linux-kernel - Re: [PATCH v2 1/8] riscv: mm: Pre-allocate vmemmap/direct map PGD entries

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Date: Tue, 14 May 2024 18:33:09 +0200
From: Björn Töpel <bjorn@...nel.org>
To: Alexandre Ghiti <alexghiti@...osinc.com>
Cc: Albert Ou <aou@...s.berkeley.edu>, David Hildenbrand <david@...hat.com>,
 Palmer Dabbelt <palmer@...belt.com>, Paul Walmsley
 <paul.walmsley@...ive.com>, linux-riscv@...ts.infradead.org,
 Björn Töpel
 <bjorn@...osinc.com>, Andrew Bresticker <abrestic@...osinc.com>, Chethan
 Seshadri <Chethan.Seshadri@...alinasystems.io>, Lorenzo Stoakes
 <lstoakes@...il.com>, Oscar Salvador <osalvador@...e.de>, Santosh Mamila
 <santosh.mamila@...alinasystems.io>, Sivakumar Munnangi
 <siva.munnangi@...alinasystems.io>, Sunil V L <sunilvl@...tanamicro.com>,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v2 1/8] riscv: mm: Pre-allocate vmemmap/direct map PGD
 entries

Alexandre Ghiti <alexghiti@...osinc.com> writes:

> Hi Björn,
>
> On Tue, May 14, 2024 at 4:05 PM Björn Töpel <bjorn@...nel.org> wrote:
>>
>> From: Björn Töpel <bjorn@...osinc.com>
>>
>> The RISC-V port copies the PGD table from init_mm/swapper_pg_dir to
>> all userland page tables, which means that if the PGD level table is
>> changed, other page tables has to be updated as well.
>>
>> Instead of having the PGD changes ripple out to all tables, the
>> synchronization can be avoided by pre-allocating the PGD entries/pages
>> at boot, avoiding the synchronization all together.
>>
>> This is currently done for the bpf/modules, and vmalloc PGD regions.
>> Extend this scheme for the PGD regions touched by memory hotplugging.
>>
>> Prepare the RISC-V port for memory hotplug by pre-allocate
>> vmemmap/direct map entries at the PGD level. This will roughly waste
>> ~128 worth of 4K pages when memory hotplugging is enabled in the
>> kernel configuration.
>>
>> Signed-off-by: Björn Töpel <bjorn@...osinc.com>
>> ---
>>  arch/riscv/include/asm/kasan.h | 4 ++--
>>  arch/riscv/mm/init.c           | 7 +++++++
>>  2 files changed, 9 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/riscv/include/asm/kasan.h b/arch/riscv/include/asm/kasan.h
>> index 0b85e363e778..e6a0071bdb56 100644
>> --- a/arch/riscv/include/asm/kasan.h
>> +++ b/arch/riscv/include/asm/kasan.h
>> @@ -6,8 +6,6 @@
>>
>>  #ifndef __ASSEMBLY__
>>
>> -#ifdef CONFIG_KASAN
>> -
>>  /*
>>   * The following comment was copied from arm64:
>>   * KASAN_SHADOW_START: beginning of the kernel virtual addresses.
>> @@ -34,6 +32,8 @@
>>   */
>>  #define KASAN_SHADOW_START     ((KASAN_SHADOW_END - KASAN_SHADOW_SIZE) & PGDIR_MASK)
>>  #define KASAN_SHADOW_END       MODULES_LOWEST_VADDR
>> +
>> +#ifdef CONFIG_KASAN
>>  #define KASAN_SHADOW_OFFSET    _AC(CONFIG_KASAN_SHADOW_OFFSET, UL)
>>
>>  void kasan_init(void);
>> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
>> index 2574f6a3b0e7..5b8cdfafb52a 100644
>> --- a/arch/riscv/mm/init.c
>> +++ b/arch/riscv/mm/init.c
>> @@ -27,6 +27,7 @@
>>
>>  #include <asm/fixmap.h>
>>  #include <asm/io.h>
>> +#include <asm/kasan.h>
>>  #include <asm/numa.h>
>>  #include <asm/pgtable.h>
>>  #include <asm/sections.h>
>> @@ -1488,10 +1489,16 @@ static void __init preallocate_pgd_pages_range(unsigned long start, unsigned lon
>>         panic("Failed to pre-allocate %s pages for %s area\n", lvl, area);
>>  }
>>
>> +#define PAGE_END KASAN_SHADOW_START
>> +
>>  void __init pgtable_cache_init(void)
>>  {
>>         preallocate_pgd_pages_range(VMALLOC_START, VMALLOC_END, "vmalloc");
>>         if (IS_ENABLED(CONFIG_MODULES))
>>                 preallocate_pgd_pages_range(MODULES_VADDR, MODULES_END, "bpf/modules");
>> +       if (IS_ENABLED(CONFIG_MEMORY_HOTPLUG)) {
>> +               preallocate_pgd_pages_range(VMEMMAP_START, VMEMMAP_END, "vmemmap");
>> +               preallocate_pgd_pages_range(PAGE_OFFSET, PAGE_END, "direct map");
>> +       }
>>  }
>>  #endif
>> --
>> 2.40.1
>>
>
> As you asked, with
> https://lore.kernel.org/linux-riscv/20240514133614.87813-1-alexghiti@rivosinc.com/T/#u,
> you will be able to remove the usage of KASAN_SHADOW_START.

Very nice -- consistency! I'll need to respin, so I'll clean this up for
the next version.

> But anyhow, you can add:
>
> Reviewed-by: Alexandre Ghiti <alexghiti@...osinc.com>


Thank you!
Björn