linux-kernel - Re: [PATCH] kexec: unmap reserved pages for each error-return way

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Date:	Thu, 28 Jan 2016 15:01:28 +0100
From:	Michael Holzheu <holzheu@...ux.vnet.ibm.com>
To:	xlpang@...hat.com
Cc:	xpang@...hat.com, Andrew Morton <akpm@...ux-foundation.org>,
	linux-s390@...r.kernel.org,
	Dmitry Safonov <dsafonov@...tuozzo.com>,
	heiko.carstens@...ibm.com, linux-kernel@...r.kernel.org,
	ebiederm@...ssion.com, 0x7f454c46@...il.com,
	schwidefsky@...ibm.com, dyoung@...hat.com,
	kexec@...ts.infradead.org
Subject: Re: [PATCH] kexec: unmap reserved pages for each error-return way

On Thu, 28 Jan 2016 21:12:54 +0800
Xunlei Pang <xpang@...hat.com> wrote:

> On 2016/01/28 at 20:44, Michael Holzheu wrote:
> > On Thu, 28 Jan 2016 19:56:56 +0800
> > Xunlei Pang <xpang@...hat.com> wrote:
> >
> >> On 2016/01/28 at 18:32, Michael Holzheu wrote:
> >>> On Wed, 27 Jan 2016 11:15:46 -0800
> >>> Andrew Morton <akpm@...ux-foundation.org> wrote:
> >>>
> >>>> On Wed, 27 Jan 2016 14:48:31 +0300 Dmitry Safonov <dsafonov@...tuozzo.com> wrote:
> >>>>
> >>>>> For allocation of kimage failure or kexec_prepare or load segments
> >>>>> errors there is no need to keep crashkernel memory mapped.
> >>>>> It will affect only s390 as map/unmap hook defined only for it.
> >>>>> As on unmap s390 also changes os_info structure let's check return code
> >>>>> and add info only on success.
> >>>>>
> >>>> This conflicts (both mechanically and somewhat conceptually) with
> >>>> Xunlei Pang's "kexec: Introduce a protection mechanism for the
> >>>> crashkernel reserved memory" and "kexec: provide
> >>>> arch_kexec_protect(unprotect)_crashkres()".
> >>>>
> >>>> http://ozlabs.org/~akpm/mmots/broken-out/kexec-introduce-a-protection-mechanism-for-the-crashkernel-reserved-memory.patch
> >>>> http://ozlabs.org/~akpm/mmots/broken-out/kexec-introduce-a-protection-mechanism-for-the-crashkernel-reserved-memory-v4.patch
> >>>>
> >>>> and
> >>>>
> >>>> http://ozlabs.org/~akpm/mmots/broken-out/kexec-provide-arch_kexec_protectunprotect_crashkres.patch
> >>>> http://ozlabs.org/~akpm/mmots/broken-out/kexec-provide-arch_kexec_protectunprotect_crashkres-v4.patch
> >>> Hmm, It looks to me that arch_kexec_(un)protect_crashkres() has exactly
> >>> the same semantics as crash_(un)map_reserved_pages().
> >>>
> >>> On s390 we don't have the crashkernel memory mapped and therefore need
> >>> crash_map_reserved_pages() before loading something into crashkernel
> >>> memory.
> >> I don't know s390, just curious, if s390 doesn't have crash kernel memory mapped,
> >> what's the purpose of the commit(558df7209e)  for s390 as the reserved crash memory
> >> with no kernel mapping already means the protection is on?
> > When we reserve crashkernel memory on s390 ("crashkernel=" kernel parameter),
> > we create a memory hole without page tables.
> >
> > Commit (558df7209e) was necessary to load a kernel/ramdisk into
> > the memory hole with the kexec() system call.
> >
> > We create a temporary mapping with crash_map_reserved_pages(), then
> > copy the kernel/ramdisk and finally remove the mapping again
> > via crash_unmap_reserved_pages().
> 
> Thanks for the explanation.
> So, on s390 the physical memory address has the same value as its kernel virtual address,
> and kmap() actually returns the value of the physical address of the page, right?

Correct. On s390 kmap() always return the physical address of the page.

We have an 1:1 mapping for all the physical memory. For this area
virtual=real. In addition to that we have the vmalloc area above
the 1:1 mapping where some of the memory is mapped a second time.

Michael