linux-kernel - Re: [PATCH v3 1/2] mm/memblock: Add reserved memory release function

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Message-ID: <20250211125211.1ce892a5@gandalf.local.home>
Date: Tue, 11 Feb 2025 12:52:11 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: "Masami Hiramatsu (Google)" <mhiramat@...nel.org>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org, Mark
 Rutland <mark.rutland@....com>, Mathieu Desnoyers
 <mathieu.desnoyers@...icios.com>, Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v3 1/2] mm/memblock: Add reserved memory release
 function

On Tue, 11 Feb 2025 23:47:03 +0900
"Masami Hiramatsu (Google)" <mhiramat@...nel.org> wrote:

> From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
> 
> Add reserve_mem_release_by_name() to release a reserved memory region
> with a given name. This allows us to release reserved memory which is
> defined by kernel cmdline, after boot.
> 
> Signed-off-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Mike Rapoport <rppt@...nel.org>
> Cc: linux-mm@...ck.org

Hi, can we get one of the Memory Management maintainers to ack this patch?

We will be having devices going out with the reserve_mem option to perform
tracing in the field. But that only happens if the user grants permission
to do so. But the kernel command line does not change between users that
granted permission and those that do not. We would like to free up the
memory for those devices where the users did not grant permission to trace,
as then the memory is just wasted.

Thanks!

-- Steve


> ---
>  Changes in v2:
>   - Rename reserved_mem_* to reserve_mem_*.
> ---
>  include/linux/mm.h |    1 +
>  mm/memblock.c      |   72 +++++++++++++++++++++++++++++++++++++++++++---------
>  2 files changed, 61 insertions(+), 12 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index f02925447e59..fe5f7711df04 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -4197,6 +4197,7 @@ void vma_pgtable_walk_begin(struct vm_area_struct *vma);
>  void vma_pgtable_walk_end(struct vm_area_struct *vma);
>  
>  int reserve_mem_find_by_name(const char *name, phys_addr_t *start, phys_addr_t *size);
> +int reserve_mem_release_by_name(const char *name);
>  
>  #ifdef CONFIG_64BIT
>  int do_mseal(unsigned long start, size_t len_in, unsigned long flags);
> diff --git a/mm/memblock.c b/mm/memblock.c
> index 095c18b5c430..c8d207ebb93c 100644
> --- a/mm/memblock.c
> +++ b/mm/memblock.c
> @@ -16,6 +16,7 @@
>  #include <linux/kmemleak.h>
>  #include <linux/seq_file.h>
>  #include <linux/memblock.h>
> +#include <linux/mutex.h>
>  
>  #include <asm/sections.h>
>  #include <linux/io.h>
> @@ -2263,6 +2264,7 @@ struct reserve_mem_table {
>  };
>  static struct reserve_mem_table reserved_mem_table[RESERVE_MEM_MAX_ENTRIES];
>  static int reserved_mem_count;
> +static DEFINE_MUTEX(reserve_mem_lock);
>  
>  /* Add wildcard region with a lookup name */
>  static void __init reserved_mem_add(phys_addr_t start, phys_addr_t size,
> @@ -2276,6 +2278,21 @@ static void __init reserved_mem_add(phys_addr_t start, phys_addr_t size,
>  	strscpy(map->name, name);
>  }
>  
> +static struct reserve_mem_table *reserve_mem_find_by_name_nolock(const char *name)
> +{
> +	struct reserve_mem_table *map;
> +	int i;
> +
> +	for (i = 0; i < reserved_mem_count; i++) {
> +		map = &reserved_mem_table[i];
> +		if (!map->size)
> +			continue;
> +		if (strcmp(name, map->name) == 0)
> +			return map;
> +	}
> +	return NULL;
> +}
> +
>  /**
>   * reserve_mem_find_by_name - Find reserved memory region with a given name
>   * @name: The name that is attached to a reserved memory region
> @@ -2289,22 +2306,53 @@ static void __init reserved_mem_add(phys_addr_t start, phys_addr_t size,
>  int reserve_mem_find_by_name(const char *name, phys_addr_t *start, phys_addr_t *size)
>  {
>  	struct reserve_mem_table *map;
> -	int i;
>  
> -	for (i = 0; i < reserved_mem_count; i++) {
> -		map = &reserved_mem_table[i];
> -		if (!map->size)
> -			continue;
> -		if (strcmp(name, map->name) == 0) {
> -			*start = map->start;
> -			*size = map->size;
> -			return 1;
> -		}
> -	}
> -	return 0;
> +	guard(mutex)(&reserve_mem_lock);
> +	map = reserve_mem_find_by_name_nolock(name);
> +	if (!map)
> +		return 0;
> +
> +	*start = map->start;
> +	*size = map->size;
> +	return 1;
>  }
>  EXPORT_SYMBOL_GPL(reserve_mem_find_by_name);
>  
> +/**
> + * reserve_mem_release_by_name - Release reserved memory region with a given name
> + * @name: The name that is attatched to a reserved memory region
> + *
> + * Forcibly release the pages in the reserved memory region so that those memory
> + * can be used as free memory. After released the reserved region size becomes 0.
> + *
> + * Returns: 1 if released or 0 if not found.
> + */
> +int reserve_mem_release_by_name(const char *name)
> +{
> +	struct reserve_mem_table *map;
> +	unsigned int page_count;
> +	phys_addr_t start;
> +
> +	guard(mutex)(&reserve_mem_lock);
> +	map = reserve_mem_find_by_name_nolock(name);
> +	if (!map)
> +		return 0;
> +
> +	start = map->start;
> +	page_count = DIV_ROUND_UP(map->size, PAGE_SIZE);
> +
> +	for (int i = 0; i < page_count; i++) {
> +		phys_addr_t addr = start + i * PAGE_SIZE;
> +		struct page *page = pfn_to_page(addr >> PAGE_SHIFT);
> +
> +		page->flags &= ~BIT(PG_reserved);
> +		__free_page(page);
> +	}
> +	map->size = 0;
> +
> +	return 1;
> +}
> +
>  /*
>   * Parse reserve_mem=nn:align:name
>   */