On Fri, Jan 31, 2014 at 03:00:58PM -0800, Andrew Morton wrote:
> On Fri, 31 Jan 2014 12:23:45 -0600 Alex Thorlton <athorlton@xxxxxxx> wrote:
>
> > This patch adds a VM_INIT_DEF_MASK, to allow us to set the default flags
> > for VMs. It also adds a prctl control which alllows us to set the THP
> > disable bit in mm->def_flags so that VMs will pick up the setting as
> > they are created.
> >
> > ...
> >
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -177,6 +177,8 @@ extern unsigned int kobjsize(const void *objp);
> > */
> > #define VM_SPECIAL (VM_IO | VM_DONTEXPAND | VM_PFNMAP)
> >
> > +#define VM_INIT_DEF_MASK VM_NOHUGEPAGE
>
> Document this here?
Can do. I suppose it's not exactly self-explanatory :)
> > /*
> > * mapping from the currently active vm_flags protection bits (the
> > * low four bits) to a page protection mask..
> > diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
> > index 289760f..58afc04 100644
> > --- a/include/uapi/linux/prctl.h
> > +++ b/include/uapi/linux/prctl.h
> > @@ -149,4 +149,7 @@
> >
> > #define PR_GET_TID_ADDRESS 40
> >
> > +#define PR_SET_THP_DISABLE 41
> > +#define PR_GET_THP_DISABLE 42
> > +
> > #endif /* _LINUX_PRCTL_H */
> > diff --git a/kernel/fork.c b/kernel/fork.c
> > index a17621c..9fc0a30 100644
> > --- a/kernel/fork.c
> > +++ b/kernel/fork.c
> > @@ -529,8 +529,6 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p)
> > atomic_set(&mm->mm_count, 1);
> > init_rwsem(&mm->mmap_sem);
> > INIT_LIST_HEAD(&mm->mmlist);
> > - mm->flags = (current->mm) ?
> > - (current->mm->flags & MMF_INIT_MASK) : default_dump_filter;
> > mm->core_state = NULL;
> > atomic_long_set(&mm->nr_ptes, 0);
> > memset(&mm->rss_stat, 0, sizeof(mm->rss_stat));
> > @@ -539,8 +537,15 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p)
> > mm_init_owner(mm, p);
> > clear_tlb_flush_pending(mm);
> >
> > - if (likely(!mm_alloc_pgd(mm))) {
> > + if (current->mm) {
> > + mm->flags = current->mm->flags & MMF_INIT_MASK;
> > + mm->def_flags = current->mm->def_flags & VM_INIT_DEF_MASK;
>
> So VM_INIT_DEF_MASK defines which vm flags a process may inherit from
> its parent?
Yep. It behaves pretty much the same way as MMF_INIT_MASK.
> > + } else {
> > + mm->flags = default_dump_filter;
> > mm->def_flags = 0;
> > + }
> > +
> > + if (likely(!mm_alloc_pgd(mm))) {
> > mmu_notifier_mm_init(mm);
> > return mm;
> > }
> > diff --git a/kernel/sys.c b/kernel/sys.c
> > index c0a58be..d59524a 100644
> > --- a/kernel/sys.c
> > +++ b/kernel/sys.c
> > @@ -1996,6 +1996,23 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
> > if (arg2 || arg3 || arg4 || arg5)
> > return -EINVAL;
> > return current->no_new_privs ? 1 : 0;
> > + case PR_GET_THP_DISABLE:
> > + if (arg2 || arg3 || arg4 || arg5)
> > + return -EINVAL;
>
> Please add
>
> /* fall through */
>
> here. So people don't think you added a bug. Also, iirc there's a
> static checking tool which will complain about this and there was talk
> about using the /* fall through */ to suppress the warning.
Understood. More comments below.
> > + case PR_SET_THP_DISABLE:
> > + if (arg3 || arg4 || arg5)
> > + return -EINVAL;
> > + down_write(&me->mm->mmap_sem);
> > + if (option == PR_SET_THP_DISABLE) {
> > + if (arg2)
> > + me->mm->def_flags |= VM_NOHUGEPAGE;
> > + else
> > + me->mm->def_flags &= ~VM_NOHUGEPAGE;
> > + } else {
> > + error = !!(me->mm->def_flags & VM_NOHUGEPAGE);
> > + }
> > + up_write(&me->mm->mmap_sem);
> > + break;
>
> I suspect it would be simpler to not try to combine the set and get
> code in the same lump.
I think you're right here. This is what we originally came up with for
this piece, but I think it will look simpler to do each check
separately. In that case, we won't need the /* fall through */ either,
so that will take care of both issues.
> The prctl() extension should be added to user-facing documentation.
> Please work with Michael Kerrisk <mtk.manpages@xxxxxxxxx> on that.
Got it. I'll make sure that gets in on the next pass.
Thanks for the input, Andrew!
- Alex
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