Re: [PATCH v4 1/2] mm/memblock: Add "reserve_mem" to reserved named memory at boot up

[Guenter Roeck <linux@roeck-us.net>]

On 6/11/24 14:56, Steven Rostedt wrote:
> From: "Steven Rostedt (Google)" <rostedt@goodmis.org>
> 
> In order to allow for requesting a memory region that can be used for
> things like pstore on multiple machines where the memory layout is not the
> same, add a new option to the kernel command line called "reserve_mem".
> 
> The format is:  reserve_mem=nn:align:name
> 
> Where it will find nn amount of memory at the given alignment of align.
> The name field is to allow another subsystem to retrieve where the memory
> was found. For example:
> 
>    reserve_mem=12M:4096:oops ramoops.mem_name=oops
> 
> Where ramoops.mem_name will tell ramoops that memory was reserved for it
> via the reserve_mem option and it can find it by calling:
> 
>    if (reserve_mem_find_by_name("oops", &start, &size)) {
> 	// start holds the start address and size holds the size given
> 
> This is typically used for systems that do not wipe the RAM, and this
> command line will try to reserve the same physical memory on soft reboots.
> Note, it is not guaranteed to be the same location. For example, if KASLR
> places the kernel at the location of where the RAM reservation was from a
> previous boot, the new reservation will be at a different location.  Any
> subsystem using this feature must add a way to verify that the contents of
> the physical memory is from a previous boot, as there may be cases where
> the memory will not be located at the same location.
> 
> Not all systems may work either. There could be bit flips if the reboot
> goes through the BIOS. Using kexec to reboot the machine is likely to
> have better results in such cases.
> 
> Link: https://lore.kernel.org/all/ZjJVnZUX3NZiGW6q@kernel.org/
> 
> Suggested-by: Mike Rapoport <rppt@kernel.org>
> Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
> ---
> Changes since v3: https://lore.kernel.org/linux-trace-kernel/20240606150316.751642266@goodmis.org
> 
> - Changed table type of start and size from unsigned long to phys_addr_t
>    (as well as the parameters to the functions that use them)
> 
> - Changed old reference to "early_reserve_mem" to "reserve_mem"
> 
> - Check before reservering memory:
>    o Size is non-zero
>    o name has text in it
> 
> - If align is less than SMP_CACHE_BYTES, make it SMP_CACHE_BYTES
> 
> - Remove the silly check of testing *p == '\0' after a p += strlen(p)
> 
>   .../admin-guide/kernel-parameters.txt         |  20 +++
>   include/linux/mm.h                            |   2 +
>   mm/memblock.c                                 | 115 ++++++++++++++++++
>   3 files changed, 137 insertions(+)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index b600df82669d..ce7de8136f2f 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -5710,6 +5710,26 @@
>   			them.  If <base> is less than 0x10000, the region
>   			is assumed to be I/O ports; otherwise it is memory.
>   
> +	reserve_mem=	[RAM]
> +			Format: nn[KNG]:<align>:<label>
> +			Reserve physical memory and label it with a name that
> +			other subsystems can use to access it. This is typically
> +			used for systems that do not wipe the RAM, and this command
> +			line will try to reserve the same physical memory on
> +			soft reboots. Note, it is not guaranteed to be the same
> +			location. For example, if KASLR places the kernel at the
> +			location of where the RAM reservation was from a previous
> +			boot, the new reservation will be at a different location.
> +			Any subsystem using this feature must add a way to verify
> +			that the contents of the physical memory is from a previous
> +			boot, as there may be cases where the memory will not be
> +			located at the same location.
> +
> +			The format is size:align:label for example, to request
> +			12 megabytes of 4096 alignment for ramoops:
> +
> +			reserve_mem=12M:4096:oops ramoops.mem_name=oops
> +
>   	reservetop=	[X86-32,EARLY]
>   			Format: nn[KMG]
>   			Reserves a hole at the top of the kernel virtual
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 9849dfda44d4..077fb589b88a 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -4263,4 +4263,6 @@ static inline bool pfn_is_unaccepted_memory(unsigned long pfn)
>   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);
> +
>   #endif /* _LINUX_MM_H */
> diff --git a/mm/memblock.c b/mm/memblock.c
> index d09136e040d3..044ddce8f085 100644
> --- a/mm/memblock.c
> +++ b/mm/memblock.c
> @@ -2244,6 +2244,121 @@ void __init memblock_free_all(void)
>   	totalram_pages_add(pages);
>   }
>   
> +/* Keep a table to reserve named memory */
> +#define RESERVE_MEM_MAX_ENTRIES		8
> +#define RESERVE_MEM_NAME_SIZE		16
> +struct reserve_mem_table {
> +	char			name[RESERVE_MEM_NAME_SIZE];
> +	phys_addr_t		start;
> +	phys_addr_t		size;
> +};
> +static struct reserve_mem_table reserved_mem_table[RESERVE_MEM_MAX_ENTRIES];
> +static int reserved_mem_count;
> +
> +/* Add wildcard region with a lookup name */
> +static int __init reserved_mem_add(phys_addr_t start, phys_addr_t size,
> +				   const char *name)
> +{
> +	struct reserve_mem_table *map;
> +
> +	if (!name || !name[0] || strlen(name) >= RESERVE_MEM_NAME_SIZE)
> +		return -EINVAL;
> +

