Re: [PATCH v1 00/11] mm/memory: optimize fork() with PTE-mapped THP

[Ryan Roberts <ryan.roberts@arm.com>]

On 23/01/2024 20:14, David Hildenbrand wrote:
> On 23.01.24 20:43, Ryan Roberts wrote:
>> On 23/01/2024 19:33, David Hildenbrand wrote:
>>> On 23.01.24 20:15, Ryan Roberts wrote:
>>>> On 22/01/2024 19:41, David Hildenbrand wrote:
>>>>> Now that the rmap overhaul[1] is upstream that provides a clean interface
>>>>> for rmap batching, let's implement PTE batching during fork when processing
>>>>> PTE-mapped THPs.
>>>>>
>>>>> This series is partially based on Ryan's previous work[2] to implement
>>>>> cont-pte support on arm64, but its a complete rewrite based on [1] to
>>>>> optimize all architectures independent of any such PTE bits, and to
>>>>> use the new rmap batching functions that simplify the code and prepare
>>>>> for further rmap accounting changes.
>>>>>
>>>>> We collect consecutive PTEs that map consecutive pages of the same large
>>>>> folio, making sure that the other PTE bits are compatible, and (a) adjust
>>>>> the refcount only once per batch, (b) call rmap handling functions only
>>>>> once per batch and (c) perform batch PTE setting/updates.
>>>>>
>>>>> While this series should be beneficial for adding cont-pte support on
>>>>> ARM64[2], it's one of the requirements for maintaining a total mapcount[3]
>>>>> for large folios with minimal added overhead and further changes[4] that
>>>>> build up on top of the total mapcount.
>>>>
>>>> I'm currently rebasing my contpte work onto this series, and have hit a
>>>> problem.
>>>> I need to expose the "size" of a pte (pte_size()) and skip forward to the start
>>>> of the next (cont)pte every time through the folio_pte_batch() loop. But
>>>> pte_next_pfn() only allows advancing by 1 pfn; I need to advance by nr pfns:
>>>>
>>>>
>>>> static inline int folio_pte_batch(struct folio *folio, unsigned long addr,
>>>>          pte_t *start_ptep, pte_t pte, int max_nr, bool *any_writable)
>>>> {
>>>>      unsigned long folio_end_pfn = folio_pfn(folio) + folio_nr_pages(folio);
>>>>      const pte_t *end_ptep = start_ptep + max_nr;
>>>>      pte_t expected_pte = __pte_batch_clear_ignored(pte_next_pfn(pte));
>>>> -    pte_t *ptep = start_ptep + 1;
>>>> +    pte_t *ptep = start_ptep;
>>>> +    int vfn, nr, i;
>>>>      bool writable;
>>>>
>>>>      if (any_writable)
>>>>          *any_writable = false;
>>>>
>>>>      VM_WARN_ON_FOLIO(!pte_present(pte), folio);
>>>>
>>>> +    vfn = addr >> PAGE_SIZE;
>>>> +    nr = pte_size(pte);
>>>> +    nr = ALIGN_DOWN(vfn + nr, nr) - vfn;
>>>> +    ptep += nr;
>>>> +
>>>>      while (ptep != end_ptep) {
>>>> +        pte = ptep_get(ptep);
>>>>          nr = pte_size(pte);
>>>>          if (any_writable)
>>>>              writable = !!pte_write(pte);
>>>>          pte = __pte_batch_clear_ignored(pte);
>>>>
>>>>          if (!pte_same(pte, expected_pte))
>>>>              break;
>>>>
>>>>          /*
>>>>           * Stop immediately once we reached the end of the folio. In
>>>>           * corner cases the next PFN might fall into a different
>>>>           * folio.
>>>>           */
>>>> -        if (pte_pfn(pte) == folio_end_pfn)
>>>> +        if (pte_pfn(pte) >= folio_end_pfn)
>>>>              break;
>>>>
>>>>          if (any_writable)
>>>>              *any_writable |= writable;
>>>>
>>>> -        expected_pte = pte_next_pfn(expected_pte);
>>>> -        ptep++;
>>>> +        for (i = 0; i < nr; i++)
>>>> +            expected_pte = pte_next_pfn(expected_pte);
>>>> +        ptep += nr;
>>>>      }
>>>>
>>>>      return ptep - start_ptep;
>>>> }
>>>>
>>>>
>>>> So I'm wondering if instead of enabling pte_next_pfn() for all the arches,
>>>> perhaps its actually better to expose pte_pgprot() for all the arches. Then we
>>>> can be much more flexible about generating ptes with pfn_pte(pfn, pgprot).
>>>>
>>>> What do you think?
>>>
>>> The pte_pgprot() stuff is just nasty IMHO.
>>
>> I dunno; we have pfn_pte() which takes a pfn and a pgprot. It seems reasonable
>> that we should be able to do the reverse.
> 
> But pte_pgprot() is only available on a handful of architectures, no? It would
> be nice to have a completely generic pte_next_pfn() / pte_advance_pfns(), though.
> 
> Anyhow, this is all "easy" to rework later. Unless I am missing something, the
> low hanging fruit is simply using PFN_PTE_SHIFT for now that exists on most
> archs already.
> 
>>
>>>
>>> Likely it's best to simply convert pte_next_pfn() to something like
>>> pte_advance_pfns(). The we could just have
>>>
>>> #define pte_next_pfn(pte) pte_advance_pfns(pte, 1)
>>>
>>> That should be fairly easy to do on top (based on PFN_PTE_SHIFT). And only 3
>>> archs (x86-64, arm64, and powerpc) need slight care to replace a hardcoded "1"
>>> by an integer we pass in.
>>
>> I thought we agreed powerpc was safe to just define PFN_PTE_SHIFT? But, yeah,
>> the principle works I guess. I guess I can do this change along with my series.
> 
> It is, if nobody insists on that micro-optimization on powerpc.
> 
> If there is good reason to invest more time and effort right now on the
> pte_pgprot approach, then please let me know :)
> 

No I think you're right. I thought pte_pgprot() was implemented by more arches,
but there are 13 without it, so clearly a lot of effort to plug that gap. I'll
take the approach you suggest with pte_advance_pfns(). It'll just require mods
to x86 and arm64, +/- ppc.