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SubjectRe: [RFC][PATCH] procfs: Add /proc/<pid>/mapped_files
On Wednesday 01/14 at 15:53 +0100, Rasmus Villemoes wrote:
> 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.

If I understand the current behavior, the "[stack]" marker will get put
next to *any* mapping that encompasses the current value in the task's
%sp, regardless of how the mapping was created or ucontext stuff. If
you use flags on the VMA structs things could potentially be marked as
stacks even though %sp points somewhere else.

It's probable that nobody cares (you'd obviously have to be doing crazy
things to be pointing %sp at arbitrary places), but that's why I was
hesitant to mess with it.

Thanks,
Calvin

> Rasmus


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