Re: [PATCH v10 1/8] mm: ksm: Export ksm_madvise()

Bharata B Rao
2019-11-06T06:57:42Z

On Wed, Nov 06, 2019 at 03:33:29PM +1100, Paul Mackerras wrote:
> On Mon, Nov 04, 2019 at 09:47:53AM +0530, Bharata B Rao wrote:
> > KVM PPC module needs ksm_madvise() for supporting secure guests.
> > Guest pages that become secure are represented as device private
> > pages in the host. Such pages shouldn't participate in KSM merging.
>
> If we don't do the ksm_madvise call, then as far as I can tell, it
> should all still work correctly, but we might have KSM pulling pages
> in unnecessarily, causing a reduction in performance. Is that right?

I thought so too. When KSM tries to merge a secure page, it should
cause a fault resulting in page-out the secure page. However I see
the below crash when KSM is enabled and KSM scan tries to kmap and
memcmp the device private page.

BUG: Unable to handle kernel data access at 0xc007fffe00010000
Faulting instruction address: 0xc0000000000ab5a0
Oops: Kernel access of bad area, sig: 11 [#1]
LE PAGE_SIZEdK MMU=Radix MMU=Hash SMP NR_CPUS 48 NUMA PowerNV
Modules linked in:
CPU: 0 PID: 22 Comm: ksmd Not tainted 5.4.0-rc2-00026-g2249c0ae4a53-dirty #376
NIP: c0000000000ab5a0 LR: c0000000003d7c3c CTR: 0000000000000004
REGS: c0000001c85d79b0 TRAP: 0300 Not tainted (5.4.0-rc2-00026-g2249c0ae4a53-dirty)
MSR: 900000000280b033 <SF,HV,VEC,VSX,EE,FP,ME,IR,DR,RI,LE> CR: 24002242 XER: 20040000
CFAR: c0000000000ab3d0 DAR: c007fffe00010000 DSISR: 40000000 IRQMASK: 0
GPR00: 0000000000000004 c0000001c85d7c40 c0000000018ce000 c0000001c3880000
GPR04: c007fffe00010000 0000000000010000 0000000000000000 ffffffffffffffff
GPR08: c000000001992298 0000603820002138 ffffffffffffffff ffffffff00003a69
GPR12: 0000000024002242 c000000002550000 c0000001c8700000 c00000000179b728
GPR16: c00c01ffff800040 c00000000179b5b8 c00c00000070e200 ffffffffffffffff
GPR20: 0000000000000000 0000000000000000 fffffffffffff000 c00000000179b648
GPR24: c0000000024464a0 c00000000249f568 c000000001118918 0000000000000000
GPR28: c0000001c804c590 c00000000249f518 0000000000000000 c0000001c8700000
NIP [c0000000000ab5a0] memcmp+0x320/0x6a0
LR [c0000000003d7c3c] memcmp_pages+0x8c/0xe0
Call Trace:
[c0000001c85d7c40] [c0000001c804c590] 0xc0000001c804c590 (unreliable)
[c0000001c85d7c70] [c0000000004591d0] ksm_scan_thread+0x960/0x21b0
[c0000001c85d7db0] [c0000000001bf328] kthread+0x198/0x1a0
[c0000001c85d7e20] [c00000000000bfbc] ret_from_kernel_thread+0x5c/0x80
Instruction dump:
ebc1fff0 eba1ffe8 eb81ffe0 eb61ffd8 4e800020 38600001 4d810020 3860ffff
4e800020 38000004 7c0903a6 7d201c28 <7d402428> 7c295040 38630008 38840008

In anycase, we wouldn't want secure guests pages to be pulled out due
to KSM, hence disabled merging.

Regards,
Bharata.