Re: [PATCH] mm: Compute mTHP order efficiently

[Dev Jain <dev.jain@arm.com>]

On 9/16/24 10:42, Barry Song wrote:
> On Fri, Sep 13, 2024 at 5:19 PM Dev Jain <dev.jain@arm.com> wrote:
>> We use pte_range_none() to determine whether contiguous PTEs are empty
>> for an mTHP allocation. Instead of iterating the while loop for every
>> order, use some information, which is the first set PTE found, from the
>> previous iteration, to eliminate some cases. The key to understanding
>> the correctness of the patch is that the ranges we want to examine
>> form a strictly decreasing sequence of nested intervals.
>>
>> Suggested-by: Ryan Roberts <ryan.roberts@arm.com>
>> Signed-off-by: Dev Jain <dev.jain@arm.com>
> I like this patch, but could we come up with a better subject for
> pte_range_none()?
> The subject is really incorrect.

Are you asking me to change "Compute mTHP order efficiently" to
something else?

>
> Also, I'd prefer the change for alloc_anon_folio() to be separated
> into its own patch.
> So, one patchset with two patches, please.

Fine by me.

>
>> ---
>>   mm/memory.c | 30 +++++++++++++++++++++++-------
>>   1 file changed, 23 insertions(+), 7 deletions(-)
>>
>> diff --git a/mm/memory.c b/mm/memory.c
>> index 3c01d68065be..ffc24a48ef15 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -4409,26 +4409,27 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>>          return ret;
>>   }
>>
>> -static bool pte_range_none(pte_t *pte, int nr_pages)
>> +static int pte_range_none(pte_t *pte, int nr_pages)
>>   {
>>          int i;
>>
>>          for (i = 0; i < nr_pages; i++) {
>>                  if (!pte_none(ptep_get_lockless(pte + i)))
>> -                       return false;
>> +                       return i;
>>          }
>>
>> -       return true;
>> +       return nr_pages;
>>   }
>>
>>   static struct folio *alloc_anon_folio(struct vm_fault *vmf)
>>   {
>>          struct vm_area_struct *vma = vmf->vma;
>>   #ifdef CONFIG_TRANSPARENT_HUGEPAGE
>> +       pte_t *first_set_pte = NULL, *align_pte, *pte;
>>          unsigned long orders;
>>          struct folio *folio;
>>          unsigned long addr;
>> -       pte_t *pte;
>> +       int max_empty;
>>          gfp_t gfp;
>>          int order;
>>
>> @@ -4463,8 +4464,23 @@ static struct folio *alloc_anon_folio(struct vm_fault *vmf)
>>          order = highest_order(orders);
>>          while (orders) {
>>                  addr = ALIGN_DOWN(vmf->address, PAGE_SIZE << order);
>> -               if (pte_range_none(pte + pte_index(addr), 1 << order))
>> +               align_pte = pte + pte_index(addr);
>> +
>> +               /* Range to be scanned known to be empty */
>> +               if (align_pte + (1 << order) <= first_set_pte)
>>                          break;
>> +
>> +               /* Range to be scanned contains first_set_pte */
>> +               if (align_pte <= first_set_pte)
>> +                       goto repeat;
>> +
>> +               /* align_pte > first_set_pte, so need to check properly */
>> +               max_empty = pte_range_none(align_pte, 1 << order);
>> +               if (max_empty == 1 << order)
>> +                       break;
>> +
>> +               first_set_pte = align_pte + max_empty;
>> +repeat:
>>                  order = next_order(&orders, order);
>>          }
>>
>> @@ -4579,7 +4595,7 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
>>          if (nr_pages == 1 && vmf_pte_changed(vmf)) {
>>                  update_mmu_tlb(vma, addr, vmf->pte);
>>                  goto release;
>> -       } else if (nr_pages > 1 && !pte_range_none(vmf->pte, nr_pages)) {
>> +       } else if (nr_pages > 1 && pte_range_none(vmf->pte, nr_pages) != nr_pages) {
>>                  update_mmu_tlb_range(vma, addr, vmf->pte, nr_pages);
>>                  goto release;
>>          }
>> @@ -4915,7 +4931,7 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
>>                  update_mmu_tlb(vma, addr, vmf->pte);
>>                  ret = VM_FAULT_NOPAGE;
>>                  goto unlock;
>> -       } else if (nr_pages > 1 && !pte_range_none(vmf->pte, nr_pages)) {
>> +       } else if (nr_pages > 1 && pte_range_none(vmf->pte, nr_pages) != nr_pages) {
>>                  update_mmu_tlb_range(vma, addr, vmf->pte, nr_pages);
>>                  ret = VM_FAULT_NOPAGE;
>>                  goto unlock;
>> --
>> 2.30.2
>>
> Thanks
> Barry