Messages in this thread | | | From | Song Liu <> | Subject | Re: [PATCH] x86/mm/pti: in pti_clone_pgtable() don't increase addr by PUD_SIZE | Date | Tue, 20 Aug 2019 13:17:02 +0000 |
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> On Aug 20, 2019, at 2:12 AM, Thomas Gleixner <tglx@linutronix.de> wrote: > > On Tue, 20 Aug 2019, 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. > > You fail to explain how this happened. The code before the 32bit support > did always increase by PMD_SIZE. The 32bit support broke that.
Will fix.
> >> 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 information is largely irrelevant. What matters is the fact that this > got broken and incorrectly forwards the address by PUD_SIZE which is wrong > if address is not PUD_SIZE aligned.
We started looking into this because we cannot explain the regression in iTLB miss rate. I guess the patch itself explains it pretty well, so the original issue doesn't matter that much?
I will remove this part.
> >> This patch fixes this issue by adding PMD_SIZE to addr for pud_none() >> case. > > git grep 'This patch' Documentation/process/submitting-patches.rst
Will fix.
>> Cc: stable@vger.kernel.org # v4.19+ >> Fixes: 16a3fe634f6a ("x86/mm/pti: Clone kernel-image on PTE level for 32 bit") >> Signed-off-by: Song Liu <songliubraving@fb.com> >> Cc: Joerg Roedel <jroedel@suse.de> >> Cc: Thomas Gleixner <tglx@linutronix.de> >> Cc: Dave Hansen <dave.hansen@linux.intel.com> >> Cc: Andy Lutomirski <luto@kernel.org> >> Cc: Peter Zijlstra <peterz@infradead.org> >> --- >> arch/x86/mm/pti.c | 2 +- >> 1 file changed, 1 insertion(+), 1 deletion(-) >> >> 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; > > The right fix is to skip forward to the next PUD boundary instead of doing > this in a loop with PMD_SIZE increments.
Agreed.
Thanks, Song
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