I looked at this patch again and found a few more (minor) issues...
On Sat, Jan 14, 2023 at 09:23:22PM +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, the size passed to it doesn't take region management overhead
> into account. Adding a memory area of "size" bytes may result in a heap
> region of less than "size" bytes truely avaliable. Thus, the new region
> may not be adequate for current allocation request, confusing
> out-of-memory handling code.
>
> This patch introduces GRUB_MM_MGMT_OVERHEAD to address the region
> management overhead (e.g. metadata, padding). The value of this new
> constant must be large enough to make sure grub_malloc(size) always
> success after a successful call to grub_mm_init_region(addr, size +
> GRUB_MM_MGMT_OVERHEAD), for any given addr and size (assuming no
> interger overflow).
>
> The size passed to grub_mm_add_region_fn() is now correctly adjusted,
> thus if grub_mm_add_region_fn() succeeded, current allocation request
> can always success.
>
> Signed-off-by: Zhang Boyang <zhangboyang...@gmail.com>
> ---
> grub-core/kern/mm.c | 64 ++++++++++++++++++++++++++++++++++++++++++---
> 1 file changed, 61 insertions(+), 3 deletions(-)
>
> diff --git a/grub-core/kern/mm.c b/grub-core/kern/mm.c
> index ae2279133..278756dea 100644
> --- a/grub-core/kern/mm.c
> +++ b/grub-core/kern/mm.c
> @@ -83,6 +83,46 @@
>
>
>
> +/*
> + * GRUB_MM_MGMT_OVERHEAD is an upper bound of management overhead of
> + * each region, with any possible padding taken into account.
> + *
> + * The value must be large enough to make sure grub_malloc(size) always
> + * success after a successful call to
> + * grub_mm_init_region(addr, size + GRUB_MM_MGMT_OVERHEAD), for any given
> + * addr and size (assuming no interger overflow).
> + *
> + * The worst case which has maximum overhead is shown in the figure below:
> + *
> + * +-- addr
> + * v |<- size ->|
> + * +---------+----------------+----------------+--------------+---------+
> + * | padding | grub_mm_region | grub_mm_header | usable bytes | padding |
> + * +---------+----------------+----------------+--------------+---------+
> + * |<- a ->|<- b ->|<- c ->|<- d ->|<- e ->|
This drawing is missing grub_memalign() align argument. It should be
put between "c" and "d". Though the code below of course takes it into
account... :-)
> + * b == sizeof (struct grub_mm_region) +--/ This will be the pointer
> + * c == sizeof (struct grub_mm_header) | returned by next
> + * d == size | grub_malloc(size),
> + * | if no other suitable free
> + * Assuming addr % GRUB_MM_ALIGN == 1, then \ block is available.
> + * a == GRUB_MM_ALIGN - 1
> + *
> + * Assuming size % GRUB_MM_ALIGN == 1, then
> + * e == GRUB_MM_ALIGN - 1
> + *
> + * Therefore, the maximum overhead is:
> + * a + b + c + e == (GRUB_MM_ALIGN - 1) + sizeof (struct grub_mm_region)
> + * + sizeof (struct grub_mm_header) + (GRUB_MM_ALIGN - 1)
> + */
> +#define GRUB_MM_MGMT_OVERHEAD ((GRUB_MM_ALIGN - 1) \
> + + sizeof (struct grub_mm_region) \
> + + sizeof (struct grub_mm_header) \
> + + (GRUB_MM_ALIGN - 1))
> +
> +/* The size passed to grub_mm_add_region_fn() is aligned up by this value. */
> +#define GRUB_MM_HEAP_GROW_ALIGN 4096
> +
> grub_mm_region_t grub_mm_base;
> grub_mm_add_region_func_t grub_mm_add_region_fn;
>
> @@ -230,6 +270,11 @@ grub_mm_init_region (void *addr, grub_size_t size)
>
> grub_dprintf ("regions", "No: considering a new region at %p of size %"
> PRIxGRUB_SIZE "\n",
> addr, size);
> + /*
> + * If you want to modify the code below, please also take a look at
> + * GRUB_MM_MGMT_OVERHEAD and make sure it is synchronized with the code.
> + */
> +
> /* Allocate a region from the head. */
> r = (grub_mm_region_t) ALIGN_UP ((grub_addr_t) addr, GRUB_MM_ALIGN);
>
> @@ -410,6 +455,7 @@ grub_memalign (grub_size_t align, grub_size_t size)
> {
> grub_mm_region_t r;
> grub_size_t n = ((size + GRUB_MM_ALIGN - 1) >> GRUB_MM_ALIGN_LOG2) + 1;
> + grub_size_t grow;
> int count = 0;
>
> if (!grub_mm_base)
> @@ -418,10 +464,22 @@ grub_memalign (grub_size_t align, grub_size_t size)
> if (size > ~(grub_size_t) align)
> goto fail;
>
> + /*
> + * Pre-calculate the necessary 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;
> +
> + /* Align up heap growth to make it friendly to CPU/MMU. */
> + if (grow > ~(grub_size_t) (GRUB_MM_HEAP_GROW_ALIGN - 1))
> + goto fail;
> + grow = ALIGN_UP (grow, GRUB_MM_HEAP_GROW_ALIGN);
ALIGN_UP() from here should be last thing of the math which is happening
above and below. Otherwise it may not give us what we expect...
> /* We currently assume at least a 32-bit grub_size_t,
> so limiting allocations to <adress space size> - 1MiB
> in name of sanity is beneficial. */
> - if ((size + align) > ~(grub_size_t) 0x100000)
> + if (grow > ~(grub_size_t) 0x100000)
> goto fail;
We do a lot of math here which we very often ditch later almost
immediately. I would do small optimization here like:
/* If failed, increase free memory somehow. */
switch (count)
{
case 0:
/* Request additional pages, contiguous */
count++;
<our_math_from_above> <----------- MOVE IT HERE...
if (grub_mm_add_region_fn != NULL &&
grub_mm_add_region_fn (size, GRUB_MM_ADD_REGION_CONSECUTIVE) ==
GRUB_ERR_NONE)
goto again;
/* fallthrough */
case 1:
/* Request additional pages, anything at all */
count++;
...
Of course this should be a separate patch in this series. I think last one.
> align = (align >> GRUB_MM_ALIGN_LOG2);
> @@ -447,7 +505,7 @@ grub_memalign (grub_size_t align, grub_size_t size)
> count++;
>
> if (grub_mm_add_region_fn != NULL &&
> - grub_mm_add_region_fn (size, GRUB_MM_ADD_REGION_CONSECUTIVE) ==
> GRUB_ERR_NONE)
> + grub_mm_add_region_fn (grow, GRUB_MM_ADD_REGION_CONSECUTIVE) ==
> GRUB_ERR_NONE)
> goto again;
>
> /* fallthrough */
> @@ -462,7 +520,7 @@ grub_memalign (grub_size_t align, grub_size_t size)
> * Try again even if this fails, in case it was able to partially
> * satisfy the request
> */
> - grub_mm_add_region_fn (size, GRUB_MM_ADD_REGION_NONE);
> + grub_mm_add_region_fn (grow, GRUB_MM_ADD_REGION_NONE);
> goto again;
> }
Daniel
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