Memblock is one of the methods of managing memory regions during the early bootstrap period while the usual kernel memory allocators are not up and
running yet. Previously it was called `Logical Memory Block`, but with the [patch](https://lkml.org/lkml/2010/7/13/68) by Yinghai Lu, it was renamed to the `memblock`. As Linux kernel for `x86_64` architecture uses this method. We already met `memblock` in the [Last preparations before the kernel entry point](http://0xax.gitbooks.io/linux-insides/content/Initialization/linux-initialization-3.html) part. And now time to get acquainted with it closer. We will see how it is implemented.
running yet. Previously it was called `Logical Memory Block`, but with the [patch](https://lkml.org/lkml/2010/7/13/68) by Yinghai Lu, it was renamed to the `memblock`. As Linux kernel for `x86_64` architecture uses this method. We already met `memblock` in the [Last preparations before the kernel entry point](http://0xax.gitbooks.io/linux-insides/content/Initialization/linux-initialization-3.html) part. And now it's time to get acquainted with it closer. We will see how it is implemented.
We will start to learn `memblock` from the data structures. Definitions of the all data structures can be found in the [include/linux/memblock.h](https://github.com/torvalds/linux/blob/master/include/linux/memblock.h) header file.
@ -85,7 +85,7 @@ These three structures: `memblock`, `memblock_type` and `memblock_region` are ma
As all API of the `memblock` described in the [include/linux/memblock.h](https://github.com/torvalds/linux/blob/master/include/linux/memblock.h) header file, all implementation of these function is in the [mm/memblock.c](https://github.com/torvalds/linux/blob/master/mm/memblock.c) source code file. Let's look at the top of the source code file and we will see the initialization of the `memblock` structure:
As all API of the `memblock`are described in the [include/linux/memblock.h](https://github.com/torvalds/linux/blob/master/include/linux/memblock.h) header file, all implementation of these function is in the [mm/memblock.c](https://github.com/torvalds/linux/blob/master/mm/memblock.c) source code file. Let's look at the top of the source code file and we will see the initialization of the `memblock` structure:
Here we can see initialization of the `memblock` structure which has the same name as structure - `memblock`. First of all note on`__initdata_memblock`. Defenition of this macro looks like:
Here we can see initialization of the `memblock` structure which has the same name as structure - `memblock`. First of all note the`__initdata_memblock`. Defenition of this macro looks like:
```C
#ifdef CONFIG_ARCH_DISCARD_MEMBLOCK
@ -137,7 +137,7 @@ Every array contains 128 memory regions. We can see it in the `INIT_MEMBLOCK_REG
#define INIT_MEMBLOCK_REGIONS 128
```
Note that all arrays are also defined with the `__initdata_memblock` macro which we already saw in the `memblock` strucutre initialization (read above if you've forgot).
Note that all arrays are also defined with the `__initdata_memblock` macro which we already saw in the `memblock` structure initialization (read above if you've forgotten).
The last two fields describe that `bottom_up` allocation is disabled and the limit of the current Memblock is:
@ -147,12 +147,12 @@ The last two fields describe that `bottom_up` allocation is disabled and the lim
which is `0xffffffffffffffff`.
On this step initialization of the `memblock` structure finished and we can look on the Memblock API.
On this step the initialization of the `memblock` structure has been finished and we can look on the Memblock API.
Ok we have finished with initilization of the `memblock` structure and now we can look on the Memblock API and its implementation. As I said above, all implementation of the `memblock` presented in the [mm/memblock.c](https://github.com/torvalds/linux/blob/master/mm/memblock.c). To understand how `memblock` works and is implemented, let's look at its usage first of all. There are a couple of [places](http://lxr.free-electrons.com/ident?i=memblock) in the linux kernel where memblock is used. For example let's take `memblock_x86_fill` function from the [arch/x86/kernel/e820.c](https://github.com/torvalds/linux/blob/master/arch/x86/kernel/e820.c#L1061). This function goes through the memory map provided by the [e820](http://en.wikipedia.org/wiki/E820) and adds memory regions reserved by the kernel to the `memblock` with the `memblock_add` function. As we met `memblock_add` function first, let's start from it.
