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

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Date:	Fri, 24 Feb 2012 11:14:57 -0500
From:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
To:	Siddhesh Poyarekar <siddhesh.poyarekar@...il.com>
CC:	KOSAKI Motohiro <kosaki.motohiro@...il.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org, Jamie Lokier <jamie@...reable.org>,
	vapier@...too.org, Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] Mark thread stack correctly in proc/<pid>/maps

>> Sigh. No, I missed one thing. If application use
>> makecontext()/swapcontext() pair,
>> ESP is not reliable way to detect pthread stack. At that time the
>> stack is still marked
>> as anonymous memory.
>
> This is not wrong, because it essentially gives the correct picture of
> the state of that task -- the task is using another vma as a stack
> during that point and not the one it was allotted by pthreads during
> thread creation.
>
> I don't think we can successfully stick to the idea of trying to mark
> stack space allocated by pthreads but not used by any task *currently*
> as stack as long as the allocation happens outside the kernel space.
> The only way to mark this is either by marking the stack as
> VM_GROWSDOWN (which will make the stack grow and break some pthreads
> functions) or create a new flag, which a simple display such as this
> does not deserve. So it's best that this sticks to what the kernel
> *knows* is being used as stack.

Oh, maybe generically you are right. but you missed one thing. Before
your patch, stack or not stack are address space property. thus, using
/proc/pid/maps makes sense. but after your patch, it's no longer memory
property. applications can use heap or mapped file as a stack. then, at
least, current your code is wrong. the code assume each memory property
are exclusive.

Moreover, if pthread stack is unimportant, I wonder why we need this patch
at all. Which application does need it? and When?

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