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Only COW on fault if the physical page has retain_count > 1.

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This makes COW pages lazily-but-transparently revert back to read/write.
This commit is contained in:
Andreas Kling 2018-11-05 14:10:18 +01:00
parent 2d045d2a64
commit 9e62eb4856
Notes: sideshowbarker 2024-07-19 18:33:36 +09:00 
Author: https://github.com/awesomekling
Commit: https://github.com/SerenityOS/serenity/commit/9e62eb48566
4 changed files with 17 additions and 2 deletions
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10
Kernel/MemoryManager.cpp
View File

@ -223,6 +223,16 @@ Region* MemoryManager::region_from_laddr(Process& process, LinearAddress laddr)
bool MemoryManager::copy_on_write(Process& process, Region& region, unsigned page_index_in_region)
{
ASSERT_INTERRUPTS_DISABLED();
if (region.physical_pages[page_index_in_region]->retain_count() == 1) {
#ifdef PAGE_FAULT_DEBUG
dbgprintf(" >> It's a COW page but nobody is sharing it anymore. Remap r/w\n");
#endif
region.cow_map.set(page_index_in_region, false);
remap_region_page(process.m_page_directory, region, page_index_in_region, true);
return true;
}
#ifdef PAGE_FAULT_DEBUG
dbgprintf(" >> It's a COW page and it's time to COW!\n");
#endif

2
Kernel/MemoryManager.h
View File

@ -37,6 +37,8 @@ public:
return_to_freelist();
}
unsigned retain_count() const { return m_retain_count; }
private:
PhysicalPage(PhysicalAddress paddr)
: m_paddr(paddr)

5
Kernel/ProcFileSystem.cpp
View File

@ -61,9 +61,10 @@ ByteBuffer procfs$pid_vm(Process& process)
region->name.characters());
for (size_t i = 0; i < region->physical_pages.size(); ++i) {
auto& physical_page = region->physical_pages[i];
ptr += ksprintf(ptr, "P%x%s ",
ptr += ksprintf(ptr, "P%x%s(%u) ",
physical_page ? physical_page->paddr().get() : 0,
region->cow_map.get(i) ? "!" : ""
region->cow_map.get(i) ? "!" : "",
physical_page->retain_count()
);
}
ptr += ksprintf(ptr, "\n");

2
Kernel/i386.cpp
View File

@ -8,6 +8,8 @@
#include "IRQHandler.h"
#include "PIC.h"
//#define PAGE_FAULT_DEBUG
struct DescriptorTablePointer {
WORD size;
void* address;