Pages are referenced by multiple page tables. Local: Replacement only considers pages owned by the faulting In the most difficult case, the page is in a memory region for that no level 3 exists yet so that we need to create new level 3, level 2 and level 1 page tables first. Then we follow another entry and reach a level 3 table, which the CPU treats as a mapped frame. Let's change that by creating a new mapping for a previously unmapped page. Our goal is to map a previously unmapped virtual page to this frame using our create_example_mapping function. Let's perform the same change in our Since the entry point is only used in test mode, we add the #[cfg(test)] attribute to all items. So while the next entry points at a level 2 table, the CPU thinks that it points to the mapped frame, which allows us to read and write the level 2 table. For devices with very small amounts of physical memory, we could map the page tables frames only temporarily when we need to access them. For accessing the level 3 table, we follow the recursive entry three times, tricking the CPU into thinking it is already on a level 1 table. on the screen is by our write to page 0, which means that we successfully created a new mapping in the page tables. } higher-level language. Before returning that frame, we increase by one so that we return the following frame on the next call. OS Involvement: What the OS has to take care of to make VM work. The next field is initialized with 0 and will be increased for every frame allocation to avoid returning the same frame twice. The bootloader does this because paging is mandatory in 64-bit mode on x86_64. Like Result, the type uses the #[must_use] attribute to emit a warning when we accidentally forget to use it. For this case, a frame allocator that always returns None suffices. An assembly-language trap handler saves the general registers, }. The create_example_mapping function looks like this: In addition to the page that should be mapped, the function expects a mutable reference to an OffsetPageTable instance and a frame_allocator. structures::paging::PageTable, This means that we can now read and write the level 1 page table because the CPU thinks that it is the mapped frame. Local requires a policy to modify the number of pages a process has. For example, the offset could be 10 TiB: By using the virtual memory in the range 10TiB..(10TiB + physical memory size) exclusively for page table mappings, we avoid the collision problems of the identity mapping. When the page is clean (after write-back or immediately), It first explores different techniques to make the physical page table frames accessible to the kernel and discusses their respective advantages and drawbacks. Global: The replacement algorithm considers all pages in memory. hlt_loop(); Set the entry back to unused thereby removing the temporary mapping again. We then use the iter function to iterate over the page table entries and the enumerate combinator to additionally add an index i to each element. For our implementation, we first manually traversed the page tables to implement a translation function, and then used the MappedPageTable type of the x86_64 crate. virt = physical_memory_offset + frame.start_address().as_u64(); // read the page table entry and update `frame`, frame, Pages on different system can map to the same data. Now we can't use any frame with a physical address between 1000 KiB and 2008 KiB anymore, because we can't identity map it. Also, it does not allow accessing page tables of other address spaces, which would be useful when creating a new process. […], // new: use the `mapper.translate_addr` method, phys = mapper.translate_addr(virt); The TLB will be flushed. But only for the pages no other process is using. The program resumes with a private, writable copy of the page. One is meant for VVIPs, second is for VIPs and the third one is for ordinary people. For s = 1MB and e = 8 bytes, optimum page size is 4KB. Before we implement the FrameAllocator trait, we add an auxiliary method that converts the memory map into an iterator of usable frames: This function uses iterator combinator methods to transform the initial MemoryMap into an iterator of usable physical frames: The return type of the function uses the impl Trait feature. /// This function is unsafe because the caller must guarantee that the The reason for this is that mapping the same frame twice could result in undefined behavior, for example when two different &mut references point to the same physical memory location. Role maintenance paging implementation Implementation of paging front end function. I'm a Rust freelancer with a master's degree in computer science. This works because tables of all levels have the exact same layout on x86_64. All of these approaches require page table modifications for their setup. removed when one process exits. We do this because it simplifies this prototype implementation. // in src/ { In page role- page.jsp introduce role.js file Initializing global functions /// memory map is valid. /// as `USABLE` in it are really unused. Implementation. page is the corresponding portion of the file. We now can use the MapperAllSizes::translate_addr method instead of our own memory::translate_addr function. Since our _start function is called externally from the bootloader, no checking of our function signature occurs. When the CPU now follows a different entry, it lands on a level 3 table but thinks it is already on a level 1 table. The different index into the level 2 table means that a different level 1 table is used for this page. particular register, or possibly copying the whole table. This avoids the need to run a translation for these addresses too, which would be bad for performance and could easily cause endless translation loops. The reserved virtual memory range has the same size as before, with the difference that it no longer contains unmapped pages. Thus, we can choose any unused page in this memory region for our example mapping, such as the page at address 0. But in reality, it is still on the level 4 table. To avoid the problem of cluttering the virtual address space, we can use a separate memory region for page table mappings. Map that entry to the physical frame of the page table that we want to access. Can effectively perform I/O by memory reference. At the end of the previous post, we tried to take a look at the page tables our kernel runs on, but failed since we couldn't access the physical frame that the CR3 register points to. The private inner function contains the real implementation: Instead of reusing our active_level_4_table function, we read the level 4 frame from the CR3 register again. It can only be queried very early in the boot process, so the bootloader already calls the respective functions for us. Then it follows the recursive entry again and thinks that it reaches a level 2 table. mapper: x86_64::structures::paging::PageTableFlags, frame = PhysFrame::containing_address(PhysAddr::new(, // FIXME: this is not safe, we do it only for testing, mapper.map_to(page, frame, flags, frame_allocator) Since each physical address can be accessed by adding the physical_memory_offset, the translation of the physical_memory_offset address itself should point to physical address 0.