linux-kernel - Re: [PATCH] x86/mm/pti: in pti_clone_pgtable() don't increase addr by PUD

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Message-ID: <CCFAB5E0-263E-4D9F-92A5-51922EF5998C@fb.com>
Date:   Tue, 20 Aug 2019 16:05:45 +0000
From:   Song Liu <songliubraving@...com>
To:     Dave Hansen <dave.hansen@...el.com>
CC:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Kernel Team <Kernel-team@...com>,
        "stable@...r.kernel.org" <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: Re: [PATCH] x86/mm/pti: in pti_clone_pgtable() don't increase addr by
 PUD_SIZE



> On Aug 20, 2019, at 7:18 AM, Dave Hansen <dave.hansen@...el.com> wrote:
> 
> On 8/20/19 7:14 AM, Song Liu wrote:
>>> *But*, that shouldn't get hit on a Skylake CPU since those have PCIDs
>>> and shouldn't have a global kernel image.  Could you confirm whether
>>> PCIDs are supported on this CPU?
>> Yes, pcid is listed in /proc/cpuinfo. 
> 
> So what's going on?  Could you confirm exactly which pti_clone_pgtable()
> is causing you problems?  Do you have a theory as to why this manifests
> as a performance problem rather than a functional one?
> 
> A diff of these:
> 
> 	/sys/kernel/debug/page_tables/current_user
> 	/sys/kernel/debug/page_tables/current_kernel
> 
> before and after your patch might be helpful.

I believe the difference is from the following entries (7 PMDs)

Before the patch:

current_kernel:	0xffffffff81000000-0xffffffff81e04000       14352K     ro                 GLB x  pte
efi:		0xffffffff81000000-0xffffffff81e04000       14352K     ro                 GLB x  pte
kernel:		0xffffffff81000000-0xffffffff81e04000       14352K     ro                 GLB x  pte


After the patch:

current_kernel:	0xffffffff81000000-0xffffffff81e00000          14M     ro         PSE     GLB x  pmd
efi:		0xffffffff81000000-0xffffffff81e00000          14M     ro         PSE     GLB x  pmd
kernel:		0xffffffff81000000-0xffffffff81e00000          14M     ro         PSE     GLB x  pmd

current_kernel and kernel show same data though. 

Thanks,
Song