On 19/09/2023 09:41, Muchun Song wrote:
>> On Sep 19, 2023, at 16:26, Joao Martins <joao.m.martins@xxxxxxxxxx> 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@xxxxxxxxxx>
>>>>
>>>> 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@xxxxxxxxxx>
>>>> Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
>>>> ---
>>>> 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?
>
Sounds good -- no point in keep trying to split if we are failing with OOM.
Perhaps a comment in both of these clauses (the early break on split and the OOM
handling in batch optimize) could help make this clear.
>>
>> Unless this isn't something worth handling
>>
>> Joao
>
>
>
