kmalloc() functionality in linux kernel

Question

I did come across through the LDD book that using the kmalloc we can allocate from high memory. I have one basic question here. 1)But to my knowledge we can't access the high memory directly from the kernel (unless it is mapped to the kernel space through the kmap()). And i didn't see any mapping area reserved for kmalloc(), But for vmalloc() it is present.So, to which part of the kernel address does the kmalloc() will map if allocated from high memory?

This is on x86 architecture,32bit system.

Answer 1

My knowledge may be out of date but the stack is something like this:

kmalloc allocates physically contiguous memory by calling get_free_pages (this is what the acronym GFP stands for). The GFP_* flags passed to kmalloc end up in get_free_pages, which is the page allocator.

Since special handling is required for highmem pages, you won't get them unless you add the GFP_HIGHMEM flag to the request.

All memory in Linux is virtual (a generalization that is not exactly true and that is architecture-dependent, but let's go with it, until the next parenthesized statement in this paragraph). There is a range of memory, however, that is not subject to the virtualization in the sense of remapping of pages: it is just a linear mapping between virtual addresses and physical addresses. The memory allocated by get_free_pages is linearly mapped, except for high memory. (On some architectures, linear mappings are supported for memory ranges without the use of a MMU: it's just a simple arithmetic translation of logical addresses to physical: add a displacement. On other architectures, linear mappings are done with the MMU.)

Anyway, if you call get_free_pages (directly, or via kmalloc) to allocate two or more pages, it has to find physically contiguous ones.

Now virtual memory is also implemented on top of get_free_pages, because we can take a page allocated that way, and install it into a virtual address space.

This is how mmap works and everything else for user space. When a piece of virtual memory is committed (becomes backed by a physical page, on a page fault or whatever), a page comes from get_free_pages. Unless that page is highmem, it has a linear mapping so that it is visible in the kernel. Additionally, it is wired into the virtual address space for which the request is being made. Some kernel data structures keep track of this, and of course it is punched into the page tables so the MMU makes it happen.

vmalloc is similar in principle to mmap, but far simpler because it doesn't deal with multiple backends (devices in the filesystem with a mmap virtual function) and doesn't deal with issues like coalescing and splitting of mappings that mmap allows. The vmalloc area consists of a reserved range of virtual addresses visible only to the kernel (whose base address and is architecture dependent and can be tweaked by you at kernel compile time). The vmalloc allocator carves out this virtual space, and populates it with pages from get_free_pages. These need not be contiguous, and so can be obtained one at a time, and wired into the allocated virtual space.

Highmem pages are physical memory that is not addressable in the kernel's linear map representing physical memory. Highmem exists because the kernel's linear "window" on physical memory isn't always large enough to cover all of memory. (E.g. suppose you have a 1GB window, but 4GB of RAM.) So, for coverage of all of memory, there is in addition to the linear map, some smaller "non linear" map where pages they are selectively made visible, on a temporary basis using kmap and kunmap. Placement of a page into this view is considered the acquisition of a precious resource that must be used sparingly and released as soon as possible.

A highmem page can be installed into a virtual memory map just like any other page, and no special "highmem" handling is needed for that view of the page. Any map: that of a process, or the vmalloc range.

If you're dealing with some virtual memory that could be a mixture of highmem and non-highmem pages, which you have to view through the kernel's linear space, you have to be prepared to use the mapping functions.