linux-kernel - Re: [RESEND][PATCH] Mark thread stack correctly in proc/<pid>/maps

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Message-ID: <CAAHN_R0O7a+RX7BDfas3+vC+mnQpp0h3y4bBa1u4T-Jt=S9J_w@mail.gmail.com>
Date:	Fri, 3 Feb 2012 12:39:17 +0530
From:	Siddhesh Poyarekar <siddhesh.poyarekar@...il.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
Cc:	Jamie Lokier <jamie@...reable.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org,
	Michael Kerrisk <mtk.manpages@...il.com>,
	linux-man@...r.kernel.org
Subject: Re: [RESEND][PATCH] Mark thread stack correctly in proc/<pid>/maps

On Fri, Feb 3, 2012 at 3:10 AM, KOSAKI Motohiro
<kosaki.motohiro@...il.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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