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Date: Sat, 16 Mar 2024 17:47:18 -0700
From: "H. Peter Anvin" <hpa@zytor.com>
To: Matthew Wilcox <willy@infradead.org>
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Subject: Re: [RFC 00/14] Dynamic Kernel Stacks
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On March 14, 2024 12:43:06 PM PDT, Matthew Wilcox <willy@infradead=2Eorg> w=
rote:
>On Tue, Mar 12, 2024 at 10:18:10AM -0700, H=2E Peter Anvin wrote:
>> Second, non-dynamic kernel memory is one of the core design decisions i=
n
>> Linux from early on=2E This means there are lot of deeply embedded assu=
mptions
>> which would have to be untangled=2E
>
>I think there are other ways of getting the benefit that Pasha is seeking
>without moving to dynamically allocated kernel memory=2E  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)=2E
>
>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=2E  I suspect Google would also be able to use thi=
s
>API for their rare cases when they need more than 8kB of kernel stack=2E
>Who knows, we might all be able to use such a thing=2E
>
>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; allocat=
e
>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=2E  No complicated "switch stacks" AP=
I
>needed, just an "ensure we have at least N bytes of stack remaining" API=
=2E

This is what stack probes basically does=2E It provides a very cheap "API"=
 that goes via the #PF (not #DF!) path in the slow case, but synchronously =
at a well-defined point, but is virtually free in the common case=2E As a s=
ide benefit, they can be compiler-generated, as some operating systems requ=
ire them=2E