[External] Re: [PATCH v2] riscv: mm: Fix alignment of phys_ram_base

[Xu Lu]

Hi Alex,

On Thu, Dec 5, 2024 at 10:34 PM Alexandre Ghiti <alex at ghiti.fr> wrote:
>
> Hi Xu,
>
> On 03/12/2024 15:49, Xu Lu wrote:
> > This commit fixes the alignment of phys_ram_base in RISC-V.
> >
> > In sparse vmemmap model, the virtual address of vmemmap is calculated as:
> > ((struct page *)VMEMMAP_START - (phys_ram_base >> PAGE_SHIFT)).
> > And the struct page's va can be calculated with an offset:
> > (vmemmap + (pfn)).
> >
> > However, when initializing struct pages, kernel actually starts from the
> > first page from the same section that phys_ram_base belongs to. If the
> > first page's physical address is not (phys_ram_base >> PAGE_SHIFT), then
> > we get an va below VMEMMAP_START when calculating va for it's struct page.
> >
> > For example, if phys_ram_base starts from 0x82000000 with pfn 0x82000, the
> > first page in the same section is actually pfn 0x80000. During
> > init_unavailable_range(), we will initialize struct page for pfn 0x80000
> > with virtual address ((struct page *)VMEMMAP_START - 0x2000), which is
> > below VMEMMAP_START as well as PCI_IO_END.
> >
> > This commit fixes this bug by aligning phys_ram_base with SECTION_SIZE.
> >
> > Fixes: c3bcc65d4d2e ("riscv: Start of DRAM should at least be aligned on PMD size for the direct mapping")
> > Signed-off-by: Xu Lu <luxu.kernel at bytedance.com>
> > ---
> >   arch/riscv/mm/init.c | 15 ++++++++++++++-
> >   1 file changed, 14 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
> > index 0e8c20adcd98..974cafa7c85e 100644
> > --- a/arch/riscv/mm/init.c
> > +++ b/arch/riscv/mm/init.c
> > @@ -33,6 +33,9 @@
> >   #include <asm/pgtable.h>
> >   #include <asm/sections.h>
> >   #include <asm/soc.h>
> > +#ifdef CONFIG_SPARSEMEM
> > +#include <asm/sparsemem.h>
> > +#endif
> >   #include <asm/tlbflush.h>
> >
> >   #include "../kernel/head.h"
> > @@ -59,6 +62,12 @@ EXPORT_SYMBOL(pgtable_l4_enabled);
> >   EXPORT_SYMBOL(pgtable_l5_enabled);
> >   #endif
> >
> > +#ifdef CONFIG_SPARSEMEM
> > +#define RISCV_MEMSTART_ALIGN (1UL << SECTION_SIZE_BITS)
> > +#else
> > +#define RISCV_MEMSTART_ALIGN PMD_SIZE
> > +#endif
> > +
> >   phys_addr_t phys_ram_base __ro_after_init;
> >   EXPORT_SYMBOL(phys_ram_base);
> >
> > @@ -239,9 +248,13 @@ static void __init setup_bootmem(void)
> >       /*
> >        * 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.
> > +      *
> > +      * Also, make sure we align the start of the memory on a SECTION boundary
> > +      * when CONFIG_SPARSEMEM_VMEMMAP is enabled to ensure the correctness of
> > +      * pfn_to_page().
> >        */
> >       if (!IS_ENABLED(CONFIG_XIP_KERNEL))
> > -             phys_ram_base = memblock_start_of_DRAM() & PMD_MASK;
> > +             phys_ram_base = round_down(memblock_start_of_DRAM(), RISCV_MEMSTART_ALIGN);
> >
> >       /*
> >        * In 64-bit, any use of __va/__pa before this point is wrong as we
>
>
> That's a good catch indeed. But I'm wondering if it would be more
> correct to fix the macro vmemmap instead of phys_ram_base since
> phys_ram_base is supposed to hold the real base of the system memory,
> which would be wrong with your patch. I mean something like that instead
> (or similar, I haven't tested):
>
> #define vmemmap         ((struct page *)VMEMMAP_START -
> (round_down(memblock_start_of_DRAM(), RISCV_MEMSTART_ALIGN) >> PAGE_SHIFT))

Thanks for your comment.

Good idea. I have thought about this. But I wasn't sure if it's OK to
introduce extra calculation whenever pfn_to_page() and page_to_pfn()
is called. So I referred to ARM which aligns memstart_addr with
SECTION size too and then made a similar modification.

If it is not appropriate to change the semantics of phys_ram_base, how
about introducing a new variable vmemmap_start_addr and use it to
calculate vmemmap:

#define vmemmap         ((struct page *)VMEMMAP_START -
(vmemmap_start_addr >> PAGE_SHIFT))

Best Regards,

Xu Lu

>
> And the fixes tag should be:
>
> Fixes: a11dd49dcb93 ("riscv: Sparse-Memory/vmemmap out-of-bounds fix")
>
> Thanks,
>
> Alex
>