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From: Rasmus Villemoes <linux@rasmusvillemoes.dk>
To: Siddhesh Poyarekar <siddhesh.poyarekar@gmail.com>
Cc: Calvin Owens <calvinowens@fb.com>,
        Andrew Morton <akpm@linux-foundation.org>,
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Subject: Re: [RFC][PATCH] procfs: Add /proc/<pid>/mapped_files
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Date: Wed, 14 Jan 2015 15:53:16 +0100
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	(Siddhesh Poyarekar's message of "Wed, 14 Jan 2015 20:07:06 +0530")
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On Wed, Jan 14 2015, Siddhesh Poyarekar <siddhesh.poyarekar@gmail.com> wrote:

> On 14 January 2015 at 19:43, Rasmus Villemoes <linux@rasmusvillemoes.dk> wrote:
>> Just thinking out loud: Could one simply mark a VMA as being used for
>> stack during the clone call (is there room in vm_flags, or does
>> VM_GROWSDOWN already tell the whole story?), and then write the TID into
>> a new field in the VMA - I think one could make a union with vm_pgoff so
>> as not to enlarge the structure.
>
> vm_flags does not have space IIRC (that was my first approach at
> implementing this) and VM_GROWSDOWN is not sufficient.

Looking at include/linux/mm.h:

#define VM_GROWSDOWN    0x00000100      /* general info on the segment */
#define VM_PFNMAP       0x00000400      /* Page-ranges managed without "struct page", just pure PFN */
#define VM_DENYWRITE    0x00000800      /* ETXTBSY on write attempts.. */

It would seem that 0x00000200 is available (unless defined and used
somewhere else).

> If we can make a union with vm_pgoff like you say, we probably don't
> need a flag value; a non-zero value could indicate that it is a thread
> stack.

Well, only when combined with checking vm_file for being NULL. One would
also need to ensure that vm_pgoff is 0 for any non-stack,
non-file-backed VMA. At which point it is somewhat ugly. 

> One problem with caching the value on clone like this though is that
> the stack could change due to a setcontext, but AFAICT we don't care
> about that for the process stack either.

If it is important, I guess one could update the info when a task calls
setcontext.

Rasmus