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

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

> On Sep 20, 2023, at 18:39, Joao Martins <joao.m.martins@oracle.com> wrote:
> 
> On 20/09/2023 03:47, Muchun Song wrote:
>>> On Sep 19, 2023, at 23:09, Joao Martins <joao.m.martins@oracle.com> wrote:
>>> On 19/09/2023 09:57, Muchun Song wrote:
>>>>> On Sep 19, 2023, at 16:55, Joao Martins <joao.m.martins@oracle.com> wrote:
>>>>> On 19/09/2023 09:41, Muchun Song wrote:
>>>>>>> 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:
>>>>>>>>>   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.
>>>> 
>>>> Make sense.
>>> 
>>> These are the changes I have so far for this patch based on the discussion so
>>> far. For next one it's at the end:
>> 
>> Code looks good to me. One nit below.
>> 
> Thanks
> 
>>> 
>>> diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
>>> index e8bc2f7567db..d9c6f2cf698c 100644
>>> --- a/mm/hugetlb_vmemmap.c
>>> +++ b/mm/hugetlb_vmemmap.c
>>> @@ -27,7 +27,8 @@
>>> * @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
>>> + * @flags:             used to modify behavior in vmemmap page table walking
>>> + *                     operations.
>>> */
>>> struct vmemmap_remap_walk {
>>>       void                    (*remap_pte)(pte_t *pte, unsigned long addr,
>>> @@ -36,6 +37,8 @@ struct vmemmap_remap_walk {
>>>       struct page             *reuse_page;
>>>       unsigned long           reuse_addr;
>>>       struct list_head        *vmemmap_pages;
>>> +
>>> +/* Skip the TLB flush when we split the PMD */
>>> #define VMEMMAP_SPLIT_NO_TLB_FLUSH     BIT(0)
>>>       unsigned long           flags;
>>> };
>>> @@ -132,7 +135,7 @@ static int vmemmap_pmd_range(pud_t *pud, unsigned long addr,
>>>               int ret;
>>> 
>>>               ret = split_vmemmap_huge_pmd(pmd, addr & PMD_MASK,
>>> -                               walk->flags & VMEMMAP_SPLIT_NO_TLB_FLUSH);
>>> +                               !(walk->flags & VMEMMAP_SPLIT_NO_TLB_FLUSH));
>>>               if (ret)
>>>                       return ret;
>>> 
>>> @@ -677,13 +680,13 @@ void hugetlb_vmemmap_optimize(const struct hstate *h,
>>> struct page *head)
>>>       free_vmemmap_page_list(&vmemmap_pages);
>>> }
>>> 
>>> -static void hugetlb_vmemmap_split(const struct hstate *h, struct page *head)
>>> +static int hugetlb_vmemmap_split(const struct hstate *h, struct page *head)
>>> {
>>>       unsigned long vmemmap_start = (unsigned long)head, vmemmap_end;
>>>       unsigned long vmemmap_reuse;
>>> 
>>>       if (!vmemmap_should_optimize(h, head))
>>> -               return;
>>> +               return 0;
>>> 
>>>       vmemmap_end     = vmemmap_start + hugetlb_vmemmap_size(h);
>>>       vmemmap_reuse   = vmemmap_start;
>>> @@ -693,7 +696,7 @@ static void hugetlb_vmemmap_split(const struct hstate *h,
>>> struct page *head)
>>>        * Split PMDs on the vmemmap virtual address range [@vmemmap_start,
>>>        * @vmemmap_end]
>>>        */
>>> -       vmemmap_remap_split(vmemmap_start, vmemmap_end, vmemmap_reuse);
>>> +       return vmemmap_remap_split(vmemmap_start, vmemmap_end, vmemmap_reuse);
>>> }
>>> 
>>> void hugetlb_vmemmap_optimize_folios(struct hstate *h, struct list_head
>>> *folio_list)
>>> @@ -701,8 +704,18 @@ void hugetlb_vmemmap_optimize_folios(struct hstate *h,
>>> struct list_head *folio_l
>>>       struct folio *folio;
>>>       LIST_HEAD(vmemmap_pages);
>>> 
>>> -       list_for_each_entry(folio, folio_list, lru)
>>> -               hugetlb_vmemmap_split(h, &folio->page);
>>> +       list_for_each_entry(folio, folio_list, lru) {
>>> +               int ret = hugetlb_vmemmap_split(h, &folio->page);
>>> +
>>> +               /*
>>> +                * Spliting the PMD requires allocating a page, thus lets fail
>>                      ^^^^                                 ^^^
>>                    Splitting                           page table page
>> 
>> I'd like to specify the functionality of the allocated page.
>> 
> OK
> 
>>> +                * early once we encounter the first OOM. No point in retrying
>>> +                * as it can be dynamically done on remap with the memory
>>> +                * we get back from the vmemmap deduplication.
>>> +                */
>>> +               if (ret == -ENOMEM)
>>> +                       break;
>>> +       }
>>> 
>>>       flush_tlb_all();
>>> 
>>> For patch 7, I only have commentary added derived from this earlier discussion
>>> above:
>>> 
>>> diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
>>> index d9c6f2cf698c..f6a1020a4b6a 100644
>>> --- a/mm/hugetlb_vmemmap.c
>>> +++ b/mm/hugetlb_vmemmap.c
>>> @@ -40,6 +40,8 @@ struct vmemmap_remap_walk {
>>> 
>>> /* Skip the TLB flush when we split the PMD */
>>> #define VMEMMAP_SPLIT_NO_TLB_FLUSH     BIT(0)
>>> +/* Skip the TLB flush when we remap the PTE */
>>> #define VMEMMAP_REMAP_NO_TLB_FLUSH     BIT(1)
>>>       unsigned long           flags;
>>> };
>>> 
>>> @@ -721,19 +739,28 @@ void hugetlb_vmemmap_optimize_folios(struct hstate *h,
>>> struct list_head *folio_l
>>> 
>>>       list_for_each_entry(folio, folio_list, lru) {
>>>               int ret = __hugetlb_vmemmap_optimize(h, &folio->page,
>>>                                              &vmemmap_pages,
>>>                                              VMEMMAP_REMAP_NO_TLB_FLUSH);
>>> 
>>>               /*
>>>                * Pages to be freed may have been accumulated.  If we
>>>                * encounter an ENOMEM,  free what we have and try again.
>>> +                * This can occur in the case that both spliting fails
>>                                                            ^^^
>>                                                         splitting
>> 
> 
> ok
> 
>>> +                * halfway and head page allocation also failed. In this
>>                                 ^^^^^^^
>>                            head vmemmap page
>> 
> ok
> 
>> Otherwise:
>> 
>> Reviewed-by: Muchun Song <songmuchun@bytedance.com>
>> 
> 
> Thanks, I assume that's for both patches?

Yes. Thanks.

> 
>> Thanks.
>> 
>>> +                * case __hugetlb_vmemmap_optimize() would free memory
>>> +                * allowing more vmemmap remaps to occur.
>>>                */
>>>               if (ret == -ENOMEM && !list_empty(&vmemmap_pages)) {