Re: [PATCH] x86/mm/pti: in pti_clone_pgtable() don't increase addr by PUD_SIZE

Peter Zijlstra
2019-08-20T10:01:02Z

On Tue, Aug 20, 2019 at 12:51:28AM -0700, Song Liu wrote:
> pti_clone_pgtable() increases addr by PUD_SIZE for pud_none(*pud) case.
> This is not accurate because addr may not be PUD_SIZE aligned.
>
> In our x86_64 kernel, pti_clone_pgtable() fails to clone 7 PMDs because
> of this issuse, including PMD for the irq entry table. For a memcache
> like workload, this introduces about 4.5x more iTLB-load and about 2.5x
> more iTLB-load-misses on a Skylake CPU.
>
> This patch fixes this issue by adding PMD_SIZE to addr for pud_none()
> case.

> diff --git a/arch/x86/mm/pti.c b/arch/x86/mm/pti.c
> index b196524759ec..5a67c3015f59 100644
> --- a/arch/x86/mm/pti.c
> +++ b/arch/x86/mm/pti.c
> @@ -330,7 +330,7 @@ pti_clone_pgtable(unsigned long start, unsigned long end,
>
> pud = pud_offset(p4d, addr);
> if (pud_none(*pud)) {
> - addr += PUD_SIZE;
> + addr += PMD_SIZE;
> continue;
> }

I'm thinking you're right in that there's a bug here, but I'm also
thinking your patch is both incomplete and broken.

What that code wants to do is skip to the end of the pud, a pmd_size
increase will not do that. And right below this, there's a second
instance of this exact pattern.

Did I get the below right?

---
diff --git a/arch/x86/mm/pti.c b/arch/x86/mm/pti.c
index b196524759ec..32b20b3cb227 100644
--- a/arch/x86/mm/pti.c
+++ b/arch/x86/mm/pti.c
@@ -330,12 +330,14 @@ pti_clone_pgtable(unsigned long start, unsigned long end,

pud = pud_offset(p4d, addr);
if (pud_none(*pud)) {
+ addr &= PUD_MASK;
addr += PUD_SIZE;
continue;
}

pmd = pmd_offset(pud, addr);
if (pmd_none(*pmd)) {
+ addr &= PMD_MASK;
addr += PMD_SIZE;
continue;
}