linux-kernel - Re: [PATCHv3 15/17] x86/mm: Implement sync_direct

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Date:   Mon, 18 Jun 2018 09:28:27 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Tom Lendacky <thomas.lendacky@....com>
Cc:     Kai Huang <kai.huang@...ux.intel.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCHv3 15/17] x86/mm: Implement sync_direct_mapping()

> index 17383f9677fa..032b9a1ba8e1 100644
> --- a/arch/x86/mm/init_64.c
> +++ b/arch/x86/mm/init_64.c
> @@ -731,6 +731,8 @@ kernel_physical_mapping_init(unsigned long paddr_start,
>  		pgd_changed = true;
>  	}
>  
> +	sync_direct_mapping();
> +
>  	if (pgd_changed)
>  		sync_global_pgds(vaddr_start, vaddr_end - 1);
>  
> @@ -1142,10 +1144,13 @@ void __ref vmemmap_free(unsigned long start, unsigned long end,
>  static void __meminit
>  kernel_physical_mapping_remove(unsigned long start, unsigned long end)
>  {
> +	int ret;
>  	start = (unsigned long)__va(start);
>  	end = (unsigned long)__va(end);
>  
>  	remove_pagetable(start, end, true, NULL);
> +	ret = sync_direct_mapping();
> +	WARN_ON(ret);
>  }

I understand why you implemented it this way, I really do.  It's
certainly the quickest way to hack something together and make a
standalone piece of code.  But, I don't think it's maintainable.

For instance, this call to sync_direct_mapping() could be entirely
replaced by a call to:

	for_each_keyid(k)...
		remove_pagetable(start + offset_per_keyid * k,
			         end   + offset_per_keyid * k,
				 true, NULL);

No?

>  int __ref arch_remove_memory(u64 start, u64 size, struct vmem_altmap *altmap)
> @@ -1290,6 +1295,7 @@ void mark_rodata_ro(void)
>  			(unsigned long) __va(__pa_symbol(rodata_end)),
>  			(unsigned long) __va(__pa_symbol(_sdata)));
>  
> +	sync_direct_mapping();
>  	debug_checkwx();

Huh, checking the return code in some cases and not others.  Curious.
Why is it that way?