[PATCH v1 3/8] kexec: export PG_offline to VMCOREINFO

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.

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@linux-foundation.org>
Cc: Dave Young <redacted>
Cc: "Kirill A. Shutemov" <redacted>
Cc: Baoquan He <redacted>
Cc: Omar Sandoval <redacted>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Lianbo Jiang <redacted>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Kazuhito Hagio <redacted>
Signed-off-by: David Hildenbrand <redacted>
---
 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)
 	VMCOREINFO_NUMBER(PAGE_BUDDY_MAPCOUNT_VALUE);
 #ifdef CONFIG_HUGETLB_PAGE
 	VMCOREINFO_NUMBER(HUGETLB_PAGE_DTOR);
+#define PAGE_OFFLINE_MAPCOUNT_VALUE	(~PG_offline)
+	VMCOREINFO_NUMBER(PAGE_OFFLINE_MAPCOUNT_VALUE);
 #endif
 
 	arch_crash_save_vmcoreinfo();

-- 
2.17.2

On Mon, Nov 19, 2018 at 11:16:11AM +0100, 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.

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@linux-foundation.org>
Cc: Dave Young <redacted>
Cc: "Kirill A. Shutemov" <redacted>
Cc: Baoquan He <redacted>
Cc: Omar Sandoval <redacted>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Lianbo Jiang <redacted>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Kazuhito Hagio <redacted>
Signed-off-by: David Hildenbrand <redacted>

Acked-by: Michael S. Tsirkin <mst@redhat.com>

On 11/19/18 at 11:16am, David Hildenbrand wrote:

Acked-by: Dave Young <redacted>

Thanks
Dave

On 11/19/18 at 11:16am, David Hildenbrand wrote:

This solution looks good to me. One small concern is why we don't
export PG_offline to vmcoreinfo directly, then define
PAGE_OFFLINE_MAPCOUNT_VALUE in makedumpfile. We have been exporting
kernel data/MACRO directly, why this one is exceptional.

Thanks
Baoquan

On 21.11.18 07:04, Baoquan He wrote:

On 11/19/18 at 11:16am, David Hildenbrand wrote:

quoted

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)
 	VMCOREINFO_NUMBER(PAGE_BUDDY_MAPCOUNT_VALUE);
 #ifdef CONFIG_HUGETLB_PAGE
 	VMCOREINFO_NUMBER(HUGETLB_PAGE_DTOR);
+#define PAGE_OFFLINE_MAPCOUNT_VALUE	(~PG_offline)
+	VMCOREINFO_NUMBER(PAGE_OFFLINE_MAPCOUNT_VALUE);
 #endif

This solution looks good to me. One small concern is why we don't
export PG_offline to vmcoreinfo directly, then define
PAGE_OFFLINE_MAPCOUNT_VALUE in makedumpfile. We have been exporting
kernel data/MACRO directly, why this one is exceptional.

1. We are much more similar to PG_buddy (in contrast to actual page
flags), and for PG_buddy it is historically handled like this (and I
think it makes sense to expose these as actual MAPCOUNT_VALUEs).

2. Right now only one page type per page is supported. Therefore only
exactly one value in mapcount indicates e.g. PageBuddy()/PageOffline().

Now, if we ever decide to change this (e.g. treat them like real flags),
it is much easier to switch to PG_offline/PG_buddy then. We can directly
see in makedumpfile that .*_MAPCOUNT_VALUE is no longer available but
instead e.g. PG_offline and PG_buddy. Instead we would no see a change
in makedumpfile and would have to rely on other properties.

If there are no strong opinions I will leave it like this.

Thanks!

Thanks
Baoquan

-- 

Thanks,

David / dhildenb