Re: [PATCHv2] x86/kdump: bugfix, make the behavior of crashkernel=X consistent with kaslr

[Pingfan Liu]

On Wed, Dec 26, 2018 at 9:47 AM Dave Young <dyoung@xxxxxxxxxx> wrote:
>
> On 12/14/18 at 12:07pm, 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@xxxxxxxxx>
> > Cc: Dave Young <dyoung@xxxxxxxxxx>
> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > Cc: Baoquan He <bhe@xxxxxxxxxx>
> > Cc: yinghai@xxxxxxxxxx,
> > Cc: vgoyal@xxxxxxxxxx
> > Cc: kexec@xxxxxxxxxxxxxxxxxxx
> >
> > ---
> > v1->v2:
> >   improve commit log
> >  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..60f12c4 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) {
> > +             if (!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);
> > +             if (!memblock_bottom_up())
> > +                     memblock_set_bottom_up(false);
>
> The previous memblock_set_bottom_up(true) set it as true, so
> "!memblock_bottom_up()" is impossible, not sure what is the point of
> this condition check.
>
> Do you want to restore the original memblock direction? If so a variable
> to save the old direction is needed.  But is this really necessary?
> Do you know any side effects of setting the bottom up as true?
>
Yes, will fix it.

Thanks
> > +
> >               if (!crash_base) {
> >                       pr_info("crashkernel reservation failed - No suitable area found.\n");
> >                       return;
> > --
> > 2.7.4
> >
> >
> > _______________________________________________
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> > kexec@xxxxxxxxxxxxxxxxxxx
> > http://lists.infradead.org/mailman/listinfo/kexec
>
> Thanks
> Dave