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Message-ID: <4f77c04b-5fe3-4618-aaaf-7bcc6058591e@infradead.org>
Date: Mon, 11 Mar 2024 12:32:09 -0700
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Subject: Re: [RFC 10/14] fork: Dynamic Kernel Stacks
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To: Pasha Tatashin <pasha.tatashin@soleen.com>, linux-kernel@vger.kernel.org,
 linux-mm@kvack.org, akpm@linux-foundation.org, x86@kernel.org, bp@alien8.de,
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References: <20240311164638.2015063-1-pasha.tatashin@soleen.com>
 <20240311164638.2015063-11-pasha.tatashin@soleen.com>
From: Randy Dunlap <rdunlap@infradead.org>
In-Reply-To: <20240311164638.2015063-11-pasha.tatashin@soleen.com>
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Hi,

just typos etc.

On 3/11/24 09:46, Pasha Tatashin wrote:
> The core implementation of dynamic kernel stacks.
> 

..

> 
> Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
> ---
>  arch/Kconfig                     |  34 +++++
>  include/linux/sched.h            |   2 +-
>  include/linux/sched/task_stack.h |  41 +++++-
>  kernel/fork.c                    | 239 +++++++++++++++++++++++++++++++
>  kernel/sched/core.c              |   1 +
>  5 files changed, 315 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/Kconfig b/arch/Kconfig
> index a5af0edd3eb8..da3df347b069 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -1241,6 +1241,40 @@ config VMAP_STACK
>  	  backing virtual mappings with real shadow memory, and KASAN_VMALLOC
>  	  must be enabled.
>  
> +config HAVE_ARCH_DYNAMIC_STACK
> +	def_bool n
> +	help
> +	  An arch should select this symbol if it can support kernel stacks
> +	  dynamic growth.
> +
> +	  - Arch must have support for HAVE_ARCH_VMAP_STACK, in order to handle
> +	    stack related page faults

	    stack-related

> +
> +	  - Arch must be able to faults from interrupt context.

	                         fault

> +	  - Arch must allows the kernel to handle stack faults gracefully, even

	              allow

> +	    during interrupt handling.
> +
> +	  - Exceptions such as no pages available should be handled the same

	                                                    handled in the same

> +	    in the consitent and predictable way. I.e. the exception should be

	    consistent

> +	    handled the same as when stack overflow occurs when guard pages are
> +	    touched with extra information about the allocation error.
> +
> +config DYNAMIC_STACK
> +	default y
> +	bool "Dynamically grow kernel stacks"
> +	depends on THREAD_INFO_IN_TASK
> +	depends on HAVE_ARCH_DYNAMIC_STACK
> +	depends on VMAP_STACK
> +	depends on !KASAN
> +	depends on !DEBUG_STACK_USAGE
> +	depends on !STACK_GROWSUP
> +	help
> +	  Dynamic kernel stacks allow to save memory on machines with a lot of
> +	  threads by starting with small stacks, and grow them only when needed.
> +	  On workloads where most of the stack depth do not reach over one page

	                                             does

> +	  the memory saving can be subsentantial. The feature requires virtually

	                           substantial.

> +	  mapped kernel stacks in order to handle page faults.
> +
>  config HAVE_ARCH_RANDOMIZE_KSTACK_OFFSET
>  	def_bool n
>  	help


> +/*
> + * This flag is used to pass information from fault handler to refill about
> + * which pages were allocated, and should be charged to memcg.
> + */
> +#define DYNAMIC_STACK_PAGE_AQUIRED_FLAG	0x1

                              ACQUIRED
please


-- 
#Randy