process descriptor address in kernel stack

[Valdis Kl=?utf-8?Q?=c4=93?=tnieks]

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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