linux-kernel - Re: [PATCH v3 04/17] kexec: Add KHO parsing support

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Message-ID: <mafs0eddc8kru.fsf@amazon.de>
Date: Fri, 16 Feb 2024 16:57:09 +0100
From: Pratyush Yadav <ptyadav@...zon.de>
To: Alexander Graf <graf@...zon.com>
CC: <linux-kernel@...r.kernel.org>, <linux-trace-kernel@...r.kernel.org>,
	<linux-mm@...ck.org>, <devicetree@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>, <kexec@...ts.infradead.org>,
	<linux-doc@...r.kernel.org>, <x86@...nel.org>, Eric Biederman
	<ebiederm@...ssion.com>, "H . Peter Anvin" <hpa@...or.com>, Andy Lutomirski
	<luto@...nel.org>, Peter Zijlstra <peterz@...radead.org>, Steven Rostedt
	<rostedt@...dmis.org>, Andrew Morton <akpm@...ux-foundation.org>, "Mark
 Rutland" <mark.rutland@....com>, Tom Lendacky <thomas.lendacky@....com>,
	Ashish Kalra <ashish.kalra@....com>, James Gowans <jgowans@...zon.com>,
	Stanislav Kinsburskii <skinsburskii@...ux.microsoft.com>, <arnd@...db.de>,
	<pbonzini@...hat.com>, <madvenka@...ux.microsoft.com>, Anthony Yznaga
	<anthony.yznaga@...cle.com>, Usama Arif <usama.arif@...edance.com>, "David
 Woodhouse" <dwmw@...zon.co.uk>, Benjamin Herrenschmidt
	<benh@...nel.crashing.org>, Rob Herring <robh+dt@...nel.org>, "Krzysztof
 Kozlowski" <krzk@...nel.org>
Subject: Re: [PATCH v3 04/17] kexec: Add KHO parsing support

Hi,

On Wed, Jan 17 2024, Alexander Graf wrote:

> When we have a KHO kexec, we get a device tree, mem cache and scratch
> region to populate the state of the system. Provide helper functions
> that allow architecture code to easily handle memory reservations based
> on them and give device drivers visibility into the KHO DT and memory
> reservations so they can recover their own state.
>
> Signed-off-by: Alexander Graf <graf@...zon.com>
>
> ---
>
[...]
> +/**
> + * kho_return_mem - Notify the kernel that initially reserved memory is no
> + * longer needed. When the last consumer of a page returns their mem, kho
> + * returns the page to the buddy allocator as free page.
> + */
> +void kho_return_mem(const struct kho_mem *mem)
> +{
> +	uint64_t start_pfn, end_pfn, pfn;
> +
> +	start_pfn = PFN_DOWN(mem->addr);
> +	end_pfn = PFN_UP(mem->addr + mem->len);
> +
> +	for (pfn = start_pfn; pfn < end_pfn; pfn++)
> +		kho_return_pfn(pfn);
> +}
> +EXPORT_SYMBOL_GPL(kho_return_mem);
> +
> +static void kho_claim_pfn(ulong pfn)
> +{
> +	struct page *page = pfn_to_page(pfn);
> +
> +	WARN_ON(!page);
> +	if (WARN_ON(page_count(page) != 1))
> +		pr_err("Claimed non kho pfn %lx", pfn);

You do sanity checks but then never actually change anything on the
page. kho_claim_mem()'s documentation says: "This function removes the
reserved state for all pages that the mem spans". So this function
should at the very least call ClearPageReserved().

Also, checking the page count is a very rough heuristic. There can be
other non-KHO pages with page count == 1. Do you think it would make
more sense to use one of the private pageflags bits to mark a page
KHO-owned? If not, shouldn't you at least also check if the page is
reserved?

> +}
> +
> +/**
> + * kho_claim_mem - Notify the kernel that a handed over memory range is now in
> + * use by a kernel subsystem and considered an allocated page. This function
> + * removes the reserved state for all pages that the mem spans.
> + */
> +void *kho_claim_mem(const struct kho_mem *mem)
> +{
> +	u64 start_pfn, end_pfn, pfn;
> +	void *va = __va(mem->addr);
> +
> +	start_pfn = PFN_DOWN(mem->addr);
> +	end_pfn = PFN_UP(mem->addr + mem->len);
> +
> +	for (pfn = start_pfn; pfn < end_pfn; pfn++)
> +		kho_claim_pfn(pfn);
> +
> +	return va;
> +}
> +EXPORT_SYMBOL_GPL(kho_claim_mem);
> +
[...]

--
Regards,
Pratyush Yadav



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