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

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Date: Sat, 16 Mar 2024 21:32:48 -0400
From: Kent Overstreet <kent.overstreet@...ux.dev>
To: Matthew Wilcox <willy@...radead.org>
Cc: Pasha Tatashin <pasha.tatashin@...een.com>, 
	"H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org, linux-mm@...ck.org, 
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Subject: Re: [RFC 00/14] Dynamic Kernel Stacks

On Sun, Mar 17, 2024 at 12:41:33AM +0000, Matthew Wilcox wrote:
> 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.

schedule() or return to userspace, I believe was mentioned