Merge tag 'docs-6.5' of git://git.lwn.net/linux

Pull documentation updates from Jonathan Corbet:
 "It's been a relatively calm cycle in docsland. We do have:

   - Some initial page-table documentation from Linus (the other Linus)

   - Regression-handling documentation improvements from Thorsten

   - Addition of kerneldoc documentation for the ERR_PTR() and related
     macros from James Seo

  ... and the usual collection of fixes and updates"

* tag 'docs-6.5' of git://git.lwn.net/linux:
  docs: consolidate storage interfaces
  Documentation: update git configuration for Link: tag
  Documentation: KVM: make corrections to vcpu-requests.rst
  Documentation: KVM: make corrections to ppc-pv.rst
  Documentation: KVM: make corrections to locking.rst
  Documentation: KVM: make corrections to halt-polling.rst
  Documentation: virt: correct location of haltpoll module params
  Documentation/mm: Initial page table documentation
  docs: crypto: async-tx-api: fix typo in struct name
  docs/doc-guide: Clarify how to write tables
  docs: handling-regressions: rework section about fixing procedures
  docs: process: fix a typoed cross-reference
  docs: submitting-patches: Discuss interleaved replies
  MAINTAINERS: direct process doc changes to a dedicated ML
  Documentation: core-api: Add error pointer functions to kernel-api
  err.h: Add missing kerneldocs for error pointer functions
  Documentation: conf.py: Add __force to c_id_attributes
  docs: clarify KVM related kernel parameters' descriptions
  docs: consolidate human interface subsystems
  docs: admin-guide: Add information about intel_pstate active mode

