Bootmem Handling

File : mm/bootmem.c

Bootmem handling means how memory is allocated at boottime prior to paging start. This initial allocator allocates memory in page granular allocations. The bootmem allocator in Linux handles this by maintaining a bitmap large enough for accommodating all the low-memory pages (>896 MB) in the system. Each bit in the bitmap represents one page and its index in the bitmap represents the page frame number. A value of 0 in any bit in the bitmap indicates the corresponding page is free, and 1 indicates that the page is in use. Later in the startup code, the bootmem allocator bitmap is used to determine which memory pages are in use. The corresponding page structures are marked as reserved. After paging is enabled, the bootmem allocator is not needed. So the allocator bitmap is freed.

The important functions here are

• init_bootmem Initializes the bootmem allocator. Initializes the bootmem bitmap.
• reserve_bootmem Reserves a region of the memory catered by the bootmem allocator. Memory marked as reserved in the E820 map will be successively reserved using this function.
• free_bootmem Frees a region of the memory catered by the bootmem allocator.
• free_all_bootmem Frees all the memory catered by the bootmem allocator.
• alloc_bootmem   hands out contiguous physical pages using a firstfit algorithm. It checks the bitmap starting from the 'prefered' page [see note below], and stops if it finds the required number of contiguous free bits (or if the allocation request cannot be satisfied). It then marks this range of pages as used and returns the starting address of the range.

Note: The 'prefered' and 'goal' variables are used to specify the position from where __alloc_bootmem() should start searching. This is useful if the caller does not want to get DMA-able memory (first 16MB physical memory).

Actually, Bootmem allocation requests need not be in multiples of PAGE_SIZE, any 'size' argument is valid. But the minimum allocation unit is a page (since only 1 bit is maintained per page). So there is a good chance that memory gets fragmented. To avoid this, the bootmem allocator re-uses fragments of the last allocation whenever possible. It keeps track of the unused portion the last allocated page (using last_offset). For the next allocation, it uses this part first.