I know I am picky, but name is never NULL, and strlen(name) is guaranteed to be > 0.
Personally I'd suggest to check for strlen(name) >= RESERVE_MEM_NAME_SIZE together
with !strlen(name) and drop the duplicate checks here (and, as side effect, avoid
the pointless memory allocation if the name is invalid).

> +	if (reserved_mem_count >= RESERVE_MEM_MAX_ENTRIES)
> +		return -1;
> +
> +	map = &reserved_mem_table[reserved_mem_count++];
> +	map->start = start;
> +	map->size = size;
> +	strscpy(map->name, name);
> +	return 0;
> +}
> +
> +/**
> + * reserve_mem_find_by_name - Find reserved memory region with a given name
> + * @name: The name that is attached to a reserved memory region
> + * @start: If found, holds the start address
> + * @size: If found, holds the size of the address.
> + *
> + * Returns: 1 if found or 0 if not found.
> + */
> +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;
> +}
> +EXPORT_SYMBOL_GPL(reserve_mem_find_by_name);
> +
> +/*
> + * Parse reserve_mem=nn:align:name
> + */
> +static int __init reserve_mem(char *p)
> +{
> +	phys_addr_t start, size, align;
> +	char *name;
> +	char *oldp;
> +	int err;
> +
> +	if (!p)
> +		return -EINVAL;
> +
> +	oldp = p;
> +	size = memparse(p, &p);
> +	if (!size || p == oldp)
> +		return -EINVAL;
> +
> +	if (*p != ':')
> +		return -EINVAL;
> +
> +	align = memparse(p+1, &p);
> +	if (*p != ':')
> +		return -EINVAL;
> +
> +	/*
> +	 * memblock_phys_alloc() doesn't like a zero size align,
> +	 * but it is OK for this command to have it.
> +	 */
> +	if (align <= SMP_CACHE_BYTES)

Any reason for using <= instead of < ?

Guenter

> +		align = SMP_CACHE_BYTES;
> +
> +	name = p + 1;
> +	if (!strlen(name))
> +		return -EINVAL;
> +
> +	/* Make sure that name has text */
> +	for (p = name; *p; p++) {
> +		if (!isspace(*p))
> +			break;
> +	}
> +	if (!*p)
> +		return -EINVAL;
> +
> +	start = memblock_phys_alloc(size, align);
> +	if (!start)
> +		return -ENOMEM;
> +
> +	err = reserved_mem_add(start, size, name);
> +	if (err) {
> +		memblock_phys_free(start, size);
> +		return err;
> +	}
> +
> +	return 0;
> +}
> +__setup("reserve_mem=", reserve_mem);
> +
>   #if defined(CONFIG_DEBUG_FS) && defined(CONFIG_ARCH_KEEP_MEMBLOCK)
>   static const char * const flagname[] = {
>   	[ilog2(MEMBLOCK_HOTPLUG)] = "HOTPLUG",