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Date:   Thu, 12 Oct 2023 10:28:24 +0900
From:   Hyesoo Yu <hyesoo.yu@samsung.com>
To:     Alexandru Elisei <alexandru.elisei@arm.com>
Cc:     catalin.marinas@arm.com, will@kernel.org, oliver.upton@linux.dev,
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Subject: Re: [PATCH RFC 04/37] mm: Add MIGRATE_METADATA allocation policy
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On Wed, Aug 23, 2023 at 02:13:17PM +0100, Alexandru Elisei wrote:
> Some architectures implement hardware memory coloring to catch incorrect
> usage of memory allocation. One such architecture is arm64, which calls its
> hardware implementation Memory Tagging Extension.
> 
> So far, the memory which stores the metadata has been configured by
> firmware and hidden from Linux. For arm64, it is impossible to to have the
> entire system RAM allocated with metadata because executable memory cannot
> be tagged. Furthermore, in practice, only a chunk of all the memory that
> can have tags is actually used as tagged. which leaves a portion of
> metadata memory unused. As such, it would be beneficial to use this memory,
> which so far has been unaccessible to Linux, to service allocation
> requests. To prepare for exposing this metadata memory a new migratetype is
> being added to the page allocator, called MIGRATE_METADATA.
> 
> One important aspect is that for arm64 the memory that stores metadata
> cannot have metadata associated with it, it can only be used to store
> metadata for other pages. This means that the page allocator will *not*
> allocate from this migratetype if at least one of the following is true:
> 
> - The allocation also needs metadata to be allocated.
> - The allocation isn't movable. A metadata page storing data must be
>   able to be migrated at any given time so it can be repurposed to store
>   metadata.
> 
> Both cases are specific to arm64's implementation of memory metadata.
> 
> For now, metadata storage pages management is disabled, and it will be
> enabled once the architecture-specific handling is added.
> 
> Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
> ---
>  arch/arm64/include/asm/memory_metadata.h | 21 ++++++++++++++++++
>  arch/arm64/mm/fault.c                    |  3 +++
>  include/asm-generic/Kbuild               |  1 +
>  include/asm-generic/memory_metadata.h    | 18 +++++++++++++++
>  include/linux/mmzone.h                   | 11 ++++++++++
>  mm/Kconfig                               |  3 +++
>  mm/internal.h                            |  5 +++++
>  mm/page_alloc.c                          | 28 ++++++++++++++++++++++++
>  8 files changed, 90 insertions(+)
>  create mode 100644 arch/arm64/include/asm/memory_metadata.h
>  create mode 100644 include/asm-generic/memory_metadata.h
> 
> diff --git a/arch/arm64/include/asm/memory_metadata.h b/arch/arm64/include/asm/memory_metadata.h
> new file mode 100644
> index 000000000000..5269be7f455f
> --- /dev/null
> +++ b/arch/arm64/include/asm/memory_metadata.h
> @@ -0,0 +1,21 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +/*
> + * Copyright (C) 2023 ARM Ltd.
> + */
> +#ifndef __ASM_MEMORY_METADATA_H
> +#define __ASM_MEMORY_METADATA_H
> +
> +#include <asm-generic/memory_metadata.h>
> +
> +#ifdef CONFIG_MEMORY_METADATA
> +static inline bool metadata_storage_enabled(void)
> +{
> +	return false;
> +}
> +static inline bool alloc_can_use_metadata_pages(gfp_t gfp_mask)
> +{
> +	return false;
> +}
> +#endif /* CONFIG_MEMORY_METADATA */
> +
> +#endif /* __ASM_MEMORY_METADATA_H  */
> diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
> index 0ca89ebcdc63..1ca421c11ebc 100644
> --- a/arch/arm64/mm/fault.c
> +++ b/arch/arm64/mm/fault.c
> @@ -13,6 +13,7 @@
>  #include <linux/kfence.h>
>  #include <linux/signal.h>
>  #include <linux/mm.h>
> +#include <linux/mmzone.h>
>  #include <linux/hardirq.h>
>  #include <linux/init.h>
>  #include <linux/kasan.h>
> @@ -956,6 +957,8 @@ struct folio *vma_alloc_zeroed_movable_folio(struct vm_area_struct *vma,
>  
>  void tag_clear_highpage(struct page *page)
>  {
> +	/* Tag storage pages cannot be tagged. */
> +	WARN_ON_ONCE(is_migrate_metadata_page(page));
>  	/* Newly allocated page, shouldn't have been tagged yet */
>  	WARN_ON_ONCE(!try_page_mte_tagging(page));
>  	mte_zero_clear_page_tags(page_address(page));
> diff --git a/include/asm-generic/Kbuild b/include/asm-generic/Kbuild
> index 941be574bbe0..048ecffc430c 100644
> --- a/include/asm-generic/Kbuild
