )[3]
> >>>
> >>> During the booting with UEFI, UEFI Memory Mapping overwrote the membl=
ock.
> >>> The phys_ram_base was set as the end address of mmoderes0 (like 0x800=
40000 for 256 KiB mmoderes0@80000000),
> >>> which resulted the VA based on 2M-aligned PA was not 2M-aligned using=
 va_pa_offset
> >>> (PAGE_OFFSET - phys_ram_base) to translate.
> >>>
> >>> The best_map_size() from commit 3335068f8721 didn't check the virtual=
 alignment
> >>> before choosing a map size. and then a "VA hole" was created where pa=
ge faults always occurred.
> >>>
> >>> This issue was fixed by commit 49a0a3731596 ("riscv: Check the virtua=
l alignment before choosing a map size"),
> >>> But this fixing commit has a side-effect ("the possible third one" as=
 Alex said in this thread).
> >>> There are numerous PTE allocations slowing down the boot time and con=
suming some system memory when UEFI booting
> >>> (Note that it's not involved when booting directly with OpenSbi, wher=
e phys_ram_base is the 2M-aligned base of DRAM).
> >>>
> >>> In my test, compared with/out reverting both commit 49a0a3731596 and =
commit 3335068f8721,
> >>> I must wait ~20s for the linear mapping creation and mem_init_print_i=
nfo() reported ~8M extra reserved memory.
> >>
> >> Indeed, phys_ram_base is not aligned on a 2MB boundary when booting
> >> with EDK2, IIRC that's because the first chunk of memory at
> >> 0x8000_0000 is marked as UC and is then completely evicted.
> >>
> >>>
> >>> To eliminate this side-effect, We should find a way to align VA and P=
A on a 2MB boundary.
> >>> The simplest way is reverting the commit 3335068f8721 ("riscv: Use PU=
D/P4D/PGD pages for the linear mapping").
> >>>
> >>
> >> I disagree, the simplest way is to align phys_ram_base on a 2MB
> >> boundary, either by aligning to the upper boundary (and then wastes
> >> memory, like we used to) or by aligning to the lower boundary (but I
> >> want to make sure it works).
> >>
> >> I'll come up with something tomorrow.
> >
> > @Song Shuai : can you test the following please?
> >
> > commit a35b5e5e3f29e415f54fea064176315e79e21fb7 (HEAD ->
> > dev/alex/align_va_pa_v1)
> > Author: Alexandre Ghiti <alexghiti@rivosinc.com>
> > Date:   Mon Jun 5 14:26:55 2023 +0000
> >
> >      riscv: Start of DRAM should at least be aligned on PMD size for
> > the direct mapping
> >
> >      So that we do not end up mapping the whole linear mapping using 4K
> >      pages, which is slow at boot time, and also very likely at runtime=
.
> >
> >      So make sure we align the start of DRAM on a PMD boundary.
> >
> >      Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
> >
> > diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
> > index 4fa420faa780..4a43ec275c6d 100644
> > --- a/arch/riscv/mm/init.c
> > +++ b/arch/riscv/mm/init.c
> > @@ -214,8 +214,13 @@ static void __init setup_bootmem(void)
> >          memblock_reserve(vmlinux_start, vmlinux_end - vmlinux_start);
> >
> >          phys_ram_end =3D memblock_end_of_DRAM();
> > +
> > +       /*
> > +        * Make sure we align the start of the memory on a PMD boundary=
 so that
> > +        * at worst, we map the linear mapping with PMD mappings.
> > +        */
> >          if (!IS_ENABLED(CONFIG_XIP_KERNEL))
> > -               phys_ram_base =3D memblock_start_of_DRAM();
> > +               phys_ram_base =3D memblock_start_of_DRAM() & PMD_MASK;
> >
> >          /*
> >           * In 64-bit, any use of __va/__pa before this point is wrong =
as we
> >
> I tested your patch and it really fixed what I concerned :
>
> `There are numerous PTE allocations slowing down the boot time and
> consuming some system memory when UEFI booting.`
>
> FYI, I posted the `ptdmp` and the available memory with/out the patch
> from my test:
>
> ```
>  >> v6.4
>
> ---[ Linear mapping ]---
> 0xff60000000000000-0xff600000001c0000    0x0000000080040000      1792K
> PTE     D A G . . W R V
