From: "Valdis Klētnieks" <valdis.kletnieks@vt.edu>
To: ", Samuel" <samuel@csie.io>
Cc: kernelnewbies@kernelnewbies.org
Subject: Re: process descriptor address in kernel stack
Date: Thu, 19 Mar 2020 05:22:32 -0400 [thread overview]
Message-ID: <22784.1584609752@turing-police> (raw)
In-Reply-To: <CAEw=bybknPF5jK-ha3agXbxryH=7=-6XHnw_XMUiAXuGgenGVg@mail.gmail.com>
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On Thu, 19 Mar 2020 16:53:32 +0800, ", Samuel" said:
> movl $0xffffe000,%ecx or 0xfffff000 for 4KB stacks
> andl %esp,%ecx
> movl %ecx,p
>
> Why is *"stack pointer(esp) & 0xffffe000"* equal to the process descriptor
> base address?
>
> That means the base address of process descriptor is always *0xXYZ...000*,
> right? It is weird.
It's not at all weird if the kernel, when allocating the stack space to begin with,
asked for 1 (or 2 contiguous) 4K chunks of memory, at a page-aligned address....
For example, see kernel/fork.c:
238 /*
239 * Allocated stacks are cached and later reused by new threads,
240 * so memcg accounting is performed manually on assigning/releasing
241 * stacks to tasks. Drop __GFP_ACCOUNT.
242 */
243 stack = __vmalloc_node_range(THREAD_SIZE, THREAD_ALIGN,
244 VMALLOC_START, VMALLOC_END,
245 THREADINFO_GFP & ~__GFP_ACCOUNT,
246 PAGE_KERNEL,
247 0, node, __builtin_return_address(0));
I'll leave figuring out what THREAD_ALIGN is set to, as an exercise for the student. :)
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prev parent reply other threads:[~2020-03-19 9:24 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-19 8:53 process descriptor address in kernel stack , Samuel
2020-03-19 9:22 ` Valdis Klētnieks [this message]
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