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    Subject[PATCHv3 2/2] x86/kdump: bugfix, make the behavior of crashkernel=X consistent with kaslr
    Date
    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);
    +
    if (!crash_base) {
    pr_info("crashkernel reservation failed - No suitable area found.\n");
    return;
    --
    2.7.4
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