linux-kernel - [PATCH v2] x86/mm/pti: in pti_clone_pgtable(), increase addr properly

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Date:   Tue, 20 Aug 2019 13:23:14 -0700
From:   Song Liu <songliubraving@...com>
To:     <linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>
CC:     <kernel-team@...com>, Song Liu <songliubraving@...com>,
        <stable@...r.kernel.org>, Joerg Roedel <jroedel@...e.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: [PATCH v2] x86/mm/pti: in pti_clone_pgtable(), increase addr properly

Before 32-bit support, pti_clone_pmds() always adds PMD_SIZE to addr.
This behavior changes after the 32-bit support:  pti_clone_pgtable()
increases addr by PUD_SIZE for pud_none(*pud) case, and increases addr by
PMD_SIZE for pmd_none(*pmd) case. However, this is not accurate because
addr may not be PUD_SIZE/PMD_SIZE aligned.

Fix this issue by properly rounding up addr to next PUD_SIZE/PMD_SIZE
in these two cases.

The following explains how we debugged this issue:

We use huge page for hot text and thus reduces iTLB misses. As we
benchmark 5.2 based kernel (vs. 4.16 based), we found ~2.5x more
iTLB misses.

To figure out the issue, I use a debug patch that dumps page table for
a pid. The following are information from the workload pid.

For the 4.16 based kernel:

host-4.16 # grep "x  pmd" /sys/kernel/debug/page_tables/dump_pid
0x0000000000600000-0x0000000000e00000           8M USR ro         PSE         x  pmd
0xffffffff81a00000-0xffffffff81c00000           2M     ro         PSE         x  pmd

For the 5.2 based kernel before this patch:

host-5.2-before # grep "x  pmd" /sys/kernel/debug/page_tables/dump_pid
0x0000000000600000-0x0000000000e00000           8M USR ro         PSE         x  pmd

The 8MB text in pmd is from user space. 4.16 kernel has 1 pmd for the
irq entry table; while 4.16 kernel doesn't have it.

For the 5.2 based kernel after this patch:

host-5.2-after # grep "x  pmd" /sys/kernel/debug/page_tables/dump_pid
0x0000000000600000-0x0000000000e00000           8M USR ro         PSE         x  pmd
0xffffffff81000000-0xffffffff81e00000          14M     ro         PSE     GLB x  pmd

So after this patch, the 5.2 based kernel has 7 PMDs instead of 1 PMD
in 4.16 kernel. This further reduces iTLB miss rate

Cc: stable@...r.kernel.org # v4.19+
Fixes: 16a3fe634f6a ("x86/mm/pti: Clone kernel-image on PTE level for 32 bit")
Reviewed-by: Rik van Riel <riel@...riel.com>
Signed-off-by: Song Liu <songliubraving@...com>
Cc: Joerg Roedel <jroedel@...e.de>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: Andy Lutomirski <luto@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>
---
 arch/x86/mm/pti.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/mm/pti.c b/arch/x86/mm/pti.c
index b196524759ec..1337494e22ef 100644
--- a/arch/x86/mm/pti.c
+++ b/arch/x86/mm/pti.c
@@ -330,13 +330,13 @@ pti_clone_pgtable(unsigned long start, unsigned long end,

 		pud = pud_offset(p4d, addr);
 		if (pud_none(*pud)) {
-			addr += PUD_SIZE;
+			addr = round_up(addr + 1, PUD_SIZE);
 			continue;
 		}

 		pmd = pmd_offset(pud, addr);
 		if (pmd_none(*pmd)) {
-			addr += PMD_SIZE;
+			addr = round_up(addr + 1, PMD_SIZE);
 			continue;
 		}

-- 
2.17.1