Ok we have finished with initilization of the `memblock` structure and now we can look on the Memblock API and its implementation. As I said above, all implementation of the `memblock`is presented in the [mm/memblock.c](https://github.com/torvalds/linux/blob/master/mm/memblock.c). To understand how `memblock` works and how it is implemented, let's look at its usage first. There are a couple of [places](http://lxr.free-electrons.com/ident?i=memblock) in the linux kernel where memblock is used. For example let's take `memblock_x86_fill` function from the [arch/x86/kernel/e820.c](https://github.com/torvalds/linux/blob/master/arch/x86/kernel/e820.c#L1061). This function goes through the memory map provided by the [e820](http://en.wikipedia.org/wiki/E820) and adds memory regions reserved by the kernel to the `memblock` with the `memblock_add` function. As we met `memblock_add` function first, let's start from it.
This function takes physical base address and size of the memory region and adds it to the `memblock`. `memblock_add` function does not do anything special in its body, but just calls:
@ -160,7 +160,7 @@ This function takes physical base address and size of the memory region and adds
function. We pass memory block type - `memory`, physical base address and size of the memory region, maximum number of nodes which are zero if `CONFIG_NODES_SHIFT` is not set in the configuration file or `CONFIG_NODES_SHIFT` if it is set, and flags. The `memblock_add_range` function adds new memory region to the memory block. It starts by checking the size of the given region and if it is zero it just returns. After this, `memblock_add_range` checks for existence of the memory regions in the `memblock` structure with the given `memblock_type`. If there are no memory regions, we just fill new `memory_region` with the given values and return (we already saw the implementation of this in the [First touch of the linux kernel memory manager framework](http://0xax.gitbooks.io/linux-insides/content/Initialization/linux-initialization-3.html)). If `memblock_type` is not empty, we start to add new memory region to the `memblock` with the given `memblock_type`.
function. We pass memory block type - `memory`, physical base address and size of the memory region, maximum number of nodes which is 1 if `CONFIG_NODES_SHIFT` is not set in the configuration file or `1 << CONFIG_NODES_SHIFT` if it is set, and flags. The `memblock_add_range` function adds new memory region to the memory block. It starts by checking the size of the given region and if it is zero it just returns. After this, `memblock_add_range` checks for existence of the memory regions in the `memblock` structure with the given `memblock_type`. If there are no memory regions, we just fill new `memory_region` with the given values and return (we already saw the implementation of this in the [First touch of the linux kernel memory manager framework](http://0xax.gitbooks.io/linux-insides/content/Initialization/linux-initialization-3.html)). If `memblock_type` is not empty, we start to add new memory region to the `memblock` with the given `memblock_type`.
First of all we get the end of the memory region with the:
@ -235,7 +235,7 @@ and copies memory area with `memmove`:
After this fills `memblock_region` fields of the new memory region base, size and etc... and increase size of the `memblock_type`. In the end of the execution, `memblock_add_range` calls `memblock_merge_regions` which merges neighboring compatible regions in the second step.
After this fills `memblock_region` fields of the new memory region base, size, etc. and increases size of the `memblock_type`. In the end of the execution, `memblock_add_range` calls `memblock_merge_regions` which merges neighboring compatible regions in the second step.
In the second case the new memory region can overlap already stored regions. For example we already have `region1` in the `memblock`:
@ -301,7 +301,7 @@ If none of these conditions are not true, we update the size of the first region
this->size += next->size;
```
As we update the size of the first memory region with the size of the next memory region, we copy every (in the loop) memory region which is after the current (`this`) memory region to the one index ago with the `memmove` function:
As we update the size of the first memory region with the size of the next memory region, we move all memory regions which are after the (`next`) memory region one index backward with the `memmove` function:
```C
memmove(next, next + 1, (type->cnt - (i + 2)) * sizeof(*next));
@ -330,7 +330,7 @@ That's all. This is the whole principle of the work of the `memblock_add_range`
There is also `memblock_reserve` function which does the same as `memblock_add`, but only with one difference. It stores `memblock_type.reserved` in the memblock instead of `memblock_type.memory`.
Of course this is not the full API. Memblock provides an API for not only adding `memory` and `reserved` memory regions, but also:
Of course this is not the full API. Memblock provides APIs for not only adding `memory` and `reserved` memory regions, but also:
* memblock_remove - removes memory region from memblock;
* memblock_find_in_range - finds free area in given range;
zhaoxiaoqiang
8 years ago