People reported that crash_notes in /proc/vmcore were corrupted and
this cause crash kdump failure. With code debugging and log we got
the root cause. This is because percpu variable crash_notes are
allocated in 2 vmalloc pages. Currently percpu is based on vmalloc
by default. Vmalloc can't guarantee 2 continuous vmalloc pages
are also on 2 continuous physical pages. So when 1st kernel exports
the starting address and size of crash_notes through sysfs like below:
/sys/devices/system/cpu/cpux/crash_notes
/sys/devices/system/cpu/cpux/crash_notes_size
kdump kernel use them to get the content of crash_notes. However the
2nd part may not be in the next neighbouring physical page as we
expected if crash_notes are allocated accross 2 vmalloc pages. That's
why nhdr_ptr->n_namesz or nhdr_ptr->n_descsz could be very huge in
update_note_header_size_elf64() and cause note header merging failure
or some warnings.
In this patch change to call __alloc_percpu() to passed in the align
value by rounding crash_notes_size up to the nearest power of two.
This makes sure the crash_notes is allocated inside one physical page
since sizeof(note_buf_t) in all ARCHS is smaller than PAGE_SIZE.
Meanwhile add a BUILD_BUG_ON to break compile if size is bigger than
PAGE_SIZE since crash_notes definitely will be in 2 pages. That need
be avoided, and need be reported if it's unavoidable.
v1->v2:
Minfei mentioned percpu can't take align value bigger then PAGE_SIZE.
So limit align to be PAGE_SIZE at most.
v2->v3:
Change code and add code comments as Andrew suggested.
Signed-off-by: Baoquan He <bhe at redhat.com>
---
kernel/kexec.c | 19 ++++++++++++++++++-
1 file changed, 18 insertions(+), 1 deletion(-)
diff --git a/kernel/kexec.c b/kernel/kexec.c
index 35d9e0a..293fe64 100644
--- a/kernel/kexec.c
+++ b/kernel/kexec.c
@@ -1620,7 +1620,24 @@ void crash_save_cpu(struct pt_regs *regs, int cpu)
static int __init crash_notes_memory_init(void)
{
/* Allocate memory for saving cpu registers. */
- crash_notes = alloc_percpu(note_buf_t);
+ size_t size, align;
+
+ /* crash_notes could be allocated across 2 vmalloc pages when percpu
+ * is vmalloc based . vmalloc doesn't guarantee 2 continuous vmalloc
+ * pages are also on 2 continuous physical pages. In this case the
+ * 2nd part of crash_notes in 2nd page could be lost since only the
+ * starting address and size of crash_notes are exported through sysfs.
+ * Here round up the size of crash_notes to the nearest power of two
+ * and pass it to __alloc_percpu as align value. This can make sure
+ * crash_notes is allocated inside one physical page. */
+ size = sizeof(note_buf_t);
+ align = min(roundup_pow_of_two(sizeof(note_buf_t)), PAGE_SIZE);
+
+ /* Break compile if size is bigger than PAGE_SIZE since crash_notes
+ * definitely will be in 2 pages with that. */
+ BUILD_BUG_ON(size > PAGE_SIZE);
+
+ crash_notes = __alloc_percpu(size, align);
if (!crash_notes) {
pr_warn("Kexec: Memory allocation for saving cpu register states failed\n");
return -ENOMEM;
--
1.9.3
