linux-kernel - Re: [PATCH V9 2/2] arm64/mm: Enable memory hot remove

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Date:   Thu, 10 Oct 2019 12:34:59 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Anshuman Khandual <anshuman.khandual@....com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, akpm@...ux-foundation.org,
        will@...nel.org, mark.rutland@....com, david@...hat.com,
        cai@....pw, logang@...tatee.com, cpandya@...eaurora.org,
        arunks@...eaurora.org, dan.j.williams@...el.com,
        mgorman@...hsingularity.net, osalvador@...e.de,
        ard.biesheuvel@....com, steve.capper@....com, broonie@...nel.org,
        valentin.schneider@....com, Robin.Murphy@....com,
        steven.price@....com, suzuki.poulose@....com, ira.weiny@...el.com
Subject: Re: [PATCH V9 2/2] arm64/mm: Enable memory hot remove

Hi Anshuman,

On Wed, Oct 09, 2019 at 01:51:48PM +0530, Anshuman Khandual wrote:
> +static void unmap_hotplug_pmd_range(pud_t *pudp, unsigned long addr,
> +				    unsigned long end, bool free_mapped)
> +{
> +	unsigned long next;
> +	pmd_t *pmdp, pmd;
> +
> +	do {
> +		next = pmd_addr_end(addr, end);
> +		pmdp = pmd_offset(pudp, addr);
> +		pmd = READ_ONCE(*pmdp);
> +		if (pmd_none(pmd))
> +			continue;
> +
> +		WARN_ON(!pmd_present(pmd));
> +		if (pmd_sect(pmd)) {
> +			pmd_clear(pmdp);
> +			flush_tlb_kernel_range(addr, next);

The range here could be a whole PMD_SIZE. Since we are invalidating a
single block entry, one TLBI should be sufficient:

			flush_tlb_kernel_range(addr, addr + PAGE_SIZE);

> +			if (free_mapped)
> +				free_hotplug_page_range(pmd_page(pmd),
> +							PMD_SIZE);
> +			continue;
> +		}
> +		WARN_ON(!pmd_table(pmd));
> +		unmap_hotplug_pte_range(pmdp, addr, next, free_mapped);
> +	} while (addr = next, addr < end);
> +}
> +
> +static void unmap_hotplug_pud_range(pgd_t *pgdp, unsigned long addr,
> +				    unsigned long end, bool free_mapped)
> +{
> +	unsigned long next;
> +	pud_t *pudp, pud;
> +
> +	do {
> +		next = pud_addr_end(addr, end);
> +		pudp = pud_offset(pgdp, addr);
> +		pud = READ_ONCE(*pudp);
> +		if (pud_none(pud))
> +			continue;
> +
> +		WARN_ON(!pud_present(pud));
> +		if (pud_sect(pud)) {
> +			pud_clear(pudp);
> +			flush_tlb_kernel_range(addr, next);
			^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
			flush_tlb_kernel_range(addr, addr + PAGE_SIZE);

[...]
> +static void free_empty_pte_table(pmd_t *pmdp, unsigned long addr,
> +				 unsigned long end, unsigned long floor,
> +				 unsigned long ceiling)
> +{
> +	pte_t *ptep, pte;
> +	unsigned long i, start = addr;
> +
> +	do {
> +		ptep = pte_offset_kernel(pmdp, addr);
> +		pte = READ_ONCE(*ptep);
> +		WARN_ON(!pte_none(pte));
> +	} while (addr += PAGE_SIZE, addr < end);

So this loop is just a sanity check (pte clearing having been done by
the unmap loops). That's fine, maybe a comment for future reference.

> +
> +	if (!pgtable_range_aligned(start, end, floor, ceiling, PMD_MASK))
> +		return;
> +
> +	ptep = pte_offset_kernel(pmdp, 0UL);
> +	for (i = 0; i < PTRS_PER_PTE; i++) {
> +		if (!pte_none(READ_ONCE(ptep[i])))
> +			return;
> +	}

We could do with a comment for this loop along the lines of:

	Check whether we can free the pte page if the rest of the
	entries are empty. Overlap with other regions have been handled
	by the floor/ceiling check.

Apart from the comments above, the rest of the patch looks fine. Once
fixed:

Reviewed-by: Catalin Marinas <catalin.marinas@....com>

Mark Rutland mentioned at some point that, as a preparatory patch to
this series, we'd need to make sure we don't hot-remove memory already
given to the kernel at boot. Any plans here?

Thanks.

-- 
Catalin