> +++ b/include/asm-generic/Kbuild
> @@ -36,6 +36,7 @@ mandatory-y += kprobes.h
>  mandatory-y += linkage.h
>  mandatory-y += local.h
>  mandatory-y += local64.h
> +mandatory-y += memory_metadata.h
>  mandatory-y += mmiowb.h
>  mandatory-y += mmu.h
>  mandatory-y += mmu_context.h
> diff --git a/include/asm-generic/memory_metadata.h b/include/asm-generic/memory_metadata.h
> new file mode 100644
> index 000000000000..dc0c84408a8e
> --- /dev/null
> +++ b/include/asm-generic/memory_metadata.h
> @@ -0,0 +1,18 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef __ASM_GENERIC_MEMORY_METADATA_H
> +#define __ASM_GENERIC_MEMORY_METADATA_H
> +
> +#include <linux/gfp.h>
> +
> +#ifndef CONFIG_MEMORY_METADATA
> +static inline bool metadata_storage_enabled(void)
> +{
> +	return false;
> +}
> +static inline bool alloc_can_use_metadata_pages(gfp_t gfp_mask)
> +{
> +	return false;
> +}
> +#endif /* !CONFIG_MEMORY_METADATA */
> +
> +#endif /* __ASM_GENERIC_MEMORY_METADATA_H */
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 5e50b78d58ea..74925806687e 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -61,6 +61,9 @@ enum migratetype {
>  	 */
>  	MIGRATE_CMA,
>  #endif
> +#ifdef CONFIG_MEMORY_METADATA
> +	MIGRATE_METADATA,
> +#endif
>  #ifdef CONFIG_MEMORY_ISOLATION
>  	MIGRATE_ISOLATE,	/* can't allocate from here */
>  #endif
> @@ -78,6 +81,14 @@ extern const char * const migratetype_names[MIGRATE_TYPES];
>  #  define is_migrate_cma_page(_page) false
>  #endif
>  
> +#ifdef CONFIG_MEMORY_METADATA
> +#  define is_migrate_metadata(migratetype) unlikely((migratetype) == MIGRATE_METADATA)
> +#  define is_migrate_metadata_page(_page) (get_pageblock_migratetype(_page) == MIGRATE_METADATA)
> +#else
> +#  define is_migrate_metadata(migratetype) false
> +#  define is_migrate_metadata_page(_page) false
> +#endif
> +
>  static inline bool is_migrate_movable(int mt)
>  {
>  	return is_migrate_cma(mt) || mt == MIGRATE_MOVABLE;
> diff --git a/mm/Kconfig b/mm/Kconfig
> index 09130434e30d..838193522e20 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -1236,6 +1236,9 @@ config LOCK_MM_AND_FIND_VMA
>  	bool
>  	depends on !STACK_GROWSUP
>  
> +config MEMORY_METADATA
> +	bool
> +
>  source "mm/damon/Kconfig"
>  
>  endmenu
> diff --git a/mm/internal.h b/mm/internal.h
> index a7d9e980429a..efd52c9f1578 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -824,6 +824,11 @@ unsigned int reclaim_clean_pages_from_list(struct zone *zone,
>  #define ALLOC_NOFRAGMENT	  0x0
>  #endif
>  #define ALLOC_HIGHATOMIC	0x200 /* Allows access to MIGRATE_HIGHATOMIC */
> +#ifdef CONFIG_MEMORY_METADATA
> +#define ALLOC_FROM_METADATA	0x400 /* allow allocations from MIGRATE_METADATA list */
> +#else
> +#define ALLOC_FROM_METADATA	0x0
> +#endif
>  #define ALLOC_KSWAPD		0x800 /* allow waking of kswapd, __GFP_KSWAPD_RECLAIM set */
>  
>  /* Flags that allow allocations below the min watermark. */
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index fdc230440a44..7baa78abf351 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -53,6 +53,7 @@
>  #include <linux/khugepaged.h>
>  #include <linux/delayacct.h>
>  #include <asm/div64.h>
> +#include <asm/memory_metadata.h>
>  #include "internal.h"
>  #include "shuffle.h"
>  #include "page_reporting.h"
> @@ -1645,6 +1646,17 @@ static inline struct page *__rmqueue_cma_fallback(struct zone *zone,
>  					unsigned int order) { return NULL; }
>  #endif
>  
> +#ifdef CONFIG_MEMORY_METADATA
> +static __always_inline struct page *__rmqueue_metadata_fallback(struct zone *zone,
> +					unsigned int order)
> +{
> +	return __rmqueue_smallest(zone, order, MIGRATE_METADATA);
> +}
> +#else
> +static inline struct page *__rmqueue_metadata_fallback(struct zone *zone,
> +					unsigned int order) { return NULL; }
> +#endif
> +
>  /*
>   * Move the free pages in a range to the freelist tail of the requested type.
>   * Note that start_page and end_pages are not aligned on a pageblock
> @@ -2144,6 +2156,15 @@ __rmqueue(struct zone *zone, unsigned int order, int migratetype,
>  		if (alloc_flags & ALLOC_CMA)
>  			page = __rmqueue_cma_fallback(zone, order);
>  
> +		/*
> +		 * Allocate data pages from MIGRATE_METADATA only if the regular
> +		 * allocation path fails to increase the chance that the
> +		 * metadata page is available when the associated data page
> +		 * needs it.
> +		 */
> +		if (!page && (alloc_flags & ALLOC_FROM_METADATA))
> +			page = __rmqueue_metadata_fallback(zone, order);
> +

