Re: [PATCH v2 1/4] mm: Allow dynamically requesting additional memory regions

[Daniel Kiper]

On Sun, Aug 08, 2021 at 03:31:41PM +0200, Patrick Steinhardt wrote:
> Currently, all platforms will set up their heap on initialization of the
> platform code. While this works mostly fine, it poses some limitations
> on memory management on us. Most notably, allocating big chunks of
> memory in the gigabyte range would require us to pre-request this many
> bytes from the firmware and add it to the heap from the beginning on
> some platforms like EFI. As this isn't needed for most configurations,
> it is inefficient and may even negatively impact some usecases when

s/when/when, e.g.,/

> chainloading. Nonetheless, allocating big chunks of memory is required
> sometimes, where one example is the upcoming support for the Argon2 key
> derival function in LUKS2.
>
> In order to avoid pre-allocating big chunks of memory, this commit
> implements a runtime mechanism to add more pages to the system. When
> a given allocation cannot be currently be satisfied, we'll call a given

s/currently be/currently/

> callback set up by the platform's own memory management subsystem,
> asking it to add a memory area with at least `n` bytes. If this
> succeeds, we retry searching for a valid memory region, which should now
> succeed.
>
> Signed-off-by: Patrick Steinhardt <ps@pks.im>
> ---
>  grub-core/kern/mm.c | 11 ++++++++++-
>  include/grub/mm.h   | 13 +++++++++++++
>  2 files changed, 23 insertions(+), 1 deletion(-)
>
> diff --git a/grub-core/kern/mm.c b/grub-core/kern/mm.c
> index c070afc62..3c003a2f9 100644
> --- a/grub-core/kern/mm.c
> +++ b/grub-core/kern/mm.c
> @@ -81,6 +81,7 @@
>  
>
>  grub_mm_region_t grub_mm_base;
> +grub_mm_region_func_t grub_mm_region_fn;
>
>  /* Get a header from the pointer PTR, and set *P and *R to a pointer
>     to the header and a pointer to its region, respectively. PTR must
> @@ -360,8 +361,16 @@ grub_memalign (grub_size_t align, grub_size_t size)
>        count++;
>        goto again;
>
> -#if 0
>      case 1:
> +      /* Request additional pages.  */
> +      count++;
> +
> +      if (grub_mm_region_fn && grub_mm_region_fn (size, 
> GRUB_MM_REGION_CONSECUTIVE) == GRUB_ERR_NONE)
> +     goto again;
> +
> +      /* fallthrough */
> +#if 0
> +    case 2:
>        /* Unload unneeded modules.  */
>        grub_dl_unload_unneeded ();

Please drop this dead code together with grub_dl_unload_unneeded()
function in a preparatory patch.

>        count++;
> diff --git a/include/grub/mm.h b/include/grub/mm.h
> index 9c38dd3ca..deff15f17 100644
> --- a/include/grub/mm.h
> +++ b/include/grub/mm.h
> @@ -20,6 +20,7 @@
>  #ifndef GRUB_MM_H
>  #define GRUB_MM_H    1
>
> +#include <grub/err.h>
>  #include <grub/types.h>
>  #include <grub/symbol.h>
>  #include <config.h>
> @@ -28,6 +29,18 @@
>  # define NULL        ((void *) 0)
>  #endif
>
> +enum {
> +  GRUB_MM_REGION_CONSECUTIVE = 1 << 0,
> +};

I prefer define instead of enum here.

> +/* Function used to request memory regions of `grub_size_t` bytes. The second
> + * parameter is a bitfield of `GRUB_MM_REGION` flags.  */

Please format comments according to [1].

> +typedef grub_err_t (*grub_mm_region_func_t) (grub_size_t, unsigned);

s/unsigned/unsigned int/

> +/* Set this function pointer to enable adding memory-regions at runtime in 
> case
> + * a memory allocation cannot be satisfied with existing regions.  */
> +extern grub_mm_region_func_t EXPORT_VAR(grub_mm_region_fn);
> +
>  void grub_mm_init_region (void *addr, grub_size_t size);
>  void *EXPORT_FUNC(grub_calloc) (grub_size_t nmemb, grub_size_t size);
>  void *EXPORT_FUNC(grub_malloc) (grub_size_t size);

Daniel

[1] https://www.gnu.org/software/grub/manual/grub-dev/grub-dev.html#Comments