Re: [PATCH 2/3] Add VM_INIT_DEF_MASK and PRCTL_THP_DISABLE

[Alex Thorlton]

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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