Re: [PATCH v3 2/2] mm: Preallocate some space when adding new regions

[Daniel Kiper]

On Thu, Dec 22, 2022 at 05:18:24PM +0800, Zhang Boyang wrote:
> When grub_memalign() encounters out-of-memory, it will try
> grub_mm_add_region_fn() to request more memory from system firmware.
> However, it doesn't preallocate memory space for future allocation
> requests. In extreme cases, it requires one call to
> grub_mm_add_region_fn() for each memory allocation request. This can be
> very slow.
>
> This patch introduces GRUB_MM_HEAP_GROW_EXTRA, the minimal heap growth
> granularity. The new region size is now set to the bigger one of its
> original value and GRUB_MM_HEAP_GROW_EXTRA. Thus, it will result in some
> memory space preallocated if current allocations request is small.
>
> The value of GRUB_MM_HEAP_GROW_EXTRA is set to 1MB. If this value is
> smaller, the cost of small memory allocations will be higher. If this
> value is larger, more memory will be wasted and it might cause
> out-of-memory on machines with small amount of RAM.
>
> Signed-off-by: Zhang Boyang <zhangboyang.id@gmail.com>
> ---
>  grub-core/kern/mm.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/grub-core/kern/mm.c b/grub-core/kern/mm.c
> index 5c0e5bbbf..16c28cab8 100644
> --- a/grub-core/kern/mm.c
> +++ b/grub-core/kern/mm.c
> @@ -120,6 +120,9 @@
>                              + sizeof (struct grub_mm_header) \
>                              + (GRUB_MM_ALIGN - 1))
>
> +/* Minimal heap growth granularity when existing heap space is exhausted. */
> +#define GRUB_MM_HEAP_GROW_EXTRA 0x100000
> +
>  grub_mm_region_t grub_mm_base;
>  grub_mm_add_region_func_t grub_mm_add_region_fn;
>
> @@ -468,11 +471,12 @@ grub_memalign (grub_size_t align, grub_size_t size)
>      goto fail;
>
>    /*
> -   * Pre-calculate the size of heap growth (if applicable),
> +   * Pre-calculate the optimal size of heap growth (if applicable),
>     * with region management overhead taken into account.
>     */
>    if (grub_add (size + align, GRUB_MM_MGMT_OVERHEAD, &grow))
>      goto fail;
> +  grow = grub_max (grow, GRUB_MM_HEAP_GROW_EXTRA);

If you update the #1 patch as I suggested then this should be

  grow = grub_max (size, GRUB_MM_HEAP_GROW_EXTRA);

Daniel