Re: [PATCHv5] x86/kdump: bugfix, make the behavior of crashkernel=X consistent with kaslr

[Mike Rapoport <rppt@linux.ibm.com>]

On Mon, Jan 07, 2019 at 04:04:59PM +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
> ---
> v4 -> v5:
>   add a wrapper of bottom up allocation func
> v3 -> v4:
>   instead of exporting the stage of parsing mem hotplug info, just using the bottom-up allocation func directly
>  arch/x86/kernel/setup.c  |  8 ++++----
>  include/linux/memblock.h |  3 +++
>  mm/memblock.c            | 29 +++++++++++++++++++++++++++++
>  3 files changed, 36 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
> index d494b9b..80e7923 100644
> --- a/arch/x86/kernel/setup.c
> +++ b/arch/x86/kernel/setup.c
> @@ -546,10 +546,10 @@ static void __init reserve_crashkernel(void)
>  		 * 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);
> +		crash_base = memblock_find_range_bottom_up(CRASH_ALIGN,
> +			(max_pfn * PAGE_SIZE), crash_size, CRASH_ALIGN,
> +			NUMA_NO_NODE);
> +
>  		if (!crash_base) {
>  			pr_info("crashkernel reservation failed - No suitable area found.\n");
>  			return;
> diff --git a/include/linux/memblock.h b/include/linux/memblock.h
> index aee299a..a35ae17 100644
> --- a/include/linux/memblock.h
> +++ b/include/linux/memblock.h
> @@ -116,6 +116,9 @@ phys_addr_t memblock_find_in_range_node(phys_addr_t size, phys_addr_t align,
>  					int nid, enum memblock_flags flags);
>  phys_addr_t memblock_find_in_range(phys_addr_t start, phys_addr_t end,
>  				   phys_addr_t size, phys_addr_t align);
> +phys_addr_t __init_memblock
> +memblock_find_range_bottom_up(phys_addr_t start, phys_addr_t end,
> +	phys_addr_t size, phys_addr_t align, int nid);
>  void memblock_allow_resize(void);
>  int memblock_add_node(phys_addr_t base, phys_addr_t size, int nid);
>  int memblock_add(phys_addr_t base, phys_addr_t size);
> diff --git a/mm/memblock.c b/mm/memblock.c
> index 81ae63c..f68287e 100644
> --- a/mm/memblock.c
> +++ b/mm/memblock.c
> @@ -192,6 +192,35 @@ __memblock_find_range_bottom_up(phys_addr_t start, phys_addr_t end,
>  	return 0;
>  }
> 
> +phys_addr_t __init_memblock
> +memblock_find_range_bottom_up(phys_addr_t start, phys_addr_t end,
> +	phys_addr_t size, phys_addr_t align, int nid)
> +{
> +	phys_addr_t ret;
> +	enum memblock_flags flags = choose_memblock_flags();
> +
> +	/* pump up @end */
> +	if (end == MEMBLOCK_ALLOC_ACCESSIBLE)
> +		end = memblock.current_limit;
> +
> +	/* avoid allocating the first page */
> +	start = max_t(phys_addr_t, start, PAGE_SIZE);
> +	end = max(start, end);
> +
> +again:
> +	ret = __memblock_find_range_bottom_up(start, end, size, align,
> +					    nid, flags);
> +
> +	if (!ret && (flags & MEMBLOCK_MIRROR)) {
> +		pr_warn("Could not allocate %pap bytes of mirrored memory\n",
> +			&size);
> +		flags &= ~MEMBLOCK_MIRROR;
> +		goto again;
> +	}

I'm not thrilled by duplicating this code (yet again).
I liked the v3 of this patch [1] more, assuming we allow bottom-up mode to
allocate [0, kernel_start) unconditionally. 
I'd just replace you first patch in v3 [2] with something like:

diff --git a/mm/memblock.c b/mm/memblock.c
index 7df468c..d1b30b9 100644
--- a/mm/memblock.c
+++ b/mm/memblock.c
@@ -274,24 +274,14 @@ phys_addr_t __init_memblock memblock_find_in_range_node(phys_addr_t size,
 	 * try bottom-up allocation only when bottom-up mode
 	 * is set and @end is above the kernel image.
 	 */
-	if (memblock_bottom_up() && end > kernel_end) {
-		phys_addr_t bottom_up_start;
-
-		/* make sure we will allocate above the kernel */
-		bottom_up_start = max(start, kernel_end);
-
+	if (memblock_bottom_up()) {
 		/* ok, try bottom-up allocation first */
-		ret = __memblock_find_range_bottom_up(bottom_up_start, end,
+		ret = __memblock_find_range_bottom_up(start, end,
 						      size, align, nid, flags);
 		if (ret)
 			return ret;
 
 		/*
-		 * we always limit bottom-up allocation above the kernel,
-		 * but top-down allocation doesn't have the limit, so
-		 * retrying top-down allocation may succeed when bottom-up
-		 * allocation failed.
-		 *
 		 * bottom-up allocation is expected to be fail very rarely,
 		 * so we use WARN_ONCE() here to see the stack trace if
 		 * fail happens.

[1] https://lore.kernel.org/lkml/1545966002-3075-3-git-send-email-kernelfans@gmail.com/
[2] https://lore.kernel.org/lkml/1545966002-3075-2-git-send-email-kernelfans@gmail.com/

> +
> +	return ret;
> +}
> +
>  /**
>   * __memblock_find_range_top_down - find free area utility, in top-down
>   * @start: start of candidate range
> -- 
> 2.7.4
> 

-- 
Sincerely yours,
Mike.