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

[Luis Chamberlain <mcgrof@xxxxxxxxxx>]

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.

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)