Subject: Re: [PATCH 4/8] Read/Write oops nvram partition via pstore

From: Aruna Balakrishnaiah <[email-masked]>
Date: Tue, 16 Apr 2013 11:50:36 +0530

Hi Michael,

Thanks for reviewing my patches.

On Monday 15 April 2013 01:25 PM, Michael Ellerman wrote:
> On Wed, Apr 10, 2013 at 12:53:03PM +0530, Aruna Balakrishnaiah wrote:
>> This patch exploits pstore infrastructure in power systems.
>> IBM's system p machines provide persistent storage for LPARs
> In the kernel we use "pseries" instead of "system p".
>

Sure, will change it.

>> through NVRAM. NVRAM's lnx,oops-log partition is used to log
>> oops messages. In case pstore registration fails it will
>> fall back to kmsg_dump mechanism.
> What are the implications of falling back to kmsg_dump()?
>

Logging oops messages to nvram should not fail in case pstore registration
fails. So it falls back to existing kmsg_dump infrastructure where
oops_to_nvram will be called. The users would need to use existing tools
to read nvram data as it is now.

> Is there any reason we would not want to enable CONFIG_PSTORE ? ie.
> should the pseries platform just select it?

Since current patchset does not support compression, selecting PSTORE by
default will lose the existing compression feature.
Once the compression feature for PSTORE is in place we can make PSTORE as
default on power.

I will post the compression patches soon. The reason for posting it
separately is stated below.

>> diff --git a/arch/powerpc/platforms/pseries/nvram.c b/arch/powerpc/platforms/pseries/nvram.c
>> index 6701b71..82d32a2 100644
>> --- a/arch/powerpc/platforms/pseries/nvram.c
>> +++ b/arch/powerpc/platforms/pseries/nvram.c
>> @@ -18,6 +18,7 @@
>>   #include <linux/spinlock.h>
>>   #include <linux/slab.h>
>>   #include <linux/kmsg_dump.h>
>> +#include <linux/pstore.h>
>>   #include <linux/ctype.h>
>>   #include <linux/zlib.h>
>>   #include <asm/uaccess.h>
>> @@ -87,6 +88,25 @@ static struct kmsg_dumper nvram_kmsg_dumper = {
>>   	.dump = oops_to_nvram
>>   };
>>   
>> +static int nvram_pstore_open(struct pstore_info *psi);
>> +
>> +static ssize_t nvram_pstore_read(u64 *id, enum pstore_type_id *type,
>> +				int *count, struct timespec *time, char **buf,
>> +				struct pstore_info *psi);
>> +
>> +static int nvram_pstore_write(enum pstore_type_id type,
>> +				enum kmsg_dump_reason reason, u64 *id,
>> +				unsigned int part, int count, size_t size,
>> +				struct pstore_info *psi);
> I think you should be able to rearrange this so that you don't need the
> forward declarations.

Sure. This would result in moving the callback functions just before pstore
registration for which I need to move clobbering_unread_rtas_event() which
is used by nvram_pstore_write.

>> +
>> +static struct pstore_info nvram_pstore_info = {
>> +	.owner = THIS_MODULE,
>> +	.name = "nvram",
>> +	.open = nvram_pstore_open,
>> +	.read = nvram_pstore_read,
>> +	.write = nvram_pstore_write,
>> +};
>> +
>>   /* See clobbering_unread_rtas_event() */
>>   #define NVRAM_RTAS_READ_TIMEOUT 5		/* seconds */
>>   static unsigned long last_unread_rtas_event;	/* timestamp */
>> @@ -121,6 +141,13 @@ static char *big_oops_buf, *oops_buf;
>>   static char *oops_data;
>>   static size_t oops_data_sz;
>>   
>> +#ifdef CONFIG_PSTORE
> If we are going to have CONFIG_PSTORE #ifdefs in this file, I don't see
> why there can't be just a single block of code that is #ifdef'ed, rather
> than several like you have.

