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

[David Hildenbrand <david@redhat.com>]

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.

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.

-- 
Cheers,

David / dhildenb