Date:   Mon, 21 Jan 2019 13:11:41 +0800
From:   Pingfan Liu <kernelfans@...il.com>
To:     Jerry.Hoemann@....com
Cc:     kexec@...ts.infradead.org, Baoquan He <bhe@...hat.com>,
        Yinghai Lu <yinghai@...nel.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dave Young <dyoung@...hat.com>, vgoyal@...hat.com
Subject: Re: [PATCHv7] x86/kdump: bugfix, make the behavior of crashkernel=X
 consistent with kaslr

On Sat, Jan 19, 2019 at 9:25 AM Jerry Hoemann <jerry.hoemann@....com> wrote:
>
> On Tue, Jan 15, 2019 at 04:07:03PM +0800, Pingfan Liu wrote:
> > People 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 without ',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 grammar 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 tries to get memory region from 896 MB firstly, then [896MB,4G],
> > finally above 4G.
>
> While allocating crashkernel from below 4G seems fine, won't we have
> problems if the crash kernel gets allocated above 4G because of the SWIOTLB?
>
It will reserve extra memory below 4G for the swiotlb purpose. You can
find the logic in reserve_crashkernel_low()
And testing with crashkernel=512M@4G, we will get:
cat /proc/iomem  | grep Crash
  aa000000-b9ffffff : Crash kernel
  100000000-11fffffff : Crash kernel

Thanks,
Pingfan

> thanks
>
>
> > Dave Young sent the original post, and I just re-post it with commit log
> > improvement as his requirement.
> > http://lists.infradead.org/pipermail/kexec/2017-October/019571.html
> > There was an old discussion below (previously posted by Chao Wang):
> > https://lkml.org/lkml/2013/10/15/601
> >
> > Signed-off-by: Pingfan Liu <kernelfans@...il.com>
> > Cc: Dave Young <dyoung@...hat.com>
> > Cc: Baoquan He <bhe@...hat.com>
> > Cc: Andrew Morton <akpm@...ux-foundation.org>
> > Cc: Mike Rapoport <rppt@...ux.vnet.ibm.com>
> > Cc: yinghai@...nel.org,
> > Cc: vgoyal@...hat.com
> > Cc: Randy Dunlap <rdunlap@...radead.org>
> > ---
> > v6 -> v7: fix spelling mistake pointed out by Randy
> >  arch/x86/kernel/setup.c | 16 ++++++++++++++++
> >  1 file changed, 16 insertions(+)
> >
> > diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
> > index 3d872a5..fa62c81 100644
> > --- a/arch/x86/kernel/setup.c
> > +++ b/arch/x86/kernel/setup.c
> > @@ -551,6 +551,22 @@ static void __init reserve_crashkernel(void)
> >                                                   high ? CRASH_ADDR_HIGH_MAX
> >                                                        : CRASH_ADDR_LOW_MAX,
> >                                                   crash_size, CRASH_ALIGN);
> > +#ifdef CONFIG_X86_64
> > +             /*
> > +              * crashkernel=X reserve below 896M fails? Try below 4G
> > +              */
> > +             if (!high && !crash_base)
> > +                     crash_base = memblock_find_in_range(CRASH_ALIGN,
> > +                                             (1ULL << 32),
> > +                                             crash_size, CRASH_ALIGN);
> > +             /*
> > +              * crashkernel=X reserve below 4G fails? Try MAXMEM
> > +              */
> > +             if (!high && !crash_base)
> > +                     crash_base = memblock_find_in_range(CRASH_ALIGN,
> > +                                             CRASH_ADDR_HIGH_MAX,
> > +                                             crash_size, CRASH_ALIGN);
> > +#endif
> >               if (!crash_base) {
> >                       pr_info("crashkernel reservation failed - No suitable area found.\n");
> >                       return;
> > --
> > 2.7.4
> >
> >
> > _______________________________________________
> > kexec mailing list
> > kexec@...ts.infradead.org
> > http://lists.infradead.org/mailman/listinfo/kexec
>
> --
>
> -----------------------------------------------------------------------------
> Jerry Hoemann                  Software Engineer   Hewlett Packard Enterprise
> -----------------------------------------------------------------------------

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