linux-kernel - Re: [Qemu-devel] [PATCH 19/23] userfaultfd: activate syscall

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Date:	Tue, 8 Sep 2015 15:13:56 +0100
From:	"Dr. David Alan Gilbert" <dgilbert@...hat.com>
To:	Bharata B Rao <bharata@...ux.vnet.ibm.com>
Cc:	Michael Ellerman <mpe@...erman.id.au>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	qemu-devel@...gnu.org, kvm@...r.kernel.org,
	linux-api@...r.kernel.org, zhang.zhanghailiang@...wei.com,
	Pavel Emelyanov <xemul@...allels.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Hugh Dickins <hughd@...gle.com>,
	Sanidhya Kashyap <sanidhya.gatech@...il.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Andres Lagar-Cavilla <andreslc@...gle.com>,
	Mel Gorman <mgorman@...e.de>,
	Paolo Bonzini <pbonzini@...hat.com>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	"Huangpeng (Peter)" <peter.huangpeng@...wei.com>,
	Andy Lutomirski <luto@...capital.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Feiner <pfeiner@...gle.com>,
	linuxppc-dev@...ts.ozlabs.org
Subject: Re: [Qemu-devel] [PATCH 19/23] userfaultfd: activate syscall

* Bharata B Rao (bharata@...ux.vnet.ibm.com) wrote:
> On Tue, Sep 08, 2015 at 01:46:52PM +0100, Dr. David Alan Gilbert wrote:
> > * Bharata B Rao (bharata@...ux.vnet.ibm.com) wrote:
> > > On Tue, Sep 08, 2015 at 09:59:47AM +0100, Dr. David Alan Gilbert wrote:
> > > > * Bharata B Rao (bharata@...ux.vnet.ibm.com) wrote:
> > > > > In fact I had successfully done postcopy migration of sPAPR guest with
> > > > > this setup.
> > > > 
> > > > Interesting - I'd not got that far myself on power; I was hitting a problem
> > > > loading htab ( htab_load() bad index 2113929216 (14848+0 entries) in htab stream (htab_shift=25) )
> > > > 
> > > > Did you have to make any changes to the qemu code to get that happy?
> > > 
> > > I should have mentioned that I tried only QEMU driven migration within
> > > the same host using wp3-postcopy branch of your tree. I don't see the
> > > above issue.
> > > 
> > > (qemu) info migrate
> > > capabilities: xbzrle: off rdma-pin-all: off auto-converge: off zero-blocks: off compress: off x-postcopy-ram: on 
> > > Migration status: completed
> > > total time: 39432 milliseconds
> > > downtime: 162 milliseconds
> > > setup: 14 milliseconds
> > > transferred ram: 1297209 kbytes
> > > throughput: 270.72 mbps
> > > remaining ram: 0 kbytes
> > > total ram: 4194560 kbytes
> > > duplicate: 734015 pages
> > > skipped: 0 pages
> > > normal: 318469 pages
> > > normal bytes: 1273876 kbytes
> > > dirty sync count: 4
> > > 
> > > I will try migration between different hosts soon and check.
> > 
> > I hit that on the same host; are you sure you've switched into postcopy mode;
> > i.e. issued a migrate_start_postcopy before the end of migration?
> 
> Sorry I was following your discussion with Li in this thread
> 
> https://www.marc.info/?l=qemu-devel&m=143035620026744&w=4
> 
> and it wasn't obvious to me that anything apart from turning on the
> x-postcopy-ram capability was required :(

OK.

> So I do see the problem now.
> 
> At the source
> -------------
> Error reading data from KVM HTAB fd: Bad file descriptor
> Segmentation fault
> 
> At the target
> -------------
> htab_load() bad index 2113929216 (14336+0 entries) in htab stream (htab_shift=25)
> qemu-system-ppc64: error while loading state section id 56(spapr/htab)
> qemu-system-ppc64: postcopy_ram_listen_thread: loadvm failed: -22
> qemu-system-ppc64: VQ 0 size 0x100 Guest index 0x0 inconsistent with Host index 0x1f: delta 0xffe1
> qemu-system-ppc64: error while loading state for instance 0x0 of device 'pci@...000020000000:00.0/virtio-net'
> *** Error in `./ppc64-softmmu/qemu-system-ppc64': corrupted double-linked list: 0x00000100241234a0 ***
> ======= Backtrace: =========
> /lib64/power8/libc.so.6Segmentation fault

Good - my current world has got rid of the segfaults/corruption in the cleanup on power - but those
are only after it stumbled over the htab problem.

I don't know the innards of power/htab, so if you've got any pointers on what upset it
I'd be happy for some pointers.

(We should probably trim the cc - since I don't think this is userfault related).

Dave

--
Dr. David Alan Gilbert / dgilbert@...hat.com / Manchester, UK
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