Re: [PATCH v2 3/8] kexec: export PG_offline to VMCOREINFO

From: David Hildenbrand
Date: Mon Mar 11 2019 - 05:55:54 EST

On 11.03.19 10:04, Dave Young wrote:
> Hi David,
> On 11/22/18 at 11:06am, David Hildenbrand wrote:
>> Right now, pages inflated as part of a balloon driver will be dumped
>> by dump tools like makedumpfile. While XEN is able to check in the
>> crash kernel whether a certain pfn is actuall backed by memory in the
>> hypervisor (see xen_oldmem_pfn_is_ram) and optimize this case, dumps of
>> other balloon inflated memory will essentially result in zero pages getting
>> allocated by the hypervisor and the dump getting filled with this data.
>> The allocation and reading of zero pages can directly be avoided if a
>> dumping tool could know which pages only contain stale information not to
>> be dumped.
>> We now have PG_offline which can be (and already is by virtio-balloon)
>> used for marking pages as logically offline. Follow up patches will
>> make use of this flag also in other balloon implementations.
>> Let's export PG_offline via PAGE_OFFLINE_MAPCOUNT_VALUE, so
>> makedumpfile can directly skip pages that are logically offline and the
>> content therefore stale. (we export is as a macro to match how it is
>> done for PG_buddy. This way it is clearer that this is not actually a flag
>> but only a very specific mapcount value to represent page types).
>> Please note that this is also helpful for a problem we were seeing under
>> Hyper-V: Dumping logically offline memory (pages kept fake offline while
>> onlining a section via online_page_callback) would under some condicions
>> result in a kernel panic when dumping them.
>> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
>> Cc: Dave Young <dyoung@xxxxxxxxxx>
>> Cc: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
>> Cc: Baoquan He <bhe@xxxxxxxxxx>
>> Cc: Omar Sandoval <osandov@xxxxxx>
>> Cc: Arnd Bergmann <arnd@xxxxxxxx>
>> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
>> Cc: Michal Hocko <mhocko@xxxxxxxx>
>> Cc: "Michael S. Tsirkin" <mst@xxxxxxxxxx>
>> Cc: Lianbo Jiang <lijiang@xxxxxxxxxx>
>> Cc: Borislav Petkov <bp@xxxxxxxxx>
>> Cc: Kazuhito Hagio <k-hagio@xxxxxxxxxxxxx>
>> Acked-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
>> Acked-by: Dave Young <dyoung@xxxxxxxxxx>
>> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
>> ---
>> kernel/crash_core.c | 2 ++
>> 1 file changed, 2 insertions(+)
>> diff --git a/kernel/crash_core.c b/kernel/crash_core.c
>> index 933cb3e45b98..093c9f917ed0 100644
>> --- a/kernel/crash_core.c
>> +++ b/kernel/crash_core.c
>> @@ -464,6 +464,8 @@ static int __init crash_save_vmcoreinfo_init(void)
>> +#define PAGE_OFFLINE_MAPCOUNT_VALUE (~PG_offline)
>> #endif
>> arch_crash_save_vmcoreinfo();
> The patch has been merged, would you mind to send a documentation patch
> for the vmcoreinfo, which is added recently in Documentation/kdump/vmcoreinfo.txt
> A brief description about how this vmcoreinfo field is used is good to
> have.

Turns out, it was already documented


Page attributes. These flags are used to filter various unnecessary for
dumping pages.


> Thanks
> Dave



David / dhildenb