Sure. I will have one #ifdef for declarations and one for function
definitions.

>> +static enum pstore_type_id nvram_type_ids[] = {
>> +	PSTORE_TYPE_DMESG,
>> +	-1
>> +};
>> +static int read_type;
> I don't understand what you're doing with read_type. It looks fishy.

read_type is an iterator to traverse the partition types. It is to know
which partition we need to read.

>> +#endif
>>   /* Compression parameters */
>>   #define COMPR_LEVEL 6
>>   #define WINDOW_BITS 12
>> @@ -455,6 +482,23 @@ static void __init nvram_init_oops_partition(int rtas_partition_exists)
>>   	oops_data = oops_buf + sizeof(struct oops_log_info);
>>   	oops_data_sz = oops_log_partition.size - sizeof(struct oops_log_info);
>>   
>> +	nvram_pstore_info.buf = oops_data;
>> +	nvram_pstore_info.bufsize = oops_data_sz;
>> +
>> +	rc = pstore_register(&nvram_pstore_info);
>> +
>> +	if (rc != 0) {
>> +		pr_err("nvram: pstore_register() failed, defaults to "
>> +				"kmsg_dump; returned %d\n", rc);
>> +		goto kmsg_dump;
> You don't need the goto.

Yeah, my bad. Will fix it.

>> +	} else {
>> +		/*TODO: Support compression when pstore is configured */
> What is the issue here?
>

Currently with this patchset, pstore is not supporting compression of oops-messages
since it involves some changes in the pstore framework.

big_oops_buf will hold the large part of oops data which will be compressed and put
to oops_buf.

big_oops_buf: (1.45 of oops_partition_size)
_________________________
|  header |   oops-text |
|_________|_____________|

<header> is added by the pstore.

So in case compression fails:

we would need to log the header + last few bytes of big_oops_buf to oops_buf.
oops_buf: (this is of oops_partition_size)

we need last few bytes of big_oops_buf as we need to log the recent messages of
printk buffer. For which we need to know the header size and it involves some
changes in the pstore framework.

I have the compression patches ready, will be posting it soon as a separate set.

>> +		pr_info("nvram: Compression of oops text supported only when "
>> +				"pstore is not configured");
>> +		return;
>> +	}
>> +
>> +kmsg_dump:
>>   	/*
>>   	 * Figure compression (preceded by elimination of each line's <n>
>>   	 * severity prefix) will reduce the oops/panic report to at most
>> @@ -663,3 +707,104 @@ static void oops_to_nvram(struct kmsg_dumper *dumper,
>>   
>>   	spin_unlock_irqrestore(&lock, flags);
>>   }
>> +
>> +#ifdef CONFIG_PSTORE
> Same comment about too many ifdefs.
>

I will reduce some of the #ifdefs, but its necessary to have one for
declarations and one for function definitions.

For instance: nvram_pstore_write calls nvram_write_os_partition.
So callback function definitions should be after some of the function
definitions which it will make use of.

>> +static int nvram_pstore_open(struct pstore_info *psi)
>> +{
>> +	read_type = -1;
> Locking?

We need to reset read_type for a remount case. When a umount and mount
is done nvram_pstore_read needs to know that it has to start reading
the partitions again. So we reset the iterator.

>> +	return 0;
>> +}
>> +
>> +/*
> Make it a kernel-doc style comment.

Sure.

>> + * Called by pstore_dump() when an oops or panic report is logged to the printk
>> + * buffer. @size bytes have been written to oops_buf, starting after the
>> + * oops_log_info header.
> "@size bytes have", or "@size bytes should be written"?

Sure.

