Message-ID: <4FBBC4EE.4040802@nod.at>
Date:	Tue, 22 May 2012 18:55:10 +0200
From:	Richard Weinberger <richard@....at>
To:	Shmulik Ladkani <shmulik.ladkani@...il.com>
CC:	linux-mtd@...ts.infradead.org, dedekind1@...il.com,
	linux-kernel@...r.kernel.org, Heinz.Egger@...utronix.de,
	tim.bird@...sony.com, tglx@...utronix.de
Subject: Re: [PATCH] [RFC] UBI: Implement Fastmap support

On 22.05.2012 17:01, Shmulik Ladkani wrote:
> Thanks Richard,
>
> Just reviewed your patch, besides fastmap.c and ubi-media.h (will get to
> them soon).
> Some comments below.

Thanks a lot!

> On Mon, 21 May 2012 16:01:56 +0200 Richard Weinberger<richard@....at>  wrote:
>> Fastmap (aka checkpointing) allows attaching of an UBI volume in nearly
>> constant time. Only a fixed number of PEBs has to be scanned.
>>
>> Signed-off-by: Richard Weinberger<richard@....at>
>> ---
>>   drivers/mtd/ubi/Makefile    |    2 +-
>>   drivers/mtd/ubi/attach.c    |   68 +++-
>>   drivers/mtd/ubi/build.c     |   23 +
>>   drivers/mtd/ubi/eba.c       |   18 +-
>>   drivers/mtd/ubi/fastmap.c   | 1132 +++++++++++++++++++++++++++++++++++++++++++
>>   drivers/mtd/ubi/ubi-media.h |  118 +++++
>>   drivers/mtd/ubi/ubi.h       |   61 +++-
>>   drivers/mtd/ubi/wl.c        |  170 +++++++-
>>   8 files changed, 1568 insertions(+), 24 deletions(-)
>>   create mode 100644 drivers/mtd/ubi/fastmap.c
>
> The Kconfig changes are not present in this patch.
>
>> @@ -977,7 +977,12 @@ static int scan_peb(struct ubi_device *ubi, struct ubi_attach_info *ai,
>>   	}
>>
>>   	vol_id = be32_to_cpu(vidh->vol_id);
>> -	if (vol_id>  UBI_MAX_VOLUMES&&  vol_id != UBI_LAYOUT_VOLUME_ID) {
>> +
>> +	if (vol_id>  UBI_MAX_VOLUMES&&
>> +		vol_id != UBI_LAYOUT_VOLUME_ID&&
>> +		((vol_id != UBI_FM_SB_VOLUME_ID&&
>> +		vol_id != UBI_FM_DATA_VOLUME_ID) ||
>> +		ubi->attached_by_scanning == true)) {
>
> I think it would be easier to understand if 'attached_by_scanning' is
> evaluated first:
>
> +		(ubi->attached_by_scanning == true ||
> +		(vol_id != UBI_FM_SB_VOLUME_ID&&
> +		vol_id != UBI_FM_DATA_VOLUME_ID))) {

Yep, fixed!

>>   /**
>> - * ubi_attach - attach an MTD device.
>> - * @ubi: UBI device descriptor
>> + * scan_fastmap - attach MTD device using fastmap.
>> + * @ubi: UBI device description object
>>    *
>> - * This function returns zero in case of success and a negative error code in
>> - * case of failure.
>> + * This function attaches a MTD device using a fastmap and returns complete
>> + * information about it in form of a "struct ubi_attach_info" object. In case
>> + * of failure, an error code is returned.
>>    */
>> -int ubi_attach(struct ubi_device *ubi)
>> +static struct ubi_attach_info *scan_fastmap(struct ubi_device *ubi)
>> +{
>> +	int fm_start;
>> +
>> +	fm_start = ubi_find_fastmap(ubi);
>> +	if (fm_start<  0)
>> +		return ERR_PTR(-ENOENT);
>> +
>> +	return ubi_read_fastmap(ubi, fm_start);
>> +}
>
> I think the fastmap should include the bad peb count.
> (formerly, it was detected by scanning all pebs).
> Otherwise UBI is unaware of the correct number of good/available
> pebs (reflected in good_peb_count/avail_pebs).

Yeah, I'll add these counters.

