[PATCH v1 00/11] mm/memory: optimize fork() with PTE-mapped THP
David Hildenbrand
david at redhat.com
Tue Jan 23 12:14:50 PST 2024
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
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