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Date: Thu, 14 Mar 2024 19:57:22 +0000
From: Matthew Wilcox <willy@infradead.org>
To: Kent Overstreet <kent.overstreet@linux.dev>
Cc: "H. Peter Anvin" <hpa@zytor.com>,
	Pasha Tatashin <pasha.tatashin@soleen.com>,
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Subject: Re: [RFC 00/14] Dynamic Kernel Stacks
Message-ID: <ZfNWojLB7qjjB0Zw@casper.infradead.org>
References: <20240311164638.2015063-1-pasha.tatashin@soleen.com>
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 <ZfNTSjfE_w50Otnz@casper.infradead.org>
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On Thu, Mar 14, 2024 at 03:53:39PM -0400, Kent Overstreet wrote:
> On Thu, Mar 14, 2024 at 07:43:06PM +0000, Matthew Wilcox wrote:
> > On Tue, Mar 12, 2024 at 10:18:10AM -0700, H. Peter Anvin wrote:
> > > Second, non-dynamic kernel memory is one of the core design decisions in
> > > Linux from early on. This means there are lot of deeply embedded assumptions
> > > which would have to be untangled.
> > 
> > I think there are other ways of getting the benefit that Pasha is seeking
> > without moving to dynamically allocated kernel memory.  One icky thing
> > that XFS does is punt work over to a kernel thread in order to use more
> > stack!  That breaks a number of things including lockdep (because the
> > kernel thread doesn't own the lock, the thread waiting for the kernel
> > thread owns the lock).
> > 
> > If we had segmented stacks, XFS could say "I need at least 6kB of stack",
> > and if less than that was available, we could allocate a temporary
> > stack and switch to it.  I suspect Google would also be able to use this
> > API for their rare cases when they need more than 8kB of kernel stack.
> > Who knows, we might all be able to use such a thing.
> > 
> > I'd been thinking about this from the point of view of allocating more
> > stack elsewhere in kernel space, but combining what Pasha has done here
> > with this idea might lead to a hybrid approach that works better; allocate
> > 32kB of vmap space per kernel thread, put 12kB of memory at the top of it,
> > rely on people using this "I need more stack" API correctly, and free the
> > excess pages on return to userspace.  No complicated "switch stacks" API
> > needed, just an "ensure we have at least N bytes of stack remaining" API.
> 
> Why would we need an "I need more stack" API? Pasha's approach seems
> like everything we need for what you're talking about.

Because double faults are hard, possibly impossible, and the FRED approach
Peter described has extra overhead?  This was all described up-thread.