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Date: Tue, 23 Jan 2024 21:14:50 +0100
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Subject: Re: [PATCH v1 00/11] mm/memory: optimize fork() with PTE-mapped THP
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To: Ryan Roberts <ryan.roberts@arm.com>, linux-kernel@vger.kernel.org
Cc: linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
 Matthew Wilcox <willy@infradead.org>, Russell King <linux@armlinux.org.uk>,
 Catalin Marinas <catalin.marinas@arm.com>, Will Deacon <will@kernel.org>,
 Dinh Nguyen <dinguyen@kernel.org>, Michael Ellerman <mpe@ellerman.id.au>,
 Nicholas Piggin <npiggin@gmail.com>,
 Christophe Leroy <christophe.leroy@csgroup.eu>,
 "Aneesh Kumar K.V" <aneesh.kumar@kernel.org>,
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From: David Hildenbrand <david@redhat.com>
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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 :)

-- 
Cheers,

David / dhildenb