Hi,
On 2023/1/12 21:52, Daniel Kiper wrote:
On Thu, Dec 22, 2022 at 05:18:23PM +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 set to "size + align +
GRUB_MM_MGMT_OVERHEAD", thus if grub_mm_add_region_fn() succeeded,
This comment is not in line with the code...
Fixed in v4.
current allocation request can always success.
Signed-off-by: Zhang Boyang <zhangboyang...@gmail.com>
---
grub-core/kern/mm.c | 54 +++++++++++++++++++++++++++++++++++++++++++--
1 file changed, 52 insertions(+), 2 deletions(-)
diff --git a/grub-core/kern/mm.c b/grub-core/kern/mm.c
index ae2279133..5c0e5bbbf 100644
--- a/grub-core/kern/mm.c
+++ b/grub-core/kern/mm.c
@@ -83,6 +83,43 @@
+/*
+ * 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 ->|
+ * ^
+ * addr % GRUB_MM_ALIGN == 1 +--/ This will be the pointer
s/addr % GRUB_MM_ALIGN == 1/assuming addr % GRUB_MM_ALIGN == 1/
Fixed in v4.
It took me some time to understand what do you mean by that.
+ * a == GRUB_MM_ALIGN - 1 | returned by next
+ * | grub_malloc(size),
+ * b == sizeof (struct grub_mm_region) | if no other suitable free
+ * c == sizeof (struct grub_mm_header) \ block is available.
+ *
+ * size % GRUB_MM_ALIGN == 1
Ditto...
+ * d == size
+ * 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)
+ */
Thank you for this comment! It clarifies a lot...
+#define GRUB_MM_MGMT_OVERHEAD ((GRUB_MM_ALIGN - 1) \
+ + sizeof (struct grub_mm_region) \
+ + sizeof (struct grub_mm_header) \
+ + (GRUB_MM_ALIGN - 1))
It is correct from math POV but the value may not be very friendly to
the CPU/MMU. Please look below for more details...
+
grub_mm_region_t grub_mm_base;
grub_mm_add_region_func_t grub_mm_add_region_fn;
@@ -230,6 +267,11 @@ 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 +452,7 @@ 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)
@@ -424,6 +467,13 @@ grub_memalign (grub_size_t align, grub_size_t size)
if ((size + align) > ~(grub_size_t) 0x100000)
goto fail;
+ /*
+ * Pre-calculate the 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;
This should go before "if ((size + align) > ~(grub_size_t) 0x100000)"
and then you should replace "size" with "grow". Additionally, as I said
above due to GRUB_MM_MGMT_OVERHEAD value final "grow" value may not be
very CPU/MMU friendly. So, I suggest to do ALIGN_UP(grow, 4096) after
grub_add() and before "if ((grow + align) > ...".
Fixed in v4. Please check if my changes matches your intention.
+
align = (align >> GRUB_MM_ALIGN_LOG2);
if (align == 0)
align = 1;
@@ -447,7 +497,7 @@ 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 +512,7 @@ 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
On 2023/1/12 21:59, Daniel Kiper wrote:
> On Thu, Dec 22, 2022 at 05:18:24PM +0800, Zhang Boyang wrote:
>> /*
>> - * 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);
>
I think this shouldn't be changed. If "size" is used here, then "grow"
will be irrelevant to GRUB_MM_MGMT_OVERHEAD.
> Daniel
Best Regards,
Zhang Boyang
_______________________________________________
Grub-devel mailing list
Grub-devel@gnu.org
https://lists.gnu.org/mailman/listinfo/grub-devel
