Re: [PATCH v4 6/8] hugetlb: batch PMD split for bulk vmemmap dedup

[Muchun Song <muchun.song@linux.dev>]

> On Sep 19, 2023, at 16:26, Joao Martins <joao.m.martins@oracle.com> wrote:
> 
> On 19/09/2023 07:42, Muchun Song wrote:
>> On 2023/9/19 07:01, Mike Kravetz wrote:
>>> From: Joao Martins <joao.m.martins@oracle.com>
>>> 
>>> In an effort to minimize amount of TLB flushes, batch all PMD splits
>>> belonging to a range of pages in order to perform only 1 (global) TLB
>>> flush.
>>> 
>>> Add a flags field to the walker and pass whether it's a bulk allocation
>>> or just a single page to decide to remap. First value
>>> (VMEMMAP_SPLIT_NO_TLB_FLUSH) designates the request to not do the TLB
>>> flush when we split the PMD.
>>> 
>>> Rebased and updated by Mike Kravetz
>>> 
>>> Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
>>> Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
>>> ---
>>>  mm/hugetlb_vmemmap.c | 79 +++++++++++++++++++++++++++++++++++++++++---
>>>  1 file changed, 75 insertions(+), 4 deletions(-)
>>> 
>>> diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
>>> index 147ed15bcae4..e8bc2f7567db 100644
>>> --- a/mm/hugetlb_vmemmap.c
>>> +++ b/mm/hugetlb_vmemmap.c
>>> @@ -27,6 +27,7 @@
>>>   * @reuse_addr:        the virtual address of the @reuse_page page.
>>>   * @vmemmap_pages:    the list head of the vmemmap pages that can be freed
>>>   *            or is mapped from.
>>> + * @flags:        used to modify behavior in bulk operations
>> 
>> Better to describe it as "used to modify behavior in vmemmap page table walking
>> operations"
>> 
> OK
> 
>>>  void hugetlb_vmemmap_optimize_folios(struct hstate *h, struct list_head
>>> *folio_list)
>>>  {
>>>      struct folio *folio;
>>>      LIST_HEAD(vmemmap_pages);
>>>  +    list_for_each_entry(folio, folio_list, lru)
>>> +        hugetlb_vmemmap_split(h, &folio->page);
>>> +
>>> +    flush_tlb_all();
>>> +
>>>      list_for_each_entry(folio, folio_list, lru) {
>>>          int ret = __hugetlb_vmemmap_optimize(h, &folio->page,
>>>                                  &vmemmap_pages);
>> 
>> This is unlikely to be failed since the page table allocation
>> is moved to the above 
> 
>> (Note that the head vmemmap page allocation
>> is not mandatory). 
> 
> Good point that I almost forgot
> 
>> So we should handle the error case in the above
>> splitting operation.
> 
> But back to the previous discussion in v2... the thinking was that /some/ PMDs
> got split, and say could allow some PTE remapping to occur and free some pages
> back (each page allows 6 more splits worst case). Then the next
> __hugetlb_vmemmap_optimize() will have to split PMD pages again for those
> hugepages that failed the batch PMD split (as we only defer the PTE remap tlb
> flush in this stage).

Oh, yes. Maybe we could break the above traversal as early as possible
once we enter an ENOMEM?

> 
> Unless this isn't something worth handling
> 
> Joao