Re: [PATCH v2 07/11] hugetlb: perform vmemmap restoration on a list of pages

[Muchun Song]

> On Sep 6, 2023, at 15:33, Muchun Song <muchun.song@xxxxxxxxx> wrote:
> 
> 
> 
> On 2023/9/6 05:44, Mike Kravetz wrote:
>> When removing hugetlb pages from the pool, we first create a list
>> of removed pages and then free those pages back to low level allocators.
>> Part of the 'freeing process' is to restore vmemmap for all base pages
>> if necessary.  Pass this list of pages to a new routine
>> hugetlb_vmemmap_restore_folios() so that vmemmap restoration can be
>> performed in bulk.
>> 
>> Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
>> ---
>>  mm/hugetlb.c         |  3 +++
>>  mm/hugetlb_vmemmap.c | 13 +++++++++++++
>>  mm/hugetlb_vmemmap.h |  5 +++++
>>  3 files changed, 21 insertions(+)
>> 
>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>> index 554be94b07bd..dd2dbc256172 100644
>> --- a/mm/hugetlb.c
>> +++ b/mm/hugetlb.c
>> @@ -1838,6 +1838,9 @@ static void update_and_free_pages_bulk(struct hstate *h, struct list_head *list)
>>  {
>>   struct folio *folio, *t_folio;
>>  + /* First restore vmemmap for all pages on list. */
>> + hugetlb_vmemmap_restore_folios(h, list);
>> +
>>   list_for_each_entry_safe(folio, t_folio, list, lru) {
>>   update_and_free_hugetlb_folio(h, folio, false);
>>   cond_resched();
>> diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
>> index ac5577d372fe..79de984919ef 100644
>> --- a/mm/hugetlb_vmemmap.c
>> +++ b/mm/hugetlb_vmemmap.c
>> @@ -481,6 +481,19 @@ int hugetlb_vmemmap_restore(const struct hstate *h, struct page *head)
>>   return ret;
>>  }
>>  +/*
>> + * This function will attempt to resore vmemmap for a list of folios.  There
>> + * is no guarantee that restoration will be successful for all or any folios.
>> + * This is used in bulk operations, and no feedback is given to the caller.
>> + */
>> +void hugetlb_vmemmap_restore_folios(const struct hstate *h, struct list_head *folio_list)
>> +{
>> + struct folio *folio;
>> +
>> + list_for_each_entry(folio, folio_list, lru)
>> + (void)hugetlb_vmemmap_restore(h, &folio->page);
> 
> I am curious about the purpose of "void" here, seems it it not necessnary,
> ritgh? We cound see so many palces where we do not add the void if the caller
> does not care about the return value of the callee.

Another question: should we stop restoring vmemmap pages when
hugetlb_vmemmap_restore() fails? In which case, I suspect there
is no memory probably, there is no need to continue, right?

> 
> Thanks.
>> +}
>> +
>>  /* Return true iff a HugeTLB whose vmemmap should and can be optimized. */
>>  static bool vmemmap_should_optimize(const struct hstate *h, const struct page *head)
>>  {
>> diff --git a/mm/hugetlb_vmemmap.h b/mm/hugetlb_vmemmap.h
>> index 036494e040ca..b4ee945dc1d4 100644
>> --- a/mm/hugetlb_vmemmap.h
>> +++ b/mm/hugetlb_vmemmap.h
>> @@ -12,6 +12,7 @@
>>    #ifdef CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP
>>  int hugetlb_vmemmap_restore(const struct hstate *h, struct page *head);
>> +void hugetlb_vmemmap_restore_folios(const struct hstate *h, struct list_head *folio_list);
>>  void hugetlb_vmemmap_optimize(const struct hstate *h, struct page *head);
>>  void hugetlb_vmemmap_optimize_folios(struct hstate *h, struct list_head *folio_list);
>>  @@ -44,6 +45,10 @@ static inline int hugetlb_vmemmap_restore(const struct hstate *h, struct page *h
>>   return 0;
>>  }
>>  +static inline void hugetlb_vmemmap_restore_folios(const struct hstate *h, struct list_head *folio_list)
>> +{
>> +}
>> +
>>  static inline void hugetlb_vmemmap_optimize(const struct hstate *h, struct page *head)
>>  {
>>  }
>