1 files changed, 149 insertions, 0 deletions

diff --git a/Documentation/mm/page_tables.rst b/Documentation/mm/page_tables.rst
index 96939571d7bc1..7840c1891751e 100644
--- a/Documentation/mm/page_tables.rst
+++ b/Documentation/mm/page_tables.rst
@@ -3,3 +3,152 @@
 ===========
 Page Tables
 ===========
+
+Paged virtual memory was invented along with virtual memory as a concept in
+1962 on the Ferranti Atlas Computer which was the first computer with paged
+virtual memory. The feature migrated to newer computers and became a de facto
+feature of all Unix-like systems as time went by. In 1985 the feature was
+included in the Intel 80386, which was the CPU Linux 1.0 was developed on.
+
+Page tables map virtual addresses as seen by the CPU into physical addresses
+as seen on the external memory bus.
+
+Linux defines page tables as a hierarchy which is currently five levels in
+height. The architecture code for each supported architecture will then
+map this to the restrictions of the hardware.
+
+The physical address corresponding to the virtual address is often referenced
+by the underlying physical page frame. The **page frame number** or **pfn**
+is the physical address of the page (as seen on the external memory bus)
+divided by `PAGE_SIZE`.
+
+Physical memory address 0 will be *pfn 0* and the highest pfn will be
+the last page of physical memory the external address bus of the CPU can
+address.
+
+With a page granularity of 4KB and a address range of 32 bits, pfn 0 is at
+address 0x00000000, pfn 1 is at address 0x00001000, pfn 2 is at 0x00002000
+and so on until we reach pfn 0xfffff at 0xfffff000. With 16KB pages pfs are
+at 0x00004000, 0x00008000 ... 0xffffc000 and pfn goes from 0 to 0x3fffff.
+
+As you can see, with 4KB pages the page base address uses bits 12-31 of the
+address, and this is why `PAGE_SHIFT` in this case is defined as 12 and
+`PAGE_SIZE` is usually defined in terms of the page shift as `(1 << PAGE_SHIFT)`
+
+Over time a deeper hierarchy has been developed in response to increasing memory
+sizes. When Linux was created, 4KB pages and a single page table called
+`swapper_pg_dir` with 1024 entries was used, covering 4MB which coincided with
+the fact that Torvald's first computer had 4MB of physical memory. Entries in
+this single table were referred to as *PTE*:s - page table entries.
+
+The software page table hierarchy reflects the fact that page table hardware has
+become hierarchical and that in turn is done to save page table memory and
+speed up mapping.
+
+One could of course imagine a single, linear page table with enormous amounts
+of entries, breaking down the whole memory into single pages. Such a page table
+would be very sparse, because large portions of the virtual memory usually
+remains unused. By using hierarchical page tables large holes in the virtual
+address space does not waste valuable page table memory, because it will suffice
+to mark large areas as unmapped at a higher level in the page table hierarchy.
+
+Additionally, on modern CPUs, a higher level page table entry can point directly
+to a physical memory range, which allows mapping a contiguous range of several
+megabytes or even gigabytes in a single high-level page table entry, taking
+shortcuts in mapping virtual memory to physical memory: there is no need to
+traverse deeper in the hierarchy when you find a large mapped range like this.
+
+The page table hierarchy has now developed into this::
+
+  +-----+
+  | PGD |
+  +-----+
+     |
+     |   +-----+
+     +-->| P4D |
+         +-----+
+            |
+            |   +-----+
+            +-->| PUD |
+                +-----+
+                   |
+                   |   +-----+
+                   +-->| PMD |
+                       +-----+
+                          |
+                          |   +-----+
+                          +-->| PTE |
+                              +-----+
+
+
+Symbols on the different levels of the page table hierarchy have the following
+meaning beginning from the bottom:
+
+- **pte**, `pte_t`, `pteval_t` = **Page Table Entry** - mentioned earlier.
+  The *pte* is an array of `PTRS_PER_PTE` elements of the `pteval_t` type, each
+  mapping a single page of virtual memory to a single page of physical memory.
+  The architecture defines the size and contents of `pteval_t`.
+
+  A typical example is that the `pteval_t` is a 32- or 64-bit value with the
+  upper bits being a **pfn** (page frame number), and the lower bits being some
+  architecture-specific bits such as memory protection.
+
+  The **entry** part of the name is a bit confusing because while in Linux 1.0
+  this did refer to a single page table entry in the single top level page
+  table, it was retrofitted to be an array of mapping elements when two-level
+  page tables were first introduced, so the *pte* is the lowermost page
+  *table*, not a page table *entry*.
+
+- **pmd**, `pmd_t`, `pmdval_t` = **Page Middle Directory**, the hierarchy right
+  above the *pte*, with `PTRS_PER_PMD` references to the *pte*:s.
+
+- **pud**, `pud_t`, `pudval_t` = **Page Upper Directory** was introduced after
+  the other levels to handle 4-level page tables. It is potentially unused,
+  or *folded* as we will discuss later.
+
+- **p4d**, `p4d_t`, `p4dval_t` = **Page Level 4 Directory** was introduced to
+  handle 5-level page tables after the *pud* was introduced. Now it was clear
+  that we needed to replace *pgd*, *pmd*, *pud* etc with a figure indicating the
+  directory level and that we cannot go on with ad hoc names any more. This
+  is only used on systems which actually have 5 levels of page tables, otherwise
+  it is folded.
+
+- **pgd**, `pgd_t`, `pgdval_t` = **Page Global Directory** - the Linux kernel
+  main page table handling the PGD for the kernel memory is still found in
+  `swapper_pg_dir`, but each userspace process in the system also has its own
+  memory context and thus its own *pgd*, found in `struct mm_struct` which
+  in turn is referenced to in each `struct task_struct`. So tasks have memory
+  context in the form of a `struct mm_struct` and this in turn has a
+  `struct pgt_t *pgd` pointer to the corresponding page global directory.
+
+To repeat: each level in the page table hierarchy is a *array of pointers*, so
+the **pgd** contains `PTRS_PER_PGD` pointers to the next level below, **p4d**
+contains `PTRS_PER_P4D` pointers to **pud** items and so on. The number of
+pointers on each level is architecture-defined.::
+
+        PMD
+  --> +-----+           PTE
+      | ptr |-------> +-----+
+      | ptr |-        | ptr |-------> PAGE
+      | ptr | \       | ptr |
+      | ptr |  \        ...
+      | ... |   \
+      | ptr |    \         PTE
+      +-----+     +----> +-----+
+                         | ptr |-------> PAGE
+                         | ptr |
+                           ...
+
+
+Page Table Folding
+==================
+
+If the architecture does not use all the page table levels, they can be *folded*
+which means skipped, and all operations performed on page tables will be
+compile-time augmented to just skip a level when accessing the next lower
+level.
+
+Page table handling code that wishes to be architecture-neutral, such as the
+virtual memory manager, will need to be written so that it traverses all of the
+currently five levels. This style should also be preferred for
+architecture-specific code, so as to be robust to future changes.