Re: [PATCH v3 2/6] mm: Allow dynamically requesting additional memory regions

[Daniel Axtens]

Patrick Steinhardt <ps@pks.im> writes:

> 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,
> 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 satisfied, we'll call a given
> 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.
>

I implemented this for ieee1275-powerpc. I set the initial memory claim
to 1MB to match EFI and to exercise the code.

Thoughts as I progressed:

 - You probably need to think about how to satisfy requests with
   particular alignments: currently there is no way to specify that with
   the current interface, and I saw powerpc-ieee1275 return bunch of
   allocations at e.g 0x2a561e which is not particularly well aligned!

 - You haven't included in the calculations the extra space required for
   mm housekeeping. For example, I'm seeing an allocation for 32kB be
   requested via grub_mm_add_region_fn. I dutifully allocate 32kB, with
   no alignment requirements, and pass that pointer to
   grub_mm_init_region. grub_mm_init_region throws away bytes at the
   start to get to GRUB_MM_ALIGN, then uses some bytes for the
   grub_mm_header_t, then any actual allocation uses bytes for the
   malloc metadata. So the actual underlying allocation cannot be
   satisfied.
   
   I think you get away with this on EFI because you use BYTES_TO_PAGES
   and get page-aligned memory, but I think you should probably round up
   to the next power of 2 for smaller allocations or to the next page or
   so for larger allocations.

 - After fixing that in the ieee1275 code, all_functional_test
   hangs trying to run the cmdline_cat test. I think this is from a slow
   algorithm somewhere - the grub allocator isn't exactly optimised for
   a proliferation of regions.

 - I noticed that nearly all the allocations were under 1MB. This seems
   inefficient for a trip out to firmware. So I made the ieee1275 code
   allocate at least max(4MB, (size of allocation rounded up nearest
   1MB) + 4kB). This makes the tests run with only the usual failures,
   at least on pseries with debug on... still chasing some bugs beyond
   that.

 - The speed impact depends on the allocation size. I'll post something
   on that tomorrow, hopefully, but larger minimum allocations work
   noticably better.

 - We only have 4GB max to play with because (at least) powerpc-ieee1275
   is technically defined to be 32 bit. So I'm a bit nervous about
   further large allocations unless we have a way to release them back
   to _firmware_, not just to grub.

I would think a better overall approach would be to allocate the 1/4 of
ram when grub starts, and create a whole new interface for large slabs
of memory that are directly allocated from, and directly returned to,
the firmware.

Kind regards,
Daniel

> Signed-off-by: Patrick Steinhardt <ps@pks.im>
> ---
>  grub-core/kern/mm.c | 10 ++++++++++
>  include/grub/mm.h   | 16 ++++++++++++++++
>  2 files changed, 26 insertions(+)
>
> diff --git a/grub-core/kern/mm.c b/grub-core/kern/mm.c
> index e0e580270..2df835392 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_add_region_func_t grub_mm_add_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
> @@ -360,6 +361,15 @@ grub_memalign (grub_size_t align, grub_size_t size)
>        count++;
>        goto again;
>  
> +    case 1:
> +      /* Request additional pages.  */
> +      count++;
> +
> +      if (grub_mm_add_region_fn && grub_mm_add_region_fn (size, 
> GRUB_MM_ADD_REGION_CONSECUTIVE) == GRUB_ERR_NONE)
> +     goto again;
> +
> +      /* fallthrough */
> +
>      default:
>        break;
>      }
> diff --git a/include/grub/mm.h b/include/grub/mm.h
> index 9c38dd3ca..afde57d2e 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 +29,21 @@
>  # define NULL        ((void *) 0)
>  #endif
>  
> +#define GRUB_MM_ADD_REGION_NONE        0
> +#define GRUB_MM_ADD_REGION_CONSECUTIVE (1 << 0)

CONSECUTIVE needs a definition here. Initially I thought it meant that
the request had to expand an existing region, but what I think it means
having read your EFI implementation is that the request needs to be
satisfied by a single region rather than by a number of smaller regions.
(NONE probably also needs a better name, because it seemed a bit odd to
type ADD_REGION_NONE. Even ADD_REGION_FLAGS_NONE would be better.)


> +
> +/*
> + * Function used to request memory regions of `grub_size_t` bytes. The second
> + * parameter is a bitfield of `GRUB_MM_ADD_REGION` flags.
> + */
> +typedef grub_err_t (*grub_mm_add_region_func_t) (grub_size_t, 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_add_region_func_t EXPORT_VAR(grub_mm_add_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);
> -- 
> 2.32.0
>
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