On Mon, Jun 15, 2020 at 12:18:48PM -0700, Linus Torvalds wrote:
> Commit 17839856fd58 ("gup: document and work around "COW can break
> either way" issue") is a real fix, but wasn't marked for stable
> because I wanted it to get more coverage testing in mainline first.
> Not because the patch is all that complex or scary, but because I was
> worried we'd find some odd case where it would make things slower by
> triggering the GUP slowpath much more often due to people doing odd
> things.
>
> It turns out my worry seems to have been misplaced. The kernel test
> robot did indeed trigger a case where this made a big difference, but
> rather than being bad, it improved the odd corner-case test-case
> performance by a factor of 20x by breaking the COW and triggering the
> fast-case code that way, rather than the other way around.
>
> See
>
> https://lore.kernel.org/lkml/20200611040453.GK12456@shao2-debian/
>
> for details.
>
> So that commit fixes a bug, isn't expected to really make any
> difference on any sane workload, and can apparently help the crazy
> cases by a huge amount. Let's just push it to stable..
Will go do that now, was waiting to see if this caused any problems,
glad to see it hasn't.
greg k-h
