Re: API for setting multiple PTEs at once

[Rolf Eike Beer <eb@emlix.com>]

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On Dienstag, 14. Februar 2023 10:55:43 CET Alexandre Ghiti wrote:
> Hi Matthew,
> 
> On 2/7/23 21:27, Matthew Wilcox wrote:
> > On Thu, Feb 02, 2023 at 09:14:23PM +0000, Matthew Wilcox wrote:
> >> For those of you not subscribed, linux-mm is currently discussing
> >> how best to handle page faults on large folios.  I simply made it work
> >> when adding large folio support.  Now Yin Fengwei is working on
> >> making it fast.
> > 
> > OK, here's an actual implementation:
> > 
> > https://lore.kernel.org/linux-mm/20230207194937.122543-3-willy@infradead.o
> > rg/
> > 
> > It survives a run of xfstests.  If your architecture doesn't store its
> > PFNs at PAGE_SHIFT, you're going to want to implement your own set_ptes(),
> > or you'll see entirely the wrong pages mapped into userspace.  You may
> > also wish to implement set_ptes() if it can be done more efficiently
> > than __pte(pteval(pte) + PAGE_SIZE).
> > 
> > Architectures that implement things like flush_icache_page() and
> > update_mmu_cache() may want to propose batched versions of those.
> > That's alpha, csky, m68k, mips, nios2, parisc, sh,
> > arm, loongarch, openrisc, powerpc, riscv, sparc and xtensa.
> > Maintainers BCC'd, mailing lists CC'd.
> > 
> > I'm happy to collect implementations and submit them as part of a v6.
> 
> Please find below the riscv implementation for set_ptes:
> 
> diff --git a/arch/riscv/include/asm/pgtable.h
> b/arch/riscv/include/asm/pgtable.h
> index ebee56d47003..10bf812776a4 100644
> --- a/arch/riscv/include/asm/pgtable.h
> +++ b/arch/riscv/include/asm/pgtable.h
> @@ -463,6 +463,20 @@ static inline void set_pte_at(struct mm_struct *mm,
>          __set_pte_at(mm, addr, ptep, pteval);
>   }
> 
> +#define set_ptes set_ptes
> +static inline void set_ptes(struct mm_struct *mm, unsigned long addr,
> +                           pte_t *ptep, pte_t pte, unsigned int nr)
> +{
> +       for (;;) {
> +               set_pte_at(mm, addr, ptep, pte);
> +               if (--nr == 0)
> +                       break;
> +               ptep++;
> +               addr += PAGE_SIZE;
> +               pte = __pte(pte_val(pte) + (1 << _PAGE_PFN_SHIFT));
> +       }
> +}

Given that this is the same code as the original version (surprise!), what 
about doing something like this in the generic code instead:

#ifndef PTE_PAGE_STEP
#define PTE_PAGE_STEP PAGE_SIZE
#endif

[…]

> +               pte = __pte(pte_val(pte) + PTE_PAGE_STEP);

The name of the define is an obvious candidate for bike-shedding, feel free to 
name it as you want.

Or if that isn't sound enough maybe introduce something like:

static inline pte_t
set_ptes_next_pte(pte_t pte)
{
	return __pte(pte_val(pte) + (1 << _PAGE_PFN_SHIFT));
}

Greetings,

Eike
-- 
Rolf Eike Beer, emlix GmbH, http://www.emlix.com
Fon +49 551 30664-0, Fax +49 551 30664-11
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