On Thu, 1 Feb 2018, Linus Torvalds wrote:

On Thu, Feb 1, 2018 at 6:18 AM, Vince Weaver [off-list ref] wrote:

quoted

I thought that to get into stable the patch had to be in upstream first.

Upstream does have the fix, it's just very very different in this case.

The upstream fix is commit 99a9dc98ba52 ("x86,perf: Disable intel_bts
when PTI"), which just disables intel_bts entirely, because upstream
doesn't do those "kaiser_add/remove_mapping()" things.

So this "backport" is ok.

           Linus

Unless I'm misunderstanding, the fix going into 4.4 actually fixes the 
issue (BTS trying to access a VM buffer that is unmapped) using the KAISER 
infrastructure.

The "fix" in 4.15 just disables BTS totally (until someone figures out how 
to fix things properly).

So while in theory both are equivelant fixes ("the kernel no longer 
crashes when using perf/bts") the mechanism is completely different.

I'm only minorly complaining because I do a lot of testing/fuzzing on 
linux-git to catch perf related issues (I'm the one who first reported 
this issue in the first place).  But I typically don't test the stable 
trees, and now we more or less have a fork where the perf/bts code is 
doing something very different in stable vs head.

Vince