>> diff --git a/drivers/mtd/ubi/build.c b/drivers/mtd/ubi/build.c
>> index 2c5ed5c..ef5d7b7 100644
>> --- a/drivers/mtd/ubi/build.c
>> +++ b/drivers/mtd/ubi/build.c
>> @@ -144,6 +144,16 @@ int ubi_volume_notify(struct ubi_device *ubi, struct ubi_volume *vol, int ntype)
>>
>>   	ubi_do_get_device_info(ubi,&nt.di);
>>   	ubi_do_get_volume_info(ubi, vol,&nt.vi);
>> +
>> +	switch (ntype) {
>> +	case UBI_VOLUME_ADDED:
>> +	case UBI_VOLUME_REMOVED:
>> +	case UBI_VOLUME_RESIZED:
>> +	case UBI_VOLUME_RENAMED:
>> +		if (ubi_update_fastmap(ubi))
>> +			ubi_err("Unable to update fastmap!");
>
> In the error case, what are the consequences leaving the older on-flash
> fastmap? Shouldn't it be invalidated?

If the fastmap is not written complete the CRC check will fail next time 
and we fall back to scanning mode.

>> +int ubi_update_fastmap(struct ubi_device *ubi)
>> +{
>> +	int ret, i;
>> +	struct ubi_fastmap *new_fm;
>> +
>> +	if (ubi->ro_mode)
>> +		return 0;
>> +
>> +	new_fm = kmalloc(sizeof(*new_fm), GFP_KERNEL);
>> +	if (!new_fm)
>> +		return -ENOMEM;
>> +
>> +	ubi->old_fm = ubi->fm;
>> +	ubi->fm = NULL;
>> +
>> +	if (ubi->old_fm) {
>> +		spin_lock(&ubi->wl_lock);
>> +		new_fm->peb[0] = ubi_wl_get_fm_peb(ubi, UBI_FM_MAX_START);
>> +		spin_unlock(&ubi->wl_lock);
>
> Same 3 lines are found later, in the 'else' clause.
> You can place these prior the 'if (ubi->old_fm)'.

Fixed.

>> diff --git a/drivers/mtd/ubi/ubi.h b/drivers/mtd/ubi/ubi.h
>> index 5275632..299a601 100644
>> --- a/drivers/mtd/ubi/ubi.h
>> +++ b/drivers/mtd/ubi/ubi.h
>> @@ -202,6 +202,39 @@ struct ubi_rename_entry {
>>   struct ubi_volume_desc;
>>
>>   /**
>> + * struct ubi_fastmap - in-memory fastmap data structure.
>> + * @peb: PEBs used by the current fastamp
>
> s/fastamp/fastmap/
>
>> + * @ec: the erase counter of each used PEB
>> + * @size: size of the fastmap in bytes
>> + * @used_blocks: number of used PEBs
>> + */
>> +struct ubi_fastmap {
>
> Suggestion: maybe ubi_fastmap_location / ubi_fastmap_layout /
> ubi_fastmap_position ? slightly more explanatory than 'ubi_fastmap'.

Good idea. ubi_fastmap_position seems good to me.

>> @@ -427,6 +467,12 @@ struct ubi_device {
>>   	struct rb_root ltree;
>>   	struct mutex alc_mutex;
>>
>> +	/* Fastmap stuff */
>> +	struct ubi_fastmap *fm;
>> +	struct ubi_fastmap *old_fm;
>> +	struct ubi_fm_pool fm_pool;
>> +	bool attached_by_scanning;
>
> Convention around MTD is 'int' for booleans.

Fixed.

>> @@ -670,6 +717,9 @@ int ubi_wl_scrub_peb(struct ubi_device *ubi, int pnum);
>>   int ubi_wl_init(struct ubi_device *ubi, struct ubi_attach_info *ai);
>>   void ubi_wl_close(struct ubi_device *ubi);
>>   int ubi_thread(void *u);
>> +int ubi_wl_get_fm_peb(struct ubi_device *ubi, int max_pnum);
>> +int ubi_wl_put_fm_peb(struct ubi_device *ubi, int pnum, int torture);
>> +int ubi_is_fm_block(struct ubi_device *ubi, int pnum);
>
> No users of 'ubi_is_fm_block' outside of wl.c (where it is defined).
> Is it deliberately a part of the ubi interface?

True. Fixed.

>> @@ -367,13 +386,70 @@ static struct ubi_wl_entry *find_wl_entry(struct rb_root *root, int diff)
>>   }
>>
>>   /**
>> - * ubi_wl_get_peb - get a physical eraseblock.
>> + * find_early_wl_entry - find wear-leveling entry with a low pnum.
>
> s/a low/the lowest/

Fixed.

>> + * @root: the RB-tree where to look for
>> + * @max_pnum: highest possible pnum
>> + *
>> + * This function looks for a wear leveling entry containing a eb which
>> + * is in front of the memory.
>
> IMO can remove the last comment.

Documentation is like sex: when it is good, it is very, very good; and 
when it is bad, it is better than nothing.
- Dick Brandon

>> +static struct ubi_wl_entry *find_early_wl_entry(struct rb_root *root,
>> +						int max_pnum)
>> +{
>> +	struct rb_node *p;
>> +	struct ubi_wl_entry *e, *victim = NULL;
>> +
>> +	ubi_rb_for_each_entry(p, e, root, u.rb) {
>> +		if (e->pnum<  max_pnum) {
>> +			victim = e;
>> +			max_pnum = e->pnum;
>> +		}
>> +	}
>> +
>> +	return victim;
>> +}
>> +
>> +/**
>> + * ubi_wl_get_fm_peb - find a physical erase block with a given maximal number.
>> + * @ubi: UBI device description object
>> + * @max_pnum: the highest acceptable erase block number
>> + *
>> + * The function returns a physical erase block with a given maximal number
>> + * and removes it from the wl subsystem.
>> + * Must be called with wl_lock held!
>> + */
>> +int ubi_wl_get_fm_peb(struct ubi_device *ubi, int max_pnum)
>> +{
>> +	int ret = -ENOSPC;
>> +	struct ubi_wl_entry *e;
>> +
>> +	if (!ubi->free.rb_node) {
>> +		ubi_err("no free eraseblocks");
>> +
>> +		goto out;
>> +	}
>> +
>> +	e = find_early_wl_entry(&ubi->free, max_pnum);
>
> This picks the eb with the lowest pnum within 'ubi->free'.
>
> When called with INT_MAX (for the FM_DATA), why do you need to pick a
> free eb with the minimal pnum? The FM_DATA EBs may reside everywhere (as
> the FM_SB holds their location).
> So why not pick the eb with a medium EC value (as done for standard
> get_peb calls)? That might be better wear-leveling wise.

Fair point.
I'll fix that.
Artem, any comments on that?

>> +/* ubi_wl_get_peb - works exaclty like __ubi_wl_get_peb but keeps track of
>> + * the fastmap pool.
>> + */
>> +int ubi_wl_get_peb(struct ubi_device *ubi)
>> +{
>> +	struct ubi_fm_pool *pool =&ubi->fm_pool;
>> +
>> +	/* pool contains no free blocks, create a new one
>> +	 * and write a fastmap */
>> +	if (pool->used == pool->size || !pool->size) {
>> +		for (pool->size = 0; pool->size<  pool->max_size;
>> +				pool->size++) {
>> +			pool->pebs[pool->size] = __ubi_wl_get_peb(ubi);
>> +			if (pool->pebs[pool->size]<  0)
>> +				break;
>> +		}
>> +
>> +		pool->used = 0;
>> +		ubi_update_fastmap(ubi);
>> +	}
>> +
>> +	/* we got not a single free PEB */
>> +	if (!pool->size)
>> +		return -1;
>
> Returning -ENOSPC is more appropriate and consistent with former
> ubi_wl_get_peb implementation.

Fixed.

>>   /**
>> + * ubi_wl_put_fm_peb - returns a PEB used in a fastmap to the wear-leveling
>> + * sub-system.
>> + *
>> + * see: ubi_wl_put_peb()
>> + */
>> +int ubi_wl_put_fm_peb(struct ubi_device *ubi, int pnum, int torture)
>> +{
>> +	int i, err = 0;
>> +	struct ubi_wl_entry *e;
>> +
>> +	dbg_wl("PEB %d", pnum);
>> +	ubi_assert(pnum>= 0);
>> +	ubi_assert(pnum<  ubi->peb_count);
>> +
>> +	spin_lock(&ubi->wl_lock);
>> +	e = ubi->lookuptbl[pnum];
>> +
>> +	/* This can happen if we recovered from a fastmap the very
>> +	 * frist time and writing now a new one. In this case the wl system
>
> s/frist/first/

Fixed.

>> +	 * has never seen any PEB used by the original fastmap.
>> +	 */
>> +	if (!e) {
>> +		ubi_assert(ubi->old_fm);
>> +		e = kmem_cache_alloc(ubi_wl_entry_slab, GFP_ATOMIC);
>
> Must it be GFP_ATOMIC?

Yes. This function is called under a spinlock.

>> +		if (!e) {
>> +			spin_unlock(&ubi->wl_lock);
>> +			return -ENOMEM;
>
> This is traumatic; you must return the PEB back to WL, but fail to do so
> due to an internal error.

This is not easy. In the !e case I need memory.

> (BTW the return value of ubi_wl_put_fm_peb is not tested at the call
> sites)
> The system is left with a missing PEB.
> Can't we guarantee ubi_wl_put_fm_peb is called only after wl_init is
> completed?

Why would that help?

Thanks,
//richard
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