Re: [PATCH mm-unstable] mm: move folio_set_compound_order() to mm/internal.h

[Sidhartha Kumar]

On 12/16/22 2:56 PM, John Hubbard wrote:
> On 12/16/22 14:27, Andrew Morton wrote:
> > On Tue, 13 Dec 2022 13:20:53 -0800 Sidhartha Kumar 
> > <sidhartha.kumar@xxxxxxxxxx> wrote:
> >
> > > folio_set_compound_order() is moved to an mm-internal location so 
> > > external
> > > folio users cannot misuse this function. Change the name of the function
> > > to folio_set_order() and use WARN_ON_ONCE() rather than BUG_ON. Also,
> > > handle the case if a non-large folio is passed and add clarifying 
> > > comments
> > > to the function.
> >
> > This differs from the version I previously merged:
> >
> > --- 
> > a/mm/internal.h~mm-move-folio_set_compound_order-to-mm-internalh-update
> > +++ a/mm/internal.h
> > @@ -384,8 +384,10 @@ int split_free_page(struct page *free_pa
> >    */
> >   static inline void folio_set_order(struct folio *folio, unsigned int 
> > order)
> >   {
> > -    if (WARN_ON_ONCE(!folio_test_large(folio)))
> > +    if (!folio_test_large(folio)) {
> > +        WARN_ON_ONCE(order);
> >           return;
> > +    }
>
> I think that's out of date?
>
> We eventually settled on the version that is (as of this a few minutes
> ago) already in mm-unstable (commit fdea060a130d: "mm: move
> folio_set_compound_order() to mm/internal.h"), which has it like this:
Hi Andrew, yes this version that is already in mm-unstable represents 
the v2 of this patch which is what we agreed on. I think the patch 
mm-move-folio_set_compound_order-to-mm-internalh-update with description 
"alter the folio_set_order() warning" which was just added to 
mm-unstable should be removed as our discussion lead us away from that 
version.


>         if (WARN_ON_ONCE(!folio_test_large(folio)))
>                 return;
>
> >       folio->_folio_order = order;
> >   #ifdef CONFIG_64BIT
> >
> > Makes sense.  But wouldn't
> >
> >     if (WARN_ON_ONCE(order && !folio_test_large(folio)))
> >
> > be clearer?
> That's a little narrower of a check. But maybe that's desirable. Could

Ya I think it would helpful to have a wider catch for the warn as any 
user calling folio_set_order() with a non-large folio should be aware as 
they could misuse the folio later on even if they passed in a 0 order 
because order itself would be an OOB access.

Thanks,
Sidhartha Kumar
> someone (Mike, Muchun, Sidhartha) comment on which behavior is
> preferable, please? I think I'm a little dizzy at this point. :)
>
>
> thanks,