> 0xff600000001c0000-0xff60000000bc0000    0x0000000080200000        10M
> PTE     D A G . . . R V
> 0xff60000000bc0000-0xff60000000fc0000    0x0000000080c00000         4M
> PTE     D A G . . W R V
> 0xff60000000fc0000-0xff600000015c0000    0x0000000081000000         6M
> PTE     D A G . . . R V
> 0xff600000015c0000-0xff600000ffdfd000    0x0000000081600000   4169972K
> PTE     D A G . . W R V
> 0xff600000fffbf000-0xff600000fffc0000    0x000000017ffff000         4K
> PTE     D A G . . W R V
>
>  >> v6.4 with the patch
>
> ---[ Linear mapping ]---
> 0xff60000000040000-0xff60000000200000    0x0000000080040000      1792K
> PTE     D A G . . W R V
> 0xff60000000200000-0xff60000000c00000    0x0000000080200000        10M
> PMD     D A G . . . R V
> 0xff60000000c00000-0xff60000001000000    0x0000000080c00000         4M
> PMD     D A G . . W R V
> 0xff60000001000000-0xff60000001600000    0x0000000081000000         6M
> PMD     D A G . . . R V
> 0xff60000001600000-0xff60000040000000    0x0000000081600000      1002M
> PMD     D A G . . W R V
> 0xff60000040000000-0xff600000c0000000    0x00000000c0000000         2G
> PUD     D A G . . W R V
> 0xff600000c0000000-0xff600000ffe00000    0x0000000140000000      1022M
> PMD     D A G . . W R V
> 0xff600000ffe00000-0xff600000ffe3d000    0x000000017fe00000       244K
> PTE     D A G . . W R V
> 0xff600000fffff000-0xff60000100000000    0x000000017ffff000         4K
> PTE     D A G . . W R V
> ```
>
> ```
>  >> v6.4
>
> Memory: 3945340K/4194048K available (8391K kernel code, 4959K rwdata,
> 4096K rodata, 2195K init, 476K bss, 248708K reserved, 0K cma-reserved)
>
>  >> v6.4 with the patch
>
> Memory: 3953524K/4194048K available (8391K kernel code, 4959K rwdata,
> 4096K rodata, 2195K init, 476K bss, 240524K reserved, 0K cma-reserved)
> ```
> > An example of output when phys_ram_base is not aligned on a 2MB boundar=
y:
> >
> > ---[ Linear mapping ]---
> > 0xffffaf8000008000-0xffffaf8000200000    0x0000000080008000      2016K
> > PTE     D A G . . W R V
> > 0xffffaf8000200000-0xffffaf8000e00000    0x0000000080200000        12M
> > PMD     D A G . . . R V
> > 0xffffaf8000e00000-0xffffaf8001200000    0x0000000080e00000         4M
> > PMD     D A G . . W R V
> > 0xffffaf8001200000-0xffffaf8001a00000    0x0000000081200000         8M
> > PMD     D A G . . . R V
> > 0xffffaf8001a00000-0xffffaf807e600000    0x0000000081a00000      1996M
> > PMD     D A G . . W R V
> > 0xffffaf807e600000-0xffffaf807e714000    0x00000000fe600000      1104K
> > PTE     D A G . . W R V
> > 0xffffaf807e715000-0xffffaf807e718000    0x00000000fe715000        12K
> > PTE     D A G . . W R V
> > 0xffffaf807e71b000-0xffffaf807e71c000    0x00000000fe71b000         4K
> > PTE     D A G . . W R V
> > 0xffffaf807e720000-0xffffaf807e800000    0x00000000fe720000       896K
> > PTE     D A G . . W R V
> > 0xffffaf807e800000-0xffffaf807fe00000    0x00000000fe800000        22M
> > PMD     D A G . . W R V
> > 0xffffaf807fe00000-0xffffaf807ff54000    0x00000000ffe00000      1360K
> > PTE     D A G . . W R V
> > 0xffffaf807ff55000-0xffffaf8080000000    0x00000000fff55000       684K
> > PTE     D A G . . W R V
> > 0xffffaf8080000000-0xffffaf83c0000000    0x0000000100000000        13G
> > PUD     D A G . . W R V
> > 0xffffaf83c0000000-0xffffaf83ffe00000    0x0000000440000000      1022M
> > PMD     D A G . . W R V
> > 0xffffaf83ffe00000-0xffffaf8400000000    0x000000047fe00000         2M
> > PTE     D A G . . W R V
> >
> > I found that it was easier to align phys_ram_base on the lower 2MB
> > boundary. Aligning on the upper boundary is more complicated to me
> > since there may be "something" between phys_ram_base and the upper 2MB
> > boundary that needs to be accessed using the linear mapping (DT is
> > accessed using fixmap so not a problem, initrd? ACPI tables? I don't
> > know actually).
> >
>
> And would there be possible influence from the diff between
> phys_ram_base and memblock_start_of_DRAM()?

