apio
e5cf32c7b3
Some pages, such as framebuffer pages, are not physical memory frames reserved for the current process. Some, such as the framebuffer, may be shared between all processes. Yet, on exit() or on exec(), a process frees all frames mapped into its address spaces. And on fork(), it copies all data between frames. So how could we map framebuffers. Simple: we use one of the bits in page table entries which are available to the OS, and mark whether that page is owned by the current process. If it is owned, it will be: - Freed on address space destruction - Its data will be copied to a new page owned by the child process on fork() If it is not owned, it will be: - Left alone on address space destruction - On fork(), the child's virtual page will be mapped to the same physical frame as the parent This still needs a bit more work, such as keeping a reference of how many processes use a page to free it when all processes using it exit/exec. This should be done for MAP_SHARED mappings, for example, since they are not permanent forever, unlike the framebuffer for example.
34 lines
744 B
C
34 lines
744 B
C
#pragma once
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#include <stdint.h>
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#ifndef PAGE_SIZE
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#define PAGE_SIZE 4096
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#endif
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struct PageDirectoryEntry
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{
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bool present : 1;
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bool read_write : 1;
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bool user : 1;
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bool write_through : 1;
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bool cache_disabled : 1;
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bool accessed : 1;
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bool ignore0 : 1;
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bool larger_pages : 1;
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bool ignore1 : 1;
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uint8_t available : 3;
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uint64_t address : 48;
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bool owned_by_task : 1; // Part of the available for OS use bits.
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uint8_t available2 : 2;
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bool no_execute : 1;
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void set_address(uint64_t addr);
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uint64_t get_address();
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} __attribute__((packed));
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struct PageTable
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{
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PageDirectoryEntry entries[512];
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} __attribute__((aligned(PAGE_SIZE)));
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static_assert(sizeof(PageDirectoryEntry) == 8UL); |