>> + */
>> +static int nvram_pstore_write(enum pstore_type_id type,
>> +				enum kmsg_dump_reason reason,
>> +				u64 *id, unsigned int part, int count,
>> +				size_t size, struct pstore_info *psi)
>> +{
>> +	struct oops_log_info *oops_hdr = (struct oops_log_info *) oops_buf;
>> +
>> +	/* part 1 has the recent messages from printk buffer */
>> +	if (part > 1 || clobbering_unread_rtas_event())
>> +		return -1;
>> +
>> +	BUG_ON(type != PSTORE_TYPE_DMESG);
>> +	BUG_ON(sizeof(*oops_hdr) + size > oops_log_partition.size);
> Why would we be called with the wrong type? Would it be better to just
> return an error, rather than causing another oops while we're trying to
> write the oops?
>
> And couldn't we just clamp the size, rather than BUG'ing.

Yeah right. Will return  an error instead of bugging here.

>> +	oops_hdr->version = OOPS_HDR_VERSION;
>> +	oops_hdr->report_length = (u16) size;
>> +	oops_hdr->timestamp = get_seconds();
>> +	(void) nvram_write_os_partition(&oops_log_partition, oops_buf,
>> +		(int) (sizeof(*oops_hdr) + size), ERR_TYPE_KERNEL_PANIC,
>> +		count);
> You definitely don't need the (void). But more to the point why aren't
> you checking the return value?

I will check for the return value here.

>> +	*id = part;
> What is this? Part of the API?

Yeah this is part of the API.

>> +	return 0;
>> +}
>> +
>> +/*
>> + * Reads the oops/panic report.
>> + * Returns the length of the data we read from each partition.
>> + * Returns 0 if we've been called before.
>> + */
>> +static ssize_t nvram_pstore_read(u64 *id, enum pstore_type_id *type,
>> +				int *count, struct timespec *time, char **buf,
>> +				struct pstore_info *psi)
>> +{
>> +	struct oops_log_info *oops_hdr;
>> +	unsigned int err_type, id_no;
>> +	struct nvram_os_partition *part = NULL;
>> +	char *buff = NULL;
>> +
>> +	read_type++;
>> +
>> +	switch (nvram_type_ids[read_type]) {
>> +	case PSTORE_TYPE_DMESG:
>> +		part = &oops_log_partition;
>> +		*type = PSTORE_TYPE_DMESG;
>> +		break;
>> +	default:
>> +		return 0;
>> +	}
>> +
>> +	buff = kmalloc(part->size, GFP_KERNEL);
>> +
>> +	if (!buff)
>> +		return -ENOMEM;
>> +
>> +	if (nvram_read_partition(part, buff, part->size, &err_type, &id_no)) {
>> +		kfree(buff);
>> +		return 0;
>> +	}
>> +
>> +	*count = 0;
>> +	*id = id_no;
> Can't you just cast in the call to nvram_read_partition() ?

Thats right, my bad will fix it.

>> +	oops_hdr = (struct oops_log_info *)buff;
>> +	*buf = buff + sizeof(*oops_hdr);
>> +	time->tv_sec = oops_hdr->timestamp;
>> +	time->tv_nsec = 0;
>> +	return oops_hdr->report_length;
>> +}
>> +#else
>> +static int nvram_pstore_open(struct pstore_info *psi)
>> +{
>> +	return 0;
>> +}
>> +
>> +static int nvram_pstore_write(enum pstore_type_id type,
>> +				enum kmsg_dump_reason reason, u64 *id,
>> +				unsigned int part, int count, size_t size,
>> +				struct pstore_info *psi)
>> +{
>> +	return 0;
>> +}
>> +
>> +static ssize_t nvram_pstore_read(u64 *id, enum pstore_type_id *type,
>> +				int *count, struct timespec *time, char **buf,
>> +				struct pstore_info *psi)
>> +{
>> +	return 0;
>> +}
>> +#endif
> I don't understand why we have empty versions of these. If CONFIG_PSTORE
> is disabled we should just not register with pstore at all.

Since pstore handles the case by returning an error code when pstore is not
configured I thought of avoiding one more #ifdef during registration.
I will remove empty callbacks and add a #ifdef during registration itself.

> cheers