I don't know why there would be (doesn't mean there is not), and arm64
also does that https://elixir.bootlin.com/linux/latest/source/arch/arm64/mm=
/init.c#L285

>
> > Weirdly simple though, I may be missing something, so any comment/test
> > is welcome, it is currently running our internal CI.
> >
> > Thanks,
> >
> > Alex
> >
> >>
> >> Thanks,
> >>
> >> Alex
> >>
> >>>
> >>>
> >>> Using PUD/P4D/PGD pages for the linear mapping to improve the perform=
ance is marginal from a recent talk [4]
> >>> from Mike Rapoport. OpenSbi had marked all the PMP-protected regions =
as "no-map" [5] to practice this talk.
> >>>
>
> I noticed best_map_size() still creates some huge-paged mapping (like
> above 2G PUD) with this patch.
> How about to revert best_map_size() to disable huge-paged mapping to
> practice the Mike's talk.
>

Mike's talk does not state that using PUD (and above) mappings is bad,
but just that it may not be worth the effort to try and keep them at
all cost during kernel life (they happen to be split by allocations)
since the performance benefit is marginal (if it even exists). On real
systems with terabytes of memory, this will help with boot duration
and memory consumption (ok, not significantly for this type of
systems).

Thanks for testing!


> >>> For all those reasons, let's revert these related commits:
> >>>
> >>> - commit 3335068f8721 ("riscv: Use PUD/P4D/PGD pages for the linear m=
apping")
> >>> - commit 49a0a3731596 ("riscv: Check the virtual alignment before cho=
osing a map size")
> >>> - commit ed309ce52218 ("RISC-V: mark hibernation as nonportable")
> >>>
> >>> [1]: https://lore.kernel.org/linux-riscv/CAAYs2=3DgQvkhTeioMmqRDVGjdt=
NF_vhB+vm_1dHJxPNi75YDQ_Q@mail.gmail.com/
> >>> [2]: https://lore.kernel.org/linux-kernel/20230530080425.18612-1-alex=
ghiti@rivosinc.com/
> >>> [3]: https://lore.kernel.org/linux-riscv/tencent_7C3B580B47C1B17C1648=
8EC1@qq.com/
> >>> [4]: https://lwn.net/Articles/931406/
> >>> [5]: https://github.com/riscv-software-src/opensbi/commit/8153b2622b0=
8802cc542f30a1fcba407a5667ab9
> >>>
> >>> Song Shuai (3):
> >>>    Revert "RISC-V: mark hibernation as nonportable"
> >>>    Revert "riscv: Check the virtual alignment before choosing a map s=
ize"
> >>>    Revert "riscv: Use PUD/P4D/PGD pages for the linear mapping"
> >>>
> >>>   arch/riscv/Kconfig            |  5 +---
> >>>   arch/riscv/include/asm/page.h | 16 -------------
> >>>   arch/riscv/mm/init.c          | 43 +++++++-------------------------=
---
> >>>   arch/riscv/mm/physaddr.c      | 16 -------------
> >>>   drivers/of/fdt.c              | 11 ++++-----
> >>>   5 files changed, 14 insertions(+), 77 deletions(-)
> >>>
> >>> --
> >>> 2.20.1
> >>>
> >
>
> --
> Thanks
> Song Shuai