Re: [PATCH] mm/page_isolation: don't putback unisolated page

[David Hildenbrand]

On 07.09.21 10:08, Vlastimil Babka wrote:
> On 9/6/21 14:49, David Hildenbrand wrote:
> > On 06.09.21 14:45, Miaohe Lin wrote:
> > > On 2021/9/6 20:11, David Hildenbrand wrote:
> > > > On 06.09.21 14:02, David Hildenbrand wrote:
> > > > > On 04.09.21 11:18, Miaohe Lin wrote:
> > > > >
> > > > > Thanks!
> > > > >
> > > > > Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>
> > > >
> > > > To make the confusion perfect (sorry) :D I tripple-checked:
> > > >
> > > > In unset_migratetype_isolate() we check that is_migrate_isolate_page(page) holds, otherwise we return.
> > > >
> > > > We call __isolate_free_page() only for such pages.
> > > >
> > > > __isolate_free_page() won't perform watermark checks on is_migrate_isolate().
> > > >
> > > > Consequently, __isolate_free_page() should never fail when called from unset_migratetype_isolate()
> > > >
> > > > If that's correct then we  could instead maybe add a VM_BUG_ON() and a comment why this can't fail.
> > > >
> > > >
> > > > Makes sense or am I missing something?
> > >
> > > I think you're right. __isolate_free_page() should never fail when called from unset_migratetype_isolate()
> > > as explained by you. But it might be too fragile to reply on the failure conditions of __isolate_free_page().
> > > If that changes, VM_BUG_ON() here might trigger unexpectedly. Or am I just over-worried as failure conditions
> > > of __isolate_free_page() can hardly change?
> >
> > Maybe
> >
> > isolated_page = !!__isolate_free_page(page, order);
> > /*
> >    * Isolating a free page in an isolated pageblock is expected to always
> >    * work as watermarks don't apply here.
> >    */
> > VM_BUG_ON(isolated_page);
> >
> >
> > VM_BUG_ON() allows us to detect any issues when testing. Combined with
> > the comment it tells everybody messing with __isolate_free_page() what
> > we expect in this function.
> >
> > In production system, we would handle it gracefully.
>
> If this can be handled gracefully, then I'd rather go with VM_WARN_ON.
> Maybe even WARN_ON_ONCE?

I think either VM_BUG_ON() or VM_WARN_ON() -- compiling the runtime 
checks out -- should be good enough.

I'd just go with VM_BUG_ON(), because anybody messing with 
__isolate_free_page() should clearly spot that we expect the current 
handling. But no strong opinion.

--
Thanks,

David / dhildenb