lkml.org link to message, because why on earth would you want to post a screenshot of text.

The email is part of a chain of responses to this RFC for adding dynamic kernel stacks, which is itself a continuation of this RFC. A high-level overview of the feature from the original RFC:

This feature allows to grow kernel stack dynamically, from 4KiB and up to the THREAD_SIZE. The intend is to save memory on fleet machines. From the initial experiments it shows to save on average 70-75% of the kernel stack memory.

And from the continuation:

The impetus for reviving this RFC is kernel stack usage on Android. On regular Android (i.e. non-wear/automotive), system processes typically have 2000-3000 threads. When adding threads from app processes, this means that systems with 4GB of memory are using 1-2% of total memory for kernel thread stacks. Dynamic kernel stacks reduce this by 65%-70%.

The main change compared to Pasha's v1 RFC is how x86_64 handles kernel stack faults.