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Date:   Fri, 11 Aug 2023 01:36:22 -0700
From:   Christoph Hellwig <hch@infradead.org>
To:     Richard Weinberger <richard@nod.at>
Cc:     Christoph Hellwig <hch@infradead.org>,
        linux-mtd <linux-mtd@lists.infradead.org>,
        Stephan Wurm <stephan.wurm@a-eberle.de>,
        stable <stable@vger.kernel.org>,
        Miquel Raynal <miquel.raynal@bootlin.com>,
        Vignesh Raghavendra <vigneshr@ti.com>,
        Oliver Neukum <oliver@neukum.org>,
        Ali Akcaagac <aliakc@web.de>,
        Jamie Lenehan <lenehan@twibble.org>,
        James Bottomley <jejb@linux.ibm.com>,
        "Martin K. Petersen" <martin.petersen@oracle.com>,
        Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        linux-scsi <linux-scsi@vger.kernel.org>
Subject: Re: [PATCH 1/7] ubi: block: Refactor sg list processing for highmem
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On Fri, Aug 11, 2023 at 10:34:52AM +0200, Richard Weinberger wrote:
> ----- Urspr?ngliche Mail -----
> > Von: "Christoph Hellwig" <hch@infradead.org>
> > On Thu, Aug 10, 2023 at 09:54:46PM +0200, Richard Weinberger wrote:
> >> > But why add the bounce buffering first if you can avoid it from the
> >> > very beginning by just using kmap_local instead of adding a new
> >> > caller for the deprecate kmap_atomic?
> >> 
> >> Because I want this fix also in all stable trees. kmap_local() is rather
> >> new. When back porting patch 1/7, bounce buffers and kmap_atomic()
> >> are needed anyway.
> >> By doing this in patch 1/7 I avoid backport troubles and keep the
> >> delta between upstream and stable trees minimal.
> > 
> > Just use plain kmap for the historic backports.
> 
> Hm, yes. For UBIblock kmap should be too expensive.

?  kmap is a no-op for !highmem.  And it will always be way cheaper
than bounce buffering for the case where you are actually fed highmem
pages.