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Message-ID: <2477a817-b482-43ed-9fd3-a7f8f948495f@pankajraghav.com>
Date: Fri, 30 Aug 2024 16:59:57 +0200
From: Pankaj Raghav <kernel@...kajraghav.com>
To: Luis Chamberlain <mcgrof@...nel.org>, Zi Yan <ziy@...dia.com>
Cc: Matthew Wilcox <willy@...radead.org>, Sven Schnelle
 <svens@...ux.ibm.com>, brauner@...nel.org, akpm@...ux-foundation.org,
 chandan.babu@...cle.com, linux-fsdevel@...r.kernel.org, djwong@...nel.org,
 hare@...e.de, gost.dev@...sung.com, linux-xfs@...r.kernel.org, hch@....de,
 david@...morbit.com, yang@...amperecomputing.com,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org, john.g.garry@...cle.com,
 cl@...amperecomputing.com, p.raghav@...sung.com, ryan.roberts@....com,
 David Howells <dhowells@...hat.com>, linux-s390@...r.kernel.org
Subject: Re: [PATCH v13 04/10] mm: split a folio in minimum folio order chunks

On 30/08/2024 01:41, 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.
> 
It feels a bit weird to pass both folio and the page in `split_page_folio_to_list()`.

How about we extract the code that returns the min order so that we don't repeat.

Something like this:


diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index c275aa9cc105..d27febd5c639 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -331,10 +331,24 @@ 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 min_order_for_split(struct folio *folio);
 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));
+       struct folio *folio = page_folio(page);
+       int ret = min_order_for_split(folio);
+
+       if (ret)
+               return ret;
+
+       /*
+        * split_huge_page() locks the page before splitting and
+        * expects the same page that has been split to be locked when
+        * returned. split_folio_to_list() cannot be used here because
+        * it converts the page to folio and passes the head page to be
+        * split.
+        */
+       return split_huge_page_to_list_to_order(page, NULL, ret);
 }
 void deferred_split_folio(struct folio *folio);

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 169f1a71c95d..b167e036d01b 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3529,12 +3529,10 @@ 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 min_order_for_split(struct folio *folio)
 {
-       unsigned int min_order = 0;
-
        if (folio_test_anon(folio))
-               goto out;
+               return 0;

        if (!folio->mapping) {
                if (folio_test_pmd_mappable(folio))
@@ -3542,10 +3540,17 @@ int split_folio_to_list(struct folio *folio, struct list_head *list)
                return -EBUSY;
        }

-       min_order = mapping_min_folio_order(folio->mapping);
-out:
-       return split_huge_page_to_list_to_order(&folio->page, list,
-                                                       min_order);
+       return mapping_min_folio_order(folio->mapping);
+}
+
+int split_folio_to_list(struct folio *folio, struct list_head *list)
+{
+       int ret = min_order_for_split(folio);
+
+       if (ret)
+               return ret;
+
+       return split_huge_page_to_list_to_order(&folio->page, list, ret);
 }

 void __folio_undo_large_rmappable(struct folio *folio)

> 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)

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