Re: [PATCH 1/7] ubi: block: Refactor sg list processing for highmem

[Christoph Hellwig]

On Fri, Aug 11, 2023 at 10:34:52AM +0200, Richard Weinberger wrote:
> ----- Ursprüngliche Mail -----
> > Von: "Christoph Hellwig" <hch@xxxxxxxxxxxxx>
> > 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.