Re: [PATCH v1 11/14] arm64/mm: Wire up PTE_CONT for user mappings

[John Hubbard]

On 6/22/23 07:42, Ryan Roberts wrote:
> With the ptep API sufficiently refactored, we can now introduce a new
> "contpte" API layer, which transparently manages the PTE_CONT bit for
> user mappings. Whenever it detects a set of PTEs that meet the
> requirements for a contiguous range, the PTEs are re-painted with the
> PTE_CONT bit.
>
> This initial change provides a baseline that can be optimized in future
> commits. That said, fold/unfold operations (which imply tlb
> invalidation) are avoided where possible with a few tricks for
> access/dirty bit management.
>
> Write-enable and write-protect modifications are likely non-optimal and
> likely incure a regression in fork() performance. This will be addressed
> separately.
>
> Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
> ---

Hi Ryan!

While trying out the full series from your gitlab features/granule_perf/all
branch, I found it necessary to EXPORT a symbol in order to build this.
Please see below:

...
> +
> +pte_t contpte_ptep_get(pte_t *ptep, pte_t orig_pte)
> +{
> +	/*
> +	 * Gather access/dirty bits, which may be populated in any of the ptes
> +	 * of the contig range. We are guarranteed to be holding the PTL, so any
> +	 * contiguous range cannot be unfolded or otherwise modified under our
> +	 * feet.
> +	 */
> +
> +	pte_t pte;
> +	int i;
> +
> +	ptep = contpte_align_down(ptep);
> +
> +	for (i = 0; i < CONT_PTES; i++, ptep++) {
> +		pte = __ptep_get(ptep);
> +
> +		/*
> +		 * Deal with the partial contpte_ptep_get_and_clear_full() case,
> +		 * where some of the ptes in the range may be cleared but others
> +		 * are still to do. See contpte_ptep_get_and_clear_full().
> +		 */
> +		if (pte_val(pte) == 0)
> +			continue;
> +
> +		if (pte_dirty(pte))
> +			orig_pte = pte_mkdirty(orig_pte);
> +
> +		if (pte_young(pte))
> +			orig_pte = pte_mkyoung(orig_pte);
> +	}
> +
> +	return orig_pte;
> +}

Here we need something like this, in order to get it to build in all
possible configurations:

EXPORT_SYMBOL_GPL(contpte_ptep_get);

(and a corresponding "#include linux/export.h" at the top of the file).

Because, the static inline functions invoke this routine, above.

thanks,
--
John Hubbard
NVIDIA