Re: [PATCH 2/2 update] X86/kdump: fall back to reserve high crashkernel memory

[Baoquan He]

On 04/22/19 at 11:19am, Dave Young wrote:
> crashkernel=xM tries to reserve crashkernel memory under 4G, which
> is enough for usual cases.  But this could fail sometimes, for example
> one tries to reserve a big chunk like 2G, it is possible to fail.
> 
> So let the crashkernel=xM just fall back to use high memory in case it
> fails to find a suitable low range.  Do not set the ,high as default
> because it allocates extra low memory for DMA buffers and swiotlb, this is
> not always necessary for all machines. Typically like crashkernel=128M
> usually work with low reservation under 4G, so still keep <4G as default.
> 
> Signed-off-by: Dave Young 
> Reviewed-by: Ingo Molnar 
> ---

Ack the whole series, thanks for the effort.

Acked-by: Baoquan He 

>  Documentation/admin-guide/kernel-parameters.txt |7 +--
>  arch/x86/kernel/setup.c |   22 ++
>  2 files changed, 19 insertions(+), 10 deletions(-)
> 
> --- linux-x86.orig/arch/x86/kernel/setup.c
> +++ linux-x86/arch/x86/kernel/setup.c
> @@ -541,21 +541,27 @@ static void __init reserve_crashkernel(v
>   }
>  
>   /* 0 means: find the address automatically */
> - if (crash_base <= 0) {
> + if (!crash_base) {
>   /*
>* 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.
> +  * crashkernel=x,high reserves memory over 4G, also allocates
> +  * 256M extra low memory for DMA buffers and swiotlb.
> +  * but the extra memory is not required for all machines.
> +  * So prefer low memory first, and fall back to high memory
> +  * 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);
> + if (!high)
> + crash_base = memblock_find_in_range(CRASH_ALIGN,
> + CRASH_ADDR_LOW_MAX,
> + crash_size, CRASH_ALIGN);
> + if (!crash_base)
> + crash_base = memblock_find_in_range(CRASH_ALIGN,
> + CRASH_ADDR_HIGH_MAX,
> + crash_size, CRASH_ALIGN);
>   if (!crash_base) {
>   pr_info("crashkernel reservation failed - No suitable 
> area found.\n");
>   return;
>   }
> -
>   } else {
>   unsigned long long start;
>  
> --- linux-x86.orig/Documentation/admin-guide/kernel-parameters.txt
> +++ linux-x86/Documentation/admin-guide/kernel-parameters.txt
> @@ -704,8 +704,11 @@
>   upon panic. This parameter reserves the physical
>   memory region [offset, offset + size] for that kernel
>   image. If '@offset' is omitted, then a suitable offset
> - is selected automatically. Check
> - Documentation/kdump/kdump.txt for further details.
> + is selected automatically.
> + [KNL, x86_64] select a region under 4G first, and
> + fall back to reserve region above 4G in case without
> + '@offset'.
> + See Documentation/kdump/kdump.txt for further details.
>  
>   crashkernel=range1:size1[,range2:size2,...][@offset]
>   [KNL] Same as above, but depends on the memory

[PATCH 2/2 update] X86/kdump: fall back to reserve high crashkernel memory

[Dave Young]

crashkernel=xM tries to reserve crashkernel memory under 4G, which
is enough for usual cases.  But this could fail sometimes, for example
one tries to reserve a big chunk like 2G, it is possible to fail.

So let the crashkernel=xM just fall back to use high memory in case it
fails to find a suitable low range.  Do not set the ,high as default
because it allocates extra low memory for DMA buffers and swiotlb, this is
not always necessary for all machines. Typically like crashkernel=128M
usually work with low reservation under 4G, so still keep <4G as default.

Signed-off-by: Dave Young 
Reviewed-by: Ingo Molnar 
---
 Documentation/admin-guide/kernel-parameters.txt |7 +--
 arch/x86/kernel/setup.c |   22 ++
 2 files changed, 19 insertions(+), 10 deletions(-)

--- linux-x86.orig/arch/x86/kernel/setup.c
+++ linux-x86/arch/x86/kernel/setup.c
@@ -541,21 +541,27 @@ static void __init reserve_crashkernel(v
}
 
/* 0 means: find the address automatically */
-   if (crash_base <= 0) {
+   if (!crash_base) {
/*
 * 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.
+* crashkernel=x,high reserves memory over 4G, also allocates
+* 256M extra low memory for DMA buffers and swiotlb.
+* but the extra memory is not required for all machines.
+* So prefer low memory first, and fall back to high memory
+* 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);
+   if (!high)
+   crash_base = memblock_find_in_range(CRASH_ALIGN,
+   CRASH_ADDR_LOW_MAX,
+   crash_size, CRASH_ALIGN);
+   if (!crash_base)
+   crash_base = memblock_find_in_range(CRASH_ALIGN,
+   CRASH_ADDR_HIGH_MAX,
+   crash_size, CRASH_ALIGN);
if (!crash_base) {
pr_info("crashkernel reservation failed - No suitable 
area found.\n");
return;
}
-
} else {
unsigned long long start;
 
--- linux-x86.orig/Documentation/admin-guide/kernel-parameters.txt
+++ linux-x86/Documentation/admin-guide/kernel-parameters.txt
@@ -704,8 +704,11 @@
upon panic. This parameter reserves the physical
memory region [offset, offset + size] for that kernel
image. If '@offset' is omitted, then a suitable offset
-   is selected automatically. Check
-   Documentation/kdump/kdump.txt for further details.
+   is selected automatically.
+   [KNL, x86_64] select a region under 4G first, and
+   fall back to reserve region above 4G in case without
+   '@offset'.
+   See Documentation/kdump/kdump.txt for further details.
 
crashkernel=range1:size1[,range2:size2,...][@offset]
[KNL] Same as above, but depends on the memory