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Message-ID: <ZfY8PSnsLtkHBBZF@casper.infradead.org>
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>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	akpm@...ux-foundation.org, x86@...nel.org, bp@...en8.de,
	brauner@...nel.org, bristot@...hat.com, bsegall@...gle.com,
	dave.hansen@...ux.intel.com, dianders@...omium.org,
	dietmar.eggemann@....com, eric.devolder@...cle.com,
	hca@...ux.ibm.com, hch@...radead.org, jacob.jun.pan@...ux.intel.com,
	jgg@...pe.ca, jpoimboe@...nel.org, jroedel@...e.de,
	juri.lelli@...hat.com, kinseyho@...gle.com,
	kirill.shutemov@...ux.intel.com, lstoakes@...il.com,
	luto@...nel.org, mgorman@...e.de, mic@...ikod.net,
	michael.christie@...cle.com, mingo@...hat.com, mjguzik@...il.com,
	mst@...hat.com, npiggin@...il.com, peterz@...radead.org,
	pmladek@...e.com, rick.p.edgecombe@...el.com, rostedt@...dmis.org,
	surenb@...gle.com, tglx@...utronix.de, urezki@...il.com,
	vincent.guittot@...aro.org, vschneid@...hat.com
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.

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