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[Patch v2] align crash_notes allocation to make it be inside one physical page

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  [Patch v2] align crash_notes allocation to make it be inside one physical page Baoquan He <bhe@redhat.com> - 2015-08-03 15:00 +0200
    Re: [Patch v2] align crash_notes allocation to make it be inside  one physical page Andrew Morton <akpm@linux-foundation.org> - 2015-08-04 00:10 +0200
      Re: [Patch v2] align crash_notes allocation to make it be inside one  physical page Baoquan He <bhe@redhat.com> - 2015-08-04 01:10 +0200

#1198831 — [Patch v2] align crash_notes allocation to make it be inside one physical page

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 make 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 WARN_ON in case it grows to be bigger than PAGE_SIZE
in the future.

v1->v2:
    Minfei mentioned percpu can't take align value bigger then PAGE_SIZE.
    So limit align to be PAGE_SIZE at most.

Signed-off-by: Baoquan He <bhe@redhat.com>
---
 kernel/kexec.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/kernel/kexec.c b/kernel/kexec.c
index a785c10..9f4b070 100644
--- a/kernel/kexec.c
+++ b/kernel/kexec.c
@@ -1620,7 +1620,16 @@ 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;
+	int order;
+
+	size = sizeof(note_buf_t);
+	order = get_count_order(size);
+	align = min_t(size_t, (1<<order), PAGE_SIZE);
+
+	WARN_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;
-- 
2.1.0

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#1199292 — Re: [Patch v2] align crash_notes allocation to make it be inside one physical page

On Mon,  3 Aug 2015 20:50:43 +0800 Baoquan He <bhe@redhat.com> wrote:

> 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 make 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 WARN_ON in case it grows to be bigger than PAGE_SIZE
> in the future.
> 
> ...
>
> --- a/kernel/kexec.c
> +++ b/kernel/kexec.c
> @@ -1620,7 +1620,16 @@ 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;
> +	int order;
> +
> +	size = sizeof(note_buf_t);
> +	order = get_count_order(size);
> +	align = min_t(size_t, (1<<order), PAGE_SIZE);
> +
> +	WARN_ON(size > PAGE_SIZE);
> +
> +	crash_notes = __alloc_percpu(size, align);

A code comment would be helpful - the reason for this code's existence
is otherwise utterly unobvious.

I think it can be done this way:

	align = min(roundup_pow_of_two(sizeof(note_buf_t)), PAGE_SIZE);


I never noticed get_count_order() before.  afaict it does the same as
order_base_2(), except get_count_order() generates better code and has
a ridiculous name.

And I think the WARN_ON can be replaced with a
BUILD_BUG_ON(sizeof>PAGE_SIZE)?  That would avoid adding runtime
overhead.

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#1199318 — Re: [Patch v2] align crash_notes allocation to make it be inside one physical page

Hi Andrew,

Thanks a lot for your reviewing and suggestiong.

On 08/03/15 at 03:04pm, Andrew Morton wrote:
> On Mon,  3 Aug 2015 20:50:43 +0800 Baoquan He <bhe@redhat.com> wrote:
> > --- a/kernel/kexec.c
> > +++ b/kernel/kexec.c
> > @@ -1620,7 +1620,16 @@ 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;
> > +	int order;
> > +
> > +	size = sizeof(note_buf_t);
> > +	order = get_count_order(size);
> > +	align = min_t(size_t, (1<<order), PAGE_SIZE);
> > +
> > +	WARN_ON(size > PAGE_SIZE);
> > +
> > +	crash_notes = __alloc_percpu(size, align);
> 
> A code comment would be helpful - the reason for this code's existence
> is otherwise utterly unobvious.

Will add in new post.

> 
> I think it can be done this way:
> 
> 	align = min(roundup_pow_of_two(sizeof(note_buf_t)), PAGE_SIZE);
> 
> 
> I never noticed get_count_order() before.  afaict it does the same as
> order_base_2(), except get_count_order() generates better code and has
> a ridiculous name.

OK, will change the code as you suggested.

> 
> And I think the WARN_ON can be replaced with a
> BUILD_BUG_ON(sizeof>PAGE_SIZE)?  That would avoid adding runtime
> overhead.

I am not sure about this. BUILD_BUG_ON will break kernel compiling.
Before we got the root cause several work around fix were introduced to
skip this kind of crash_note.

  c4082f3 vmcore: continue vmcore initialization if PT_NOTE is found empty
  38dfac8 vmcore: prevent PT_NOTE p_memsz overflow during header update

That means if (sizeof(note_buf_t)>PAGE_SIZE) really happened, normal
kernel works well, kdump kernel can work but we will lose those
crash_notes. And if on one certain ARCH sizeof(note_buf_t) is bigger
than PAGE_SIZE, the design here must be changed to avoid using percpu
variable or adjust their note_buf_t. That may take a not short time to
discuss and review. Comparing with this it may be better to tolerate the
dumping vmcore with uncomplete crash_notes for a while until new design
is taken.

Thanks
Baoquan
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