linux-kernel - Re: [RESEND PATCH v4 5/6] arm64/mm: Populate the swapper_pg

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Date:   Thu, 13 Sep 2018 18:50:21 +0800
From:   Jun Yao <yaojun8558363@...il.com>
To:     James Morse <james.morse@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, catalin.marinas@....com,
        will.deacon@....com, linux-kernel@...r.kernel.org
Subject: Re: [RESEND PATCH v4 5/6] arm64/mm: Populate the swapper_pg_dir by
 fixmap.

Hi James,

On Fri, Sep 07, 2018 at 10:58:22AM +0100, James Morse wrote:
> On 22/08/18 10:54, Jun Yao wrote:
> >  	WRITE_ONCE(*pmdp, pmd);
> >  	dsb(ishst);
> >  }
> > @@ -480,6 +511,19 @@ static inline phys_addr_t pmd_page_paddr(pmd_t pmd)
> >  
> >  static inline void set_pud(pud_t *pudp, pud_t pud)
> >  {
> > +#ifdef __PAGETABLE_PUD_FOLDED
> > +	if (in_swapper_pgdir(pudp)) {
> > +		pud_t *fixmap_pudp;
> > +
> > +		spin_lock(&swapper_pgdir_lock);
> > +		fixmap_pudp = (pud_t *)pgd_set_fixmap(__pa(pudp));
>
> I see this called 68 times during boot on a 64K/42bit-VA, 65 of which appear to
> be during paging_init(). What do you think to keeping paging_init()s use of the
> pgd fixmap for swapper_pg_dir, deliberately to skip the in_swapper_pgdir() test
> during paging_init()?

I find that the __create_pgd_mapping() is used to set up the page table
during paging_init(). And there are six functions calling it with
different pgdps:

update_mapping_prot()		init_mm.pgd(swapper_pg_dir)
create_mapping_noalloc()	init_mm.pgd(swapper_pg_dir)
__map_memblock()		pgdp(swapper_pg_dir)
map_kernel_segment()		pgdp(swapper_pg_dir)
create_pgd_mapping()		!(init_mm.pgd)
map_entry_trampoline()		tramp_pg_dir

In order to skip the in_swapper_pgdir() test during paging_init(), We
need a way to determine if we are currently in paging_init(). The way I
can think of is to create a function similar to __create_pgd_mapping().
And it is used to create the page table during paging_init(). It differs
from the __create_pgd_mapping() only in that it calls
p?d_populate_without_test(). However, in this way, I'm worried that I am
reinventing the wheel.

Thanks,

Jun