linux-kernel - Re: [PATCH v1 11/14] arm64/mm: Wire up PTE

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Date:   Thu, 29 Jun 2023 18:54:27 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Ryan Roberts <ryan.roberts@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Marc Zyngier <maz@...nel.org>,
        Oliver Upton <oliver.upton@...ux.dev>,
        James Morse <james.morse@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Zenghui Yu <yuzenghui@...wei.com>,
        Andrey Ryabinin <ryabinin.a.a@...il.com>,
        Alexander Potapenko <glider@...gle.com>,
        "Andrey Konovalov" <andreyknvl@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Matthew Wilcox <willy@...radead.org>,
        Yu Zhao <yuzhao@...gle.com>,
        "Mark Rutland" <mark.rutland@....com>
CC:     <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>
Subject: Re: [PATCH v1 11/14] arm64/mm: Wire up PTE_CONT for user mappings

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@....com>
> ---

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