Re: [PATCH v13 04/10] mm: split a folio in minimum folio order chunks

[Daniel Gomez <da.gomez@samsung.com>]

On Thu, Aug 29, 2024 at 04:41:48PM -0700, Luis Chamberlain wrote:
> On Thu, Aug 29, 2024 at 06:12:26PM -0400, Zi Yan wrote:
> > The issue is that the change to split_huge_page() makes split_huge_page_to_list_to_order()
> > unlocks the wrong subpage. split_huge_page() used to pass the “page” pointer
> > to split_huge_page_to_list_to_order(), which keeps that “page” still locked.
> > But this patch changes the “page” passed into split_huge_page_to_list_to_order()
> > always to the head page.
> > 
> > This fixes the crash on my x86 VM, but it can be improved:
> > 
> > diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> > index 7c50aeed0522..eff5d2fb5d4e 100644
> > --- a/include/linux/huge_mm.h
> > +++ b/include/linux/huge_mm.h
> > @@ -320,10 +320,7 @@ bool can_split_folio(struct folio *folio, int *pextra_pins);
> >  int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
> >                 unsigned int new_order);
> >  int split_folio_to_list(struct folio *folio, struct list_head *list);
> > -static inline int split_huge_page(struct page *page)
> > -{
> > -       return split_folio(page_folio(page));
> > -}
> > +int split_huge_page(struct page *page);
> >  void deferred_split_folio(struct folio *folio);
> > 
> >  void __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index c29af9451d92..4d723dab4336 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -3297,6 +3297,25 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
> >         return ret;
> >  }
> > 
> > +int split_huge_page(struct page *page)
> > +{
> > +       unsigned int min_order = 0;
> > +       struct folio *folio = page_folio(page);
> > +
> > +       if (folio_test_anon(folio))
> > +               goto out;
> > +
> > +       if (!folio->mapping) {
> > +               if (folio_test_pmd_mappable(folio))
> > +                       count_vm_event(THP_SPLIT_PAGE_FAILED);
> > +               return -EBUSY;
> > +       }
> > +
> > +       min_order = mapping_min_folio_order(folio->mapping);
> > +out:
> > +       return split_huge_page_to_list_to_order(page, NULL, min_order);
> > +}
> > +
> >  int split_folio_to_list(struct folio *folio, struct list_head *list)
> >  {
> >         unsigned int min_order = 0;
> 
> 
> Confirmed, and also although you suggest it can be improved, I thought
> that we could do that by sharing more code and putting things in the
> headers, the below also fixes this but tries to share more code, but
> I think it is perhaps less easier to understand than your patch.
> 
> So I think your patch is cleaner and easier as a fix.

I reproduced it in arm64 as well. And both fixes provided solved the issue.

> 
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index c275aa9cc105..99cd9c7bf55b 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -97,6 +97,7 @@ extern struct kobj_attribute thpsize_shmem_enabled_attr;
>  	(!!thp_vma_allowable_orders(vma, vm_flags, tva_flags, BIT(order)))
>  
>  #define split_folio(f) split_folio_to_list(f, NULL)
> +#define split_folio_to_list(f, list) split_page_folio_to_list(&f->page, f, list)
>  
>  #ifdef CONFIG_PGTABLE_HAS_HUGE_LEAVES
>  #define HPAGE_PMD_SHIFT PMD_SHIFT
> @@ -331,10 +332,11 @@ unsigned long thp_get_unmapped_area_vmflags(struct file *filp, unsigned long add
>  bool can_split_folio(struct folio *folio, int caller_pins, int *pextra_pins);
>  int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
>  		unsigned int new_order);
> -int split_folio_to_list(struct folio *folio, struct list_head *list);
> +int split_page_folio_to_list(struct page *page, struct folio *folio,
> +			     struct list_head *list);
>  static inline int split_huge_page(struct page *page)
>  {
> -	return split_folio(page_folio(page));
> +	return split_page_folio_to_list(page, page_folio(page), NULL);
>  }
>  void deferred_split_folio(struct folio *folio);
>  
> @@ -511,7 +513,9 @@ static inline int split_huge_page(struct page *page)
>  	return 0;
>  }
>  
> -static inline int split_folio_to_list(struct folio *folio, struct list_head *list)
> +static inline int split_page_folio_to_list(struct page *page,
> +					   struct folio *folio,
> +					   struct list_head *list)
>  {
>  	return 0;
>  }
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 169f1a71c95d..b115bfe63b52 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -3529,7 +3529,8 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
>  	return ret;
>  }
>  
> -int split_folio_to_list(struct folio *folio, struct list_head *list)
> +int split_page_folio_to_list(struct page *page, struct folio *folio,
> +			     struct list_head *list)
>  {
>  	unsigned int min_order = 0;
>  
> @@ -3544,8 +3545,7 @@ int split_folio_to_list(struct folio *folio, struct list_head *list)
>  
>  	min_order = mapping_min_folio_order(folio->mapping);
>  out:
> -	return split_huge_page_to_list_to_order(&folio->page, list,
> -							min_order);
> +	return split_huge_page_to_list_to_order(page, list, min_order);
>  }
>  
>  void __folio_undo_large_rmappable(struct folio *folio)