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

Ryan Roberts
2024-01-23T19:43:33Z

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.

>
> 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.