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

[Ard Biesheuvel]

On Tue, 4 Jun 2024 at 01:35, Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>
> From: "Steven Rostedt (Google)" <rostedt@xxxxxxxxxxx>
>
> 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
>
> Link: https://lore.kernel.org/all/ZjJVnZUX3NZiGW6q@xxxxxxxxxx/
>
> Suggested-by: Mike Rapoport <rppt@xxxxxxxxxx>
> Signed-off-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>

You failed to point out in the commit message that the assumption here
is that this memory will retain its contents across a soft reboot. Or
am I misunderstanding this?

In any case, as I pointed out before, playing these games unilaterally
from the kernel side, i.e., without any awareness whatsoever from the
firmware and bootloader (which will not attempt to preserve RAM
contents), is likely to have a rather disappointing success ratio in
the general case. I understand this may be different for vertically
integrated software stacks like ChromeOS so perhaps it should live
there as a feature.

Then, as Kees points out, there is also the risk that the kernel
itself may be stepping on this memory before having realized that it
is reserved. At least ARM and x86 have decompressors with a
substantial amount of non-trivial placement logic that would need to
be made aware of this reservation. Note that EFI vs. non-EFI boot also
makes a difference here.


> ---
>  include/linux/mm.h |  2 +
>  mm/memblock.c      | 97 ++++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 99 insertions(+)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 9849dfda44d4..b4455cc02f2c 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, unsigned long *start, unsigned long *size);
> +
>  #endif /* _LINUX_MM_H */
> diff --git a/mm/memblock.c b/mm/memblock.c
> index d09136e040d3..a8bf0ee9e2b4 100644
> --- a/mm/memblock.c
> +++ b/mm/memblock.c
> @@ -2244,6 +2244,103 @@ 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];
> +       unsigned long           start;
> +       unsigned long           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(unsigned long start, unsigned long size,
> +                                  const char *name)
> +{
> +       struct reserve_mem_table *map;
> +
> +       if (!name || !name[0] || strlen(name) >= RESERVE_MEM_NAME_SIZE)
> +               return -EINVAL;
> +
> +       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, unsigned long *start, unsigned long *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;
> +}
> +
> +/*
> + * Parse early_reserve_mem=nn:align:name
> + */
> +static int __init reserve_mem(char *p)
> +{
> +       phys_addr_t start, size, align;
> +       char *oldp;
> +       int err;
> +
> +       if (!p)
> +               return -EINVAL;
> +
> +       oldp = p;
> +       size = memparse(p, &p);
> +       if (p == oldp)
> +               return -EINVAL;
> +
> +       if (*p != ':')
> +               return -EINVAL;
> +
> +       align = memparse(p+1, &p);
> +       if (*p != ':')
> +               return -EINVAL;
> +
> +       start = memblock_phys_alloc(size, align);
> +       if (!start)
> +               return -ENOMEM;
> +
> +       p++;
> +       err = reserved_mem_add(start, size, p);
> +       if (err) {
> +               memblock_phys_free(start, size);
> +               return err;
> +       }
> +
> +       p += strlen(p);
> +
> +       return *p == '\0' ? 0: -EINVAL;
> +}
> +__setup("reserve_mem=", reserve_mem);
> +
>  #if defined(CONFIG_DEBUG_FS) && defined(CONFIG_ARCH_KEEP_MEMBLOCK)
>  static const char * const flagname[] = {
>         [ilog2(MEMBLOCK_HOTPLUG)] = "HOTPLUG",
> --
> 2.43.0
>
>
>