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