Re: [PATCHv3 2/2] x86/kdump: bugfix, make the behavior of crashkernel=X consistent with kaslr

[Pingfan Liu <kernelfans@gmail.com>]

On Mon, Dec 31, 2018 at 4:46 PM Mike Rapoport <rppt@linux.ibm.com> wrote:
>
> On Fri, Dec 28, 2018 at 11:00:02AM +0800, Pingfan Liu wrote:
> > Customer reported a bug on a high end server with many pcie devices, where
> > kernel bootup with crashkernel=384M, and kaslr is enabled. Even
> > though we still see much memory under 896 MB, the finding still failed
> > intermittently. Because currently we can only find region under 896 MB,
> > if w/0 ',high' specified. Then KASLR breaks 896 MB into several parts
> > randomly, and crashkernel reservation need be aligned to 128 MB, that's
> > why failure is found. It raises confusion to the end user that sometimes
> > crashkernel=X works while sometimes fails.
> > If want to make it succeed, customer can change kernel option to
> > "crashkernel=384M, high". Just this give "crashkernel=xx@yy" a very
> > limited space to behave even though its grammer looks more generic.
> > And we can't answer questions raised from customer that confidently:
> > 1) why it doesn't succeed to reserve 896 MB;
> > 2) what's wrong with memory region under 4G;
> > 3) why I have to add ',high', I only require 384 MB, not 3840 MB.
> >
> > This patch simplifies the method suggested in the mail [1]. It just goes
> > bottom-up to find a candidate region for crashkernel. The bottom-up may be
> > better compatible with the old reservation style, i.e. still want to get
> > memory region from 896 MB firstly, then [896 MB, 4G], finally above 4G.
> >
> > There is one trivial thing about the compatibility with old kexec-tools:
> > if the reserved region is above 896M, then old tool will fail to load
> > bzImage. But without this patch, the old tool also fail since there is no
> > memory below 896M can be reserved for crashkernel.
> >
> > [1]: http://lists.infradead.org/pipermail/kexec/2017-October/019571.html
> > Signed-off-by: Pingfan Liu <kernelfans@gmail.com>
> > Cc: Tang Chen <tangchen@cn.fujitsu.com>
> > Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>
> > Cc: Len Brown <lenb@kernel.org>
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
> > Cc: Michal Hocko <mhocko@suse.com>
> > Cc: Jonathan Corbet <corbet@lwn.net>
> > Cc: Yaowei Bai <baiyaowei@cmss.chinamobile.com>
> > Cc: Pavel Tatashin <pasha.tatashin@oracle.com>
> > Cc: Nicholas Piggin <npiggin@gmail.com>
> > Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> > Cc: Daniel Vacek <neelx@redhat.com>
> > Cc: Mathieu Malaterre <malat@debian.org>
> > Cc: Stefan Agner <stefan@agner.ch>
> > Cc: Dave Young <dyoung@redhat.com>
> > Cc: Baoquan He <bhe@redhat.com>
> > Cc: yinghai@kernel.org,
> > Cc: vgoyal@redhat.com
> > Cc: linux-kernel@vger.kernel.org
> > ---
> >  arch/x86/kernel/setup.c | 9 ++++++---
> >  1 file changed, 6 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
> > index d494b9b..165f9c3 100644
> > --- a/arch/x86/kernel/setup.c
> > +++ b/arch/x86/kernel/setup.c
> > @@ -541,15 +541,18 @@ static void __init reserve_crashkernel(void)
> >
> >       /* 0 means: find the address automatically */
> >       if (crash_base <= 0) {
> > +             bool bottom_up = memblock_bottom_up();
> > +
> > +             memblock_set_bottom_up(true);
> >
> >               /*
> >                * Set CRASH_ADDR_LOW_MAX upper bound for crash memory,
> >                * as old kexec-tools loads bzImage below that, unless
> >                * "crashkernel=size[KMG],high" is specified.
> >                */
> >               crash_base = memblock_find_in_range(CRASH_ALIGN,
> > -                                                 high ? CRASH_ADDR_HIGH_MAX
> > -                                                      : CRASH_ADDR_LOW_MAX,
> > -                                                 crash_size, CRASH_ALIGN);
> > +                     (max_pfn * PAGE_SIZE), crash_size, CRASH_ALIGN);
> > +             memblock_set_bottom_up(bottom_up);
>
> Using bottom-up does not guarantee that the allocation won't fall into a
> removable memory, it only makes it highly probable.
>
> I think that the 'max_pfn * PAGE_SIZE' limit should be replaced with the
> end of the non-removable memory node.
>
Since passing MEMBLOCK_NONE, memblock_find_in_range() ->...->
__next_mem_range(), there is a logic to guarantee hotmovable memory
will not be stamped over.
if (movable_node_is_enabled() && memblock_is_hotpluggable(m))
continue;

Thanks,
Pingfan

> > +
> >               if (!crash_base) {
> >                       pr_info("crashkernel reservation failed - No suitable area found.\n");
> >                       return;
> > --
> > 2.7.4
> >
>
> --
> Sincerely yours,
> Mike.
>