linux-kernel - Re: [PATCH v3 27/34] mm: pagewalk: Add 'depth' parameter to pte

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Date:   Thu, 28 Feb 2019 11:00:17 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Steven Price <steven.price@....com>, linux-mm@...ck.org
Cc:     Mark Rutland <Mark.Rutland@....com>, x86@...nel.org,
        Arnd Bergmann <arnd@...db.de>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Will Deacon <will.deacon@....com>,
        linux-kernel@...r.kernel.org,
        Jérôme Glisse <jglisse@...hat.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        James Morse <james.morse@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-arm-kernel@...ts.infradead.org,
        "Liang, Kan" <kan.liang@...ux.intel.com>
Subject: Re: [PATCH v3 27/34] mm: pagewalk: Add 'depth' parameter to pte_hole

On 2/28/19 3:28 AM, Steven Price wrote:
> static int get_level(unsigned long addr, unsigned long end)
> {
> 	/* Add 1 to account for ~0ULL */
> 	unsigned long size = (end - addr) + 1;
> 	if (size < PMD_SIZE)
> 		return 4;
> 	else if (size < PUD_SIZE)
> 		return 3;
> 	else if (size < P4D_SIZE)
> 		return 2;
> 	else if (size < PGD_SIZE)
> 		return 1;
> 	return 0;
> }
> 
> There are two immediate problems with that:
> 
>  * The "+1" to deal with ~0ULL is fragile
> 
>  * PGD_SIZE isn't what you might expect, it's not defined for most
> architectures and arm64/x86 use it as the size of the PGD table.
> Although that's easy enough to fix up.
> 
> Do you think a function like above would be preferable?

The question still stands of why we *need* the depth/level in the first
place.  As I said, we obviously need it for printing out the "name" of
the level.  Is that it?

> The other option would of course be to just drop the information from
> the debugfs file about at which level the holes are. But it can be
> useful information to see whether there are empty levels in the page
> table structure. Although this is an area where x86 and arm64 differ
> currently (x86 explicitly shows the gaps, arm64 doesn't), so if x86
> doesn't mind losing that functionality that would certainly simplify things!

I think I'd actually be OK with the holes just not showing up.  I
actually find it kinda hard to read sometimes with the holes in there.
I'd be curious what others think though.