On Thu, Jun 08, 2023 at 01:49:28PM +0200, Linus Walleij wrote: > diff --git a/Documentation/mm/page_tables.rst b/Documentation/mm/page_tables.rst > index 96939571d7bc..315d295d1740 100644 > --- a/Documentation/mm/page_tables.rst > +++ b/Documentation/mm/page_tables.rst > @@ -3,3 +3,134 @@ > =========== > 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 program counter 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 target architecture code for each supported architecture will then > +map this to the restrictions of the target 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 0x00004000, pfn 2 is at 0x00008000 > +and so on until we reach pfn 0x3ffff at 0xffffc000. > + > +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 was referred to as *PTE*:s - page table entries. > + > +Over time the page table hierarchy has 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. LGTM, thanks! Reviewed-by: Bagas Sanjaya <bagasdotme@xxxxxxxxx> -- An old man doll... just what I always wanted! - Clara
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