linux-kernel - Re: [PATCH v6 12/15] ubi: fastmap: Add all fastmap pebs into 'ai->fastmap' when fm->used

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Date:   Tue, 11 Jan 2022 19:44:49 +0800
From:   Zhihao Cheng <chengzhihao1@...wei.com>
To:     Richard Weinberger <richard@....at>
CC:     Miquel Raynal <miquel.raynal@...tlin.com>,
        Vignesh Raghavendra <vigneshr@...com>,
        mcoquelin stm32 <mcoquelin.stm32@...il.com>,
        "kirill shutemov" <kirill.shutemov@...ux.intel.com>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        linux-mtd <linux-mtd@...ts.infradead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: 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                           * find any outdated fastmap anchors, hence we erase the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                           * outdated fastmap anchor PEBs synchronously here.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outdated fastmap PEBs cannot be erased.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'