linux-kernel - Re: [PATCH v6 3/5] vmcore: Introduce remap_oldmem_pfn

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Message-ID: <20130716173709.3fed4c42@holzheu>
Date:	Tue, 16 Jul 2013 17:37:09 +0200
From:	Michael Holzheu <holzheu@...ux.vnet.ibm.com>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	HATAYAMA Daisuke <d.hatayama@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jan Willeke <willeke@...ibm.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	linux-kernel@...r.kernel.org, kexec@...ts.infradead.org
Subject: Re: [PATCH v6 3/5] vmcore: Introduce remap_oldmem_pfn_range()

On Tue, 16 Jul 2013 10:04:18 -0400
Vivek Goyal <vgoyal@...hat.com> wrote:

> On Tue, Jul 16, 2013 at 11:25:27AM +0200, Michael Holzheu wrote:
> 
> [..]
> > > > Hello Vivek and Andrew,
> > > > 
> > > > We just realized that Hatayama's mmap patches went into v3.11-rc1. This currently
> > > > breaks s390 kdump because of the following two issues:
> > > > 
> > > > 1) The copy_oldmem_page() is now used for copying to vmalloc memory
> > > > 2) The mmap() implementation is not compatible with the current
> > > >    s390 crashkernel swap:
> > > >    See: http://marc.info/?l=kexec&m=136940802511603&w=2
> > > > 
> > > > The "kdump: Introduce ELF header in new memory feature" patch series will
> > > > fix both issues for s390.
> > > > 
> > > > There is the one small open discussion left:
> > > > 
> > > > http://www.mail-archive.com/linux-kernel@vger.kernel.org/msg464856.html
> > > > 
> > > > But once we have finished that, would it be possible to get the
> > > > patches in 3.11?
> > > 
> > > How about taking mmap() fault handler patches in 3.12. And in 3.11, deny
> > > mmap() on s390 forcing makedumpfile to fall back on read() interface. That
> > > way there will be no regression and mmap() related speedup will show up
> > > in next release on s390.
> > 
> > Hello Vivek and Hatayama,
> > 
> > But then we still would have to somehow fix the copy_oldmem_page() issue (1).
> > 
> > We would prefer to add the current patch series with "#ifndef CONFIG_S390" in
> > the fault handler.
> > 
> > @Vivek:
> > 
> > Since you are the kdump maintainer, could you tell us which of the both
> > variants you would like to have?
> > 
> > static int mmap_vmcore_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
> > {
> > ...
> > 
> > #ifndef CONFIG_S390
> > 	return VM_FAULT_SIGBUS;
> > #endif
> 
> I think let us use this one. Kill the process on non-s390 arch if it goes
> into mmap() fault handler.
> 
> copy_oldmem_page() using real mode in s390 is an issue for vmalloc region.
> I think post the series again and let us see if Andrew is comfortable
> with it.

Ok great, I will send Andrew the current patch series.
Let's see what happens...

> I suspect it is late.
> IIUC, we do copying in real mode only to cope with HSA region in zfcpdump
> case.

The problem is that with the mmap patches we now use copy_oldmem_page()
to copy the notes from oldmem into the notes_buf which has been allocated
with vmalloc. The s390 version of copy_oldmem_page() bypasses the page
tables and uses real copy. And this does not work on vmalloc memory.

Best Regards,
Michael

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