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

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Date:   Fri, 1 Mar 2019 11:24:25 +0000
From:   Steven Price <steven.price@....com>
To:     Dave Hansen <dave.hansen@...el.com>, linux-mm@...ck.org
Cc:     Mark Rutland <Mark.Rutland@....com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Arnd Bergmann <arnd@...db.de>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Catalin Marinas <catalin.marinas@....com>, x86@...nel.org,
        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 28/02/2019 19:00, Dave Hansen wrote:
> 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?

That is the only use I'm currently aware of.

>> 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.

If no-one has any objections to dropping the holes in the output, then I
can rebase on something like below and drop this 'depth' patch.

Steve

----8<----
>From a9eabadfc212389068ec5cc60265c7a55585bb76 Mon Sep 17 00:00:00 2001
From: Steven Price <steven.price@....com>
Date: Fri, 1 Mar 2019 10:06:33 +0000
Subject: [PATCH] x86: mm: Hide page table holes in debugfs

For the /sys/kernel/debug/page_tables/ files, rather than outputing a
mostly empty line when a block of memory isn't present just skip the
line. This keeps the output shorter and will help with a future change
switching to using the generic page walk code as we no longer care about
the 'level' that the page table holes are at.

Signed-off-by: Steven Price <steven.price@....com>
---
 arch/x86/mm/dump_pagetables.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/arch/x86/mm/dump_pagetables.c b/arch/x86/mm/dump_pagetables.c
index e3cdc85ce5b6..a0f4139631dd 100644
--- a/arch/x86/mm/dump_pagetables.c
+++ b/arch/x86/mm/dump_pagetables.c
@@ -304,8 +304,8 @@ static void note_page(struct seq_file *m, struct
pg_state *st,
 		/*
 		 * Now print the actual finished series
 		 */
-		if (!st->marker->max_lines ||
-		    st->lines < st->marker->max_lines) {
+		if ((cur & _PAGE_PRESENT) && (!st->marker->max_lines ||
+		    st->lines < st->marker->max_lines)) {
 			pt_dump_seq_printf(m, st->to_dmesg,
 					   "0x%0*lx-0x%0*lx   ",
 					   width, st->start_address,
@@ -321,7 +321,9 @@ static void note_page(struct seq_file *m, struct
pg_state *st,
 			printk_prot(m, st->current_prot, st->level,
 				    st->to_dmesg);
 		}
-		st->lines++;
+		if (cur & _PAGE_PRESENT) {
+			st->lines++;
+		}

 		/*
 		 * We print markers for special areas of address space,
-- 
2.20.1