On Wednesday 01/14 at 15:53 +0100, Rasmus Villemoes wrote: > On Wed, Jan 14 2015, Siddhesh Poyarekar <siddhesh.poyare...@gmail.com> wrote: > > > On 14 January 2015 at 19:43, Rasmus Villemoes <li...@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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
