Re: [PATCH v6 12/15] ubi: fastmap: Add all fastmap pebs into 'ai->fastmap' when fm->used_blocks>=2

[Richard Weinberger <richard@nod.at>]

----- Ursprüngliche Mail -----
> Von: "chengzhihao1" <chengzhihao1@huawei.com>
> An: "richard" <richard@nod.at>
> CC: "Miquel Raynal" <miquel.raynal@bootlin.com>, "Vignesh Raghavendra" <vigneshr@ti.com>, "mcoquelin stm32"
> <mcoquelin.stm32@gmail.com>, "kirill shutemov" <kirill.shutemov@linux.intel.com>, "Sascha Hauer"
> <s.hauer@pengutronix.de>, "linux-mtd" <linux-mtd@lists.infradead.org>, "linux-kernel" <linux-kernel@vger.kernel.org>
> Gesendet: Dienstag, 11. Januar 2022 12:44:49
> Betreff: Re: [PATCH v6 12/15] ubi: fastmap: Add all fastmap pebs into 'ai->fastmap' when fm->used_blocks>=2

> Hi Richard,
>> In ubi_wl_init() there is another corner case documented:
>>                          /*
>>                           * The fastmap update code might not find a free PEB for
>>                           * writing the fastmap anchor to and then reuses the
>>                           * current fastmap anchor PEB. When this PEB gets erased
>>                           * and a power cut happens before it is written again we
>>                           * must make sure that the fastmap attach code doesn't
>>                           * find any outdated fastmap anchors, hence we erase the
>>                           * outdated fastmap anchor PEBs synchronously here.
>>                           */
>>                          if (aeb->vol_id == UBI_FM_SB_VOLUME_ID)
>>                                  sync = true;
>> 
>> So ubi_wl_init() makes sure that all old fastmap anchors get erased before UBI
>> starts to operate. With your change this is no longer satisfied
> I seem to understand the another case. But I'm still confused that why
> outdated fastmap PEBs cannot be erased. When UBI comes to this point, it
> means UBI is attached by **full scanning mode**, scan_fast() returns two
> values:
>   UBI_NO_FASTMAP: scan all pebs from pnum UBI_FM_MAX_START, ai is
> assigned with scan_ai, at last, all fastmap pebs are added into
> 'ai->fastmap'
>   UBI_BAD_FASTMAP: scan all pebs from pnum 0, all fastmap pebs are
> added into 'ai->fastmap'

I fear, I'm unable to follow your thoughts.
ubi_wl_init() is called in both cases, with and without fastmap.
And ai->fastmap contains all anchor PEBs that scan_fast() found.
This can be the most recent but also outdated anchor PEBs.

Thanks,
//richard