linux-kernel - Re: [RFC 00/14] Dynamic Kernel Stacks

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Date: Sun, 17 Mar 2024 00:41:33 +0000
From: Matthew Wilcox <willy@...radead.org>
To: Pasha Tatashin <pasha.tatashin@...een.com>
Cc: "H. Peter Anvin" <hpa@...or.com>,
	Kent Overstreet <kent.overstreet@...ux.dev>,
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Subject: Re: [RFC 00/14] Dynamic Kernel Stacks

On Sat, Mar 16, 2024 at 03:17:57PM -0400, Pasha Tatashin wrote:
> Expanding on Mathew's idea of an interface for dynamic kernel stack
> sizes, here's what I'm thinking:
> 
> - Kernel Threads: Create all kernel threads with a fully populated
> THREAD_SIZE stack.  (i.e. 16K)
> - User Threads: Create all user threads with THREAD_SIZE kernel stack
> but only the top page mapped. (i.e. 4K)
> - In enter_from_user_mode(): Expand the thread stack to 16K by mapping
> three additional pages from the per-CPU stack cache. This function is
> called early in kernel entry points.
> - exit_to_user_mode(): Unmap the extra three pages and return them to
> the per-CPU cache. This function is called late in the kernel exit
> path.
> 
> Both of the above hooks are called with IRQ disabled on all kernel
> entries whether through interrupts and syscalls, and they are called
> early/late enough that 4K is enough to handle the rest of entry/exit.

At what point do we replenish the per-CPU stash of pages?  If we're
12kB deep in the stack and call mutex_lock(), we can be scheduled out,
and then the new thread can make a syscall.  Do we just assume that
get_free_page() can sleep at kernel entry (seems reasonable)?  I don't
think this is an infeasible problem, I'd just like it to be described.