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

[Song Liu <songliubraving@fb.com>]

> On Aug 20, 2019, at 1:23 PM, Song Liu <songliubraving@fb.com> wrote:
> 
> 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.

After poking around more, I found the following doesn't really make 
sense. 

Sorry for the noise. 
Song


<nonsense> 

> 
> 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

</nonsense>

> 
> Cc: stable@vger.kernel.org # v4.19+
> Fixes: 16a3fe634f6a ("x86/mm/pti: Clone kernel-image on PTE level for 32 bit")
> Reviewed-by: Rik van Riel <riel@surriel.com>
> 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 | 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
>