Hi!

I guess it would cause non-movable page starving issue as CMA.
The metadata pages cannot be used for non-movable allocations.
Metadata pages are utilized poorly, non-movable allocations may end up
getting starved if all regular movable pages are allocated and the only
pages left are metadata. If the system has a lot of CMA pages, then
this problem would become more bad. I think it would be better to make
use of it in places where performance is not critical, including some
GFP_METADATA ?

Thanks,
Hyesoo Yu.

>  		if (!page && __rmqueue_fallback(zone, order, migratetype,
>  								alloc_flags))
>  			goto retry;
> @@ -3088,6 +3109,13 @@ static inline unsigned int gfp_to_alloc_flags_fast(gfp_t gfp_mask,
>  	if (gfp_migratetype(gfp_mask) == MIGRATE_MOVABLE)
>  		alloc_flags |= ALLOC_CMA;
>  #endif
> +#ifdef CONFIG_MEMORY_METADATA
> +	if (metadata_storage_enabled() &&
> +	    gfp_migratetype(gfp_mask) == MIGRATE_MOVABLE &&
> +	    alloc_can_use_metadata_pages(gfp_mask))
> +		alloc_flags |= ALLOC_FROM_METADATA;
> +#endif
> +
>  	return alloc_flags;
>  }
>  
> -- 
> 2.41.0
> 
> 

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