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Date: Fri, 3 Feb 2012 12:39:17 +0530
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Subject: Re: [RESEND][PATCH] Mark thread stack correctly in proc/<pid>/maps
From: Siddhesh Poyarekar <siddhesh.poyarekar@gmail.com>
To: KOSAKI Motohiro <kosaki.motohiro@gmail.com>
Cc: Jamie Lokier <jamie@shareable.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Alexander Viro <viro@zeniv.linux.org.uk>,
        linux-fsdevel@vger.kernel.org,
        Michael Kerrisk <mtk.manpages@gmail.com>, linux-man@vger.kernel.org
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On Fri, Feb 3, 2012 at 3:10 AM, KOSAKI Motohiro
<kosaki.motohiro@gmail.com> wrote:
>> ?extern unsigned long move_page_tables(struct vm_area_struct *vma,
>> diff --git a/mm/mmap.c b/mm/mmap.c
>> index 3f758c7..2f9f540 100644
>> --- a/mm/mmap.c
>> +++ b/mm/mmap.c
>> @@ -992,6 +992,9 @@ unsigned long do_mmap_pgoff(struct file *file,
>> unsigned long addr,
>> ? ? ? ?vm_flags = calc_vm_prot_bits(prot) | calc_vm_flag_bits(flags) |
>> ? ? ? ? ? ? ? ? ? ? ? ?mm->def_flags | VM_MAYREAD | VM_MAYWRITE |
>> VM_MAYEXEC;
>>
>> + ? ? ? if (flags& ?MAP_STACK)
>> + ? ? ? ? ? ? ? vm_flags |= VM_STACK_FLAGS;
>
>
> ??
> MAP_STACK doesn't mean auto stack expansion. Why do you turn on
> VM_GROWSDOWN?
> Seems incorrect.
>

Right now MAP_STACK does not mean anything since it is ignored. The
intention of this behaviour change is to make MAP_STACK mean that the
map is going to be used as a stack and hence, set it up like a stack
ought to be. I could not really think of a valid case for fixed size
stacks; it looks like a limitation in the pthread implementation in
glibc rather than a feature. So this patch will actually result in
uniform behaviour across threads when it comes to stacks.

This does change vm accounting since thread stacks were earlier
accounted as anon memory.

-- 
Siddhesh Poyarekar
http://siddhesh.in
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