linux-kernel - Re: [PATCH] x86/mm: Split vmalloc_sync

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Date:   Mon, 7 Oct 2019 08:30:51 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Joerg Roedel <joro@...tes.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>
Cc:     hpa@...or.com, x86@...nel.org, rjw@...ysocki.net, lenb@...nel.org,
        james.morse@....com, tony.luck@...el.com,
        linux-kernel@...r.kernel.org, linux-acpi@...r.kernel.org,
        linux-mm@...ck.org, Joerg Roedel <jroedel@...e.de>
Subject: Re: [PATCH] x86/mm: Split vmalloc_sync_all()

On 10/7/19 8:16 AM, Joerg Roedel wrote:
> @@ -318,7 +328,7 @@ static void dump_pagetable(unsigned long address)
>  
>  #else /* CONFIG_X86_64: */
>  
> -void vmalloc_sync_all(void)
> +void vmalloc_sync_mappings(void)
>  {
>  	sync_global_pgds(VMALLOC_START & PGDIR_MASK, VMALLOC_END);
>  }

FWIW, I generally detest the use of __weak. :)

In this case, it ends up letting us gloss over the fact that we have a
32/64-bit asymmetry.  It would probably be nice to actually have a
64-bit implementation that comes along with a nice comment.  Maybe this
in vmalloc_sync_mappings():

	/*
	 * 64-bit mappings might allocate new p4d/pud pages
	 * that need to be propagated to all tasks' PGDs.
	 */

which would pair nicely with:

void vmalloc_sync_unmappings(void)
{
	/*
	 * Unmappings never allocate or free p4d/pud pages.
	 * No work is required here.
	 */
}