Question about backport output generation with linux-4.9

Hi,

I am a newbie on linux-backport and I have a problem generating a
backport output with linux-kernel 4.9.y checkout. These are what I ran
to generate backport output against linux-4.9 kernel.

1) "git clone git://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git;
git checkout linux-4.9.y"        # linux-stable git repo clone for
source
2) "git clone git://git.kernel.org/pub/scm/linux/kernel/git/backports/backports.git";
git checkout linux-4.19.y" # linux-backports
3) In backports directory, ran "./gentree --copy-list copy-list
--clean /abs-path/linux-stable /abs-path/output"

I hit many gentree errors like

> File doesn't exists
> Patch error (refcount.c lock, rhashtable related things...)
> Another patch errors

I resolved by removing the patch files and directly modified the
source in linux-4.9 stable repo as well as modified Kconfig file under
backports/compat.
Also, remove files that don't exist.

I am really wondering whether these procedures are the right way to
generate the backport output.
I need to generate mac80211.ko / cfg80211.ko / and wifi-driver.ko
against our linux-4.9 kernel source code to support a lower version of
kernel.

If I checked out linux-5.4.y kernel from linux-stable repo and checked
out linux-5.4.y from backport repo, I don't see these types of errors.
Please advise me.

Thank you.
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