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POAllocating copy on write memory within a process
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copied!<p>I have a memory segment which was obtained via <code>mmap</code> with <code>MAP_ANONYMOUS</code>.</p> <p>How can I allocate a second memory segment of the same size which references the first one and make both copy-on write in Linux (Working Linux 2.6.36 at the moment)?</p> <p>I want to have exactly the same effect as <code>fork</code>, just without creating a new process. I want the new mapping to stay in the same process.</p> <p>The whole process has to be repeatable on both the origin and copy pages (as if parent and child would continue to <code>fork</code>).</p> <p>The reason why I don't want to allocate a straight copy of the whole segment is because they are multiple gigabytes large and I don't want to use memory which could be copy-on-write shared.</p> <p>What I have tried:</p> <p><code>mmap</code> the segment shared, anonymous. On duplication <code>mprotect</code> it to read-only and create a second mapping with <code>remap_file_pages</code> also read-only.</p> <p>Then use <code>libsigsegv</code> to intercept write attempts, manually make a copy of the page and then <code>mprotect</code> both to read-write.</p> <p>Does the trick, but is very dirty. I am essentially implementing my own VM.</p> <p>Sadly <code>mmap</code>ing <code>/proc/self/mem</code> is not supported on current Linux, otherwise a <code>MAP_PