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

Ryan Roberts
2024-01-23T19:16:04Z

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?