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Message-ID: <ed4f4589f322bd3871f825239985410b535df30e.camel@kernel.org>
Date: Thu, 17 Feb 2022 11:49:56 +0100
From: nicolas saenz julienne <nsaenz@...nel.org>
To: Vijay Balakrishna <vijayb@...ux.microsoft.com>,
Catalin Marinas <catalin.marinas@....com>,
Will Deacon <will@...nel.org>
Cc: Anshuman Khandual <anshuman.khandual@....com>,
Mike Rapoport <rppt@...nel.org>,
Ard Biesheuvel <ardb@...nel.org>,
Pavel Tatashin <pasha.tatashin@...een.com>,
linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] arm64: Do not defer reserve_crashkernel() for platforms
with no DMA memory zones
On Wed, 2022-02-16 at 16:04 -0800, Vijay Balakrishna wrote:
> The following patches resulted in deferring crash kernel reservation to
> mem_init(), mainly aimed at platforms with DMA memory zones (no IOMMU),
> in particular Raspberry Pi 4.
>
> commit 1a8e1cef7603 ("arm64: use both ZONE_DMA and ZONE_DMA32")
> commit 8424ecdde7df ("arm64: mm: Set ZONE_DMA size based on devicetree's dma-ranges")
> commit 0a30c53573b0 ("arm64: mm: Move reserve_crashkernel() into mem_init()")
> commit 2687275a5843 ("arm64: Force NO_BLOCK_MAPPINGS if crashkernel reservation is required")
>
> Above changes introduced boot slowdown due to linear map creation for
> all the memory banks with NO_BLOCK_MAPPINGS, see discussion[1]. The proposed
> changes restore crash kernel reservation to earlier behavior thus avoids
> slow boot, particularly for platforms with IOMMU (no DMA memory zones).
>
> [1] https://lore.kernel.org/all/9436d033-579b-55fa-9b00-6f4b661c2dd7@linux.microsoft.com/
>
> Signed-off-by: Vijay Balakrishna <vijayb@...ux.microsoft.com>
> Cc: stable@...r.kernel.org
> ---
> Tested changes to confirm no ~150ms boot slowdown on our SoC with IOMMU
> and 8GB memory. Also tested with ZONE_DMA and/or ZONE_DMA32 configs to confirm
> no regression to deferring scheme of crash kernel memory reservation.
> In both cases successfully collected kernel crash dump.
> ---
> arch/arm64/mm/init.c | 14 +++++++++++---
> arch/arm64/mm/mmu.c | 24 +++++++++++++++++++++++-
> 2 files changed, 34 insertions(+), 4 deletions(-)
>
> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> index db63cc885771..f2a982c19b75 100644
> --- a/arch/arm64/mm/init.c
> +++ b/arch/arm64/mm/init.c
> @@ -62,7 +62,11 @@ EXPORT_SYMBOL(memstart_addr);
> * In such case, ZONE_DMA32 covers the rest of the 32-bit addressable memory,
> * otherwise it is empty.
> */
> -phys_addr_t arm64_dma_phys_limit __ro_after_init;
> +#if !defined(CONFIG_ZONE_DMA) && !defined(CONFIG_ZONE_DMA32)
> +phys_addr_t __ro_after_init arm64_dma_phys_limit = PHYS_MASK + 1;
> +#else
> +phys_addr_t __ro_after_init arm64_dma_phys_limit;
> +#endif
>
> #ifdef CONFIG_KEXEC_CORE
> /*
> @@ -153,8 +157,6 @@ static void __init zone_sizes_init(unsigned long min, unsigned long max)
> if (!arm64_dma_phys_limit)
> arm64_dma_phys_limit = dma32_phys_limit;
> #endif
> - if (!arm64_dma_phys_limit)
> - arm64_dma_phys_limit = PHYS_MASK + 1;
> max_zone_pfns[ZONE_NORMAL] = max;
>
> free_area_init(max_zone_pfns);
> @@ -315,6 +317,10 @@ void __init arm64_memblock_init(void)
>
> early_init_fdt_scan_reserved_mem();
>
> +#if !defined(CONFIG_ZONE_DMA) && !defined(CONFIG_ZONE_DMA32)
> + reserve_crashkernel();
> +#endif
> +
> high_memory = __va(memblock_end_of_DRAM() - 1) + 1;
> }
>
> @@ -357,11 +363,13 @@ void __init bootmem_init(void)
> */
> dma_contiguous_reserve(arm64_dma_phys_limit);
>
> +#if defined(CONFIG_ZONE_DMA) || defined(CONFIG_ZONE_DMA32)
> /*
> * request_standard_resources() depends on crashkernel's memory being
> * reserved, so do it here.
> */
> reserve_crashkernel();
> +#endif
>
> memblock_dump_all();
> }
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index acfae9b41cc8..e7faf5edccfc 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -517,7 +517,7 @@ static void __init map_mem(pgd_t *pgdp)
> */
> BUILD_BUG_ON(pgd_index(direct_map_end - 1) == pgd_index(direct_map_end));
>
> - if (can_set_direct_map() || crash_mem_map || IS_ENABLED(CONFIG_KFENCE))
> + if (can_set_direct_map() || IS_ENABLED(CONFIG_KFENCE))
> flags |= NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS;
>
> /*
> @@ -528,6 +528,14 @@ static void __init map_mem(pgd_t *pgdp)
> */
> memblock_mark_nomap(kernel_start, kernel_end - kernel_start);
>
> +#ifdef CONFIG_KEXEC_CORE
> + if (crash_mem_map && !crashk_res.end)
> + flags |= NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS;
Using IS_ENABLED(ZONE_DMA/DMA32) instead of '!crashk_res.end' would be more
efficient and a bit more explicit IMO.
> /* map all the memory banks */
> for_each_mem_range(i, &start, &end) {
> if (start >= end)
> @@ -554,6 +562,20 @@ static void __init map_mem(pgd_t *pgdp)
> __map_memblock(pgdp, kernel_start, kernel_end,
> PAGE_KERNEL, NO_CONT_MAPPINGS);
> memblock_clear_nomap(kernel_start, kernel_end - kernel_start);
> +#ifdef CONFIG_KEXEC_CORE
> + /*
> + * Use page-level mappings here so that we can shrink the region
> + * in page granularity and put back unused memory to buddy system
> + * through /sys/kernel/kexec_crash_size interface.
> + */
> + if (crashk_res.end) {
Same here.
> + __map_memblock(pgdp, crashk_res.start, crashk_res.end + 1,
> + PAGE_KERNEL,
> + NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS);
> + memblock_clear_nomap(crashk_res.start,
> + resource_size(&crashk_res));
> + }
> +#endif
Now, I carefully reviewed the patch and it seems to be doing the right thing.
But even while knowlegable on the topic, it took a good amount of effort to
untangle the possible code paths. I suspect it's going to be painful to
maintain. I'd suggest at least introducing a comment explaining the situation.
If there approach if deemed acceptable, I'll test is on the RPi4.
Regards,
Nicolas
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