Linux如何优化部分内存

我们也可以修改RISC-V Linux的内核源码，解除2M对齐的限制。我们只需要在setup_vm()函数中，将原来的 二级页表改为三级页表 ，这样kernel入口地址只需要4K对齐，因此就能将kernel往前挪，从而利用前面的内存。

修改代码

路径：arch/riscv/mm/init.c

注释原来的2M对齐检查：

对kernel的前2M页表映射由二级页表改为三级页表：

//新增一个PTE
pte_t trampoline_pte[PTRS_PER_PTE] __page_aligned_bss;

create_pgd_mapping(trampoline_pg_dir,PAGE_OFFSET,
                   (uintptr_t)trampoline_pmd,PGDIR_SIZE,PAGE_TABLE);
create_pmd_mapping(trampoline_pmd,PAGE_OFFSET,
                   (uintptr_t)trampoline_pte,PMD_SIZE,PAGE_TABLE);

end_va = PAGE_OFFSET + PMD_SIZE;
for (va = PAGE_OFFSET; va < end_va; va += PAGE_SIZE)
{
    create_pte_mapping(trampoline_pte,PAGE_OFFSET,
                   load_pa + (va - PAGE_OFFSET),
                       PAGE_SIZE,PAGE_KERNEL_EXEC);
}

对整个kernel的页表映射由二级页表改为三级页表：

假设kernel大小为4M+

//定义三个PTE
pte_t load_sz_pte[PTRS_PER_PTE] __page_aligned_bss;
pte_t load_sz_pte1[PTRS_PER_PTE] __page_aligned_bss;
pte_t load_sz_pte2[PTRS_PER_PTE] __page_aligned_bss;

//=======0-2M======
create_pgd_mapping(early_pg_dir,PAGE_OFFSET,
                   (uintptr_t)early_pmd,PGDIR_SIZE,PAGE_TABLE);
create_pmd_mapping(early_pmd,PAGE_OFFSET,
                   (uintptr_t)load_sz_pte,PMD_SIZE,PAGE_TABLE);

end_va = PAGE_OFFSET + PMD_SIZE;
for (va = PAGE_OFFSET; va < end_va; va += PAGE_SIZE)
{
    create_pte_mapping(load_sz_pte,PAGE_OFFSET,
                   load_pa + (va - PAGE_OFFSET),
                       PAGE_SIZE,PAGE_KERNEL_EXEC);
}

//=======2-4M==========
create_pgd_mapping(early_pg_dir,PAGE_OFFSET + PMD_SIZE,
                   (uintptr_t)early_pmd,PGDIR_SIZE,PAGE_TABLE);
create_pmd_mapping(early_pmd,PAGE_OFFSET,
                   (uintptr_t)load_sz_pte1,PMD_SIZE,PAGE_TABLE);

end_va = PAGE_OFFSET + (PMD_SIZE * 2);
for (va = PAGE_OFFSET + PMD_SIZE; va < end_va; va += PAGE_SIZE)
{
    create_pte_mapping(load_sz_pte1,va,
                   load_pa + (va - PAGE_OFFSET),
                       PAGE_SIZE,PAGE_KERNEL_EXEC);
}

//=======4-6M==========
create_pgd_mapping(early_pg_dir,PAGE_OFFSET + (PMD_SIZE*2),
                   (uintptr_t)early_pmd,PGDIR_SIZE,PAGE_TABLE);
create_pmd_mapping(early_pmd,PAGE_OFFSET,
                   (uintptr_t)load_sz_pte2,PMD_SIZE,PAGE_TABLE);

end_va = PAGE_OFFSET + (PMD_SIZE * 3);
for (va = PAGE_OFFSET + (PMD_SIZE*2); va < end_va; va += PAGE_SIZE)
{
    create_pte_mapping(load_sz_pte2,va,
                   load_pa + (va - PAGE_OFFSET),
                       PAGE_SIZE,PAGE_KERNEL_EXEC);
}

通过以上的代码修改，就能将Kernel入口地址往前挪1.5M，只给opensbi预留512KB，这样RISC-V Linux启动之后，可用物理内存就会增加。