Signed-off-by: Mike Rapoport <rppt@xxxxxxxxxxxxxxxxxx> --- Documentation/vm/highmem.txt | 87 ++++++++++++++++++-------------------------- 1 file changed, 36 insertions(+), 51 deletions(-) diff --git a/Documentation/vm/highmem.txt b/Documentation/vm/highmem.txt index 4324d24..0f69a9f 100644 --- a/Documentation/vm/highmem.txt +++ b/Documentation/vm/highmem.txt @@ -1,25 +1,14 @@ +.. _highmem: - ==================== - HIGH MEMORY HANDLING - ==================== +==================== +High Memory Handling +==================== By: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx> -Contents: - - (*) What is high memory? - - (*) Temporary virtual mappings. - - (*) Using kmap_atomic. - - (*) Cost of temporary mappings. - - (*) i386 PAE. +.. contents:: :local: - -==================== -WHAT IS HIGH MEMORY? +What Is High Memory? ==================== High memory (highmem) is used when the size of physical memory approaches or @@ -38,7 +27,7 @@ kernel entry/exit. This means the available virtual memory space (4GiB on i386) has to be divided between user and kernel space. The traditional split for architectures using this approach is 3:1, 3GiB for -userspace and the top 1GiB for kernel space: +userspace and the top 1GiB for kernel space:: +--------+ 0xffffffff | Kernel | @@ -58,40 +47,38 @@ and user maps. Some hardware (like some ARMs), however, have limited virtual space when they use mm context tags. -========================== -TEMPORARY VIRTUAL MAPPINGS +Temporary Virtual Mappings ========================== The kernel contains several ways of creating temporary mappings: - (*) vmap(). This can be used to make a long duration mapping of multiple - physical pages into a contiguous virtual space. It needs global - synchronization to unmap. +* vmap(). This can be used to make a long duration mapping of multiple + physical pages into a contiguous virtual space. It needs global + synchronization to unmap. - (*) kmap(). This permits a short duration mapping of a single page. It needs - global synchronization, but is amortized somewhat. It is also prone to - deadlocks when using in a nested fashion, and so it is not recommended for - new code. +* kmap(). This permits a short duration mapping of a single page. It needs + global synchronization, but is amortized somewhat. It is also prone to + deadlocks when using in a nested fashion, and so it is not recommended for + new code. - (*) kmap_atomic(). This permits a very short duration mapping of a single - page. Since the mapping is restricted to the CPU that issued it, it - performs well, but the issuing task is therefore required to stay on that - CPU until it has finished, lest some other task displace its mappings. +* kmap_atomic(). This permits a very short duration mapping of a single + page. Since the mapping is restricted to the CPU that issued it, it + performs well, but the issuing task is therefore required to stay on that + CPU until it has finished, lest some other task displace its mappings. - kmap_atomic() may also be used by interrupt contexts, since it is does not - sleep and the caller may not sleep until after kunmap_atomic() is called. + kmap_atomic() may also be used by interrupt contexts, since it is does not + sleep and the caller may not sleep until after kunmap_atomic() is called. - It may be assumed that k[un]map_atomic() won't fail. + It may be assumed that k[un]map_atomic() won't fail. -================= -USING KMAP_ATOMIC +Using kmap_atomic ================= When and where to use kmap_atomic() is straightforward. It is used when code wants to access the contents of a page that might be allocated from high memory (see __GFP_HIGHMEM), for example a page in the pagecache. The API has two -functions, and they can be used in a manner similar to the following: +functions, and they can be used in a manner similar to the following:: /* Find the page of interest. */ struct page *page = find_get_page(mapping, offset); @@ -109,7 +96,7 @@ Note that the kunmap_atomic() call takes the result of the kmap_atomic() call not the argument. If you need to map two pages because you want to copy from one page to -another you need to keep the kmap_atomic calls strictly nested, like: +another you need to keep the kmap_atomic calls strictly nested, like:: vaddr1 = kmap_atomic(page1); vaddr2 = kmap_atomic(page2); @@ -120,8 +107,7 @@ another you need to keep the kmap_atomic calls strictly nested, like: kunmap_atomic(vaddr1); -========================== -COST OF TEMPORARY MAPPINGS +Cost of Temporary Mappings ========================== The cost of creating temporary mappings can be quite high. The arch has to @@ -136,25 +122,24 @@ If CONFIG_MMU is not set, then there can be no temporary mappings and no highmem. In such a case, the arithmetic approach will also be used. -======== i386 PAE ======== The i386 arch, under some circumstances, will permit you to stick up to 64GiB of RAM into your 32-bit machine. This has a number of consequences: - (*) Linux needs a page-frame structure for each page in the system and the - pageframes need to live in the permanent mapping, which means: +* Linux needs a page-frame structure for each page in the system and the + pageframes need to live in the permanent mapping, which means: - (*) you can have 896M/sizeof(struct page) page-frames at most; with struct - page being 32-bytes that would end up being something in the order of 112G - worth of pages; the kernel, however, needs to store more than just - page-frames in that memory... +* you can have 896M/sizeof(struct page) page-frames at most; with struct + page being 32-bytes that would end up being something in the order of 112G + worth of pages; the kernel, however, needs to store more than just + page-frames in that memory... - (*) PAE makes your page tables larger - which slows the system down as more - data has to be accessed to traverse in TLB fills and the like. One - advantage is that PAE has more PTE bits and can provide advanced features - like NX and PAT. +* PAE makes your page tables larger - which slows the system down as more + data has to be accessed to traverse in TLB fills and the like. One + advantage is that PAE has more PTE bits and can provide advanced features + like NX and PAT. The general recommendation is that you don't use more than 8GiB on a 32-bit machine - although more might work for you and your workload, you're pretty -- 2.7.4
