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

[Sidhartha Kumar <sidhartha.kumar@xxxxxxxxxx>]

On 12/14/22 12:43 AM, John Hubbard wrote:
> On 12/13/22 13:20, Sidhartha Kumar 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.
> >
> > Link: https://lore.kernel.org/lkml/20221207223731.32784-1-sidhartha.kumar@xxxxxxxxxx/T/
> > Fixes: 9fd330582b2f ("mm: add folio dtor and order setter functions")
> >
> > Signed-off-by: Sidhartha Kumar <sidhartha.kumar@xxxxxxxxxx>
> > Suggested-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> > Suggested-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> > Suggested-by: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> > Suggested-by: John Hubbard <jhubbard@xxxxxxxxxx>
> > ---
> >   include/linux/mm.h | 16 ----------------
> >   mm/hugetlb.c       |  6 +++---
> >   mm/internal.h      | 21 +++++++++++++++++++++
> >   3 files changed, 24 insertions(+), 19 deletions(-)
>
> I think this looks good. One small question below.
>
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index 7dc376052d40..300d92d2b49d 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -1019,22 +1019,6 @@ static inline void set_compound_order(struct page *page, unsigned int order)
> >   #endif
> >   }
> >   
> > -/*
> > - * folio_set_compound_order is generally passed a non-zero order to
> > - * initialize a large folio.  However, hugetlb code abuses this by
> > - * passing in zero when 'dissolving' a large folio.
> > - */
> > -static inline void folio_set_compound_order(struct folio *folio,
> > -		unsigned int order)
> > -{
> > -	VM_BUG_ON_FOLIO(!folio_test_large(folio), folio);
> > -
> > -	folio->_folio_order = order;
> > -#ifdef CONFIG_64BIT
> > -	folio->_folio_nr_pages = order ? 1U << order : 0;
> > -#endif
> > -}
> > -
> >   /* Returns the number of pages in this potentially compound page. */
> >   static inline unsigned long compound_nr(struct page *page)
> >   {
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index 7cdbcc22587b..810e840bb4f1 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -1344,7 +1344,7 @@ static void __destroy_compound_gigantic_folio(struct folio *folio,
> >   			set_page_refcounted(p);
> >   	}
> >   
> > -	folio_set_compound_order(folio, 0);
> > +	folio_set_order(folio, 0);
> >   	__folio_clear_head(folio);
> >   }
> >   
> > @@ -1808,7 +1808,7 @@ static bool __prep_compound_gigantic_folio(struct folio *folio,
> >   	__folio_clear_reserved(folio);
> >   	__folio_set_head(folio);
> >   	/* we rely on prep_new_hugetlb_folio to set the destructor */
> > -	folio_set_compound_order(folio, order);
> > +	folio_set_order(folio, order);
> >   	for (i = 0; i < nr_pages; i++) {
> >   		p = folio_page(folio, i);
> >   
> > @@ -1872,7 +1872,7 @@ static bool __prep_compound_gigantic_folio(struct folio *folio,
> >   		p = folio_page(folio, j);
> >   		__ClearPageReserved(p);
> >   	}
> > -	folio_set_compound_order(folio, 0);
> > +	folio_set_order(folio, 0);
> >   	__folio_clear_head(folio);
> >   	return false;
> >   }
> > diff --git a/mm/internal.h b/mm/internal.h
> > index bcf75a8b032d..829b6a60ceb7 100644
> > --- a/mm/internal.h
> > +++ b/mm/internal.h
> > @@ -378,6 +378,27 @@ extern void *memmap_alloc(phys_addr_t size, phys_addr_t align,
> >   int split_free_page(struct page *free_page,
> >   			unsigned int order, unsigned long split_pfn_offset);
> >   
> > +/*
> > + * This will have no effect, other than possibly generating a warning, if the
> > + * caller passes in a non-large folio.
> > + */
> > +static inline void folio_set_order(struct folio *folio, unsigned int order)
> > +{
> > +	if (!folio_test_large(folio)) {
> > +		WARN_ON_ONCE(order);
> > +		return;
> > +	}
>
> Would it be better to do this (below)? I'm not sure of the value of
> warning on "order"--it's a little odd and unexplained and doesn't really
> do anything more helpful than simply warning about what why the code is
> failing, which is really about !large, rather than order. Unless I'm
> missing something?
>
> 	if (WARN_ON_ONCE(!folio_test_large(folio)))
> 		return;
> I agree that warning this way is clearer, I will change in a v2.

> Sorry to drive you crazy over nits. This is the last one from me. :)
No worries, I appreciate the feedback.

Thanks,
Sidhartha Kumar

> thanks,