[Patch v3] align crash_notes allocation to make it be inside one physical page

[Baoquan He]

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