Re: [PATCH 5/8] thp, mm: locking tail page is a bug

[Hugh Dickins]

On Wed, 17 Jul 2013, Dave Jones wrote:
> On Wed, Jul 17, 2013 at 05:58:13PM -0700, Hugh Dickins wrote:
>  > On Wed, 17 Jul 2013, Dave Hansen wrote:
>  > > On 07/17/2013 02:09 PM, Andrew Morton wrote:
>  > > > lock_page() is a pretty commonly called function, and I assume quite a
>  > > > lot of people run with CONFIG_DEBUG_VM=y.
>  > > > 
>  > > > Is the overhead added by this patch really worthwhile?
>  > > 
>  > > I always thought of it as a developer-only thing.  I don't think any of
>  > > the big distros turn it on by default.
>  > 
>  > That's how I think of it too (and the problem is often that too few mm
>  > developers turn it on); but Dave Jones did confirm last November that
>  > Fedora turns it on.
>  > 
>  > I believe Fedora turns it on to help us all, and wouldn't mind a mere
>  > VM_BUG_ON(PageTail(page)) in __lock_page() if it's helpful to Kirill.
>  > 
>  > But if VM_BUG_ONs become expensive, I do think it's for Fedora to
>  > turn off CONFIG_DEBUG_VM, rather than for mm developers to avoid it.
> 
> I'm ambivalent about whether we keep it on or off, we have no shortage
> of bugs to fix already, though I think as mentioned above, very few people
> actually enable it, so we're going to lose a lot of testing.
> 
> Another idea, perhaps is an extra config option for more expensive debug options ?

I can't think of any expensive debug under CONFIG_DEBUG_VM at present,
just a little overhead as you'd expect.  I don't think checking a page
flag or two in __lock_page() will change that much.

We do tend to make specific debug options for the more expensive ones,
rather than a generic heavy debug option.  For example CONFIG_DEBUG_VM_RB,
which checks an mm's entire vma tree whenever a change is made there.

We could group the heavy debug options, like that and CONFIG_PROVE_LOCKING,
into a menu of their own; but I've no appetite for such a change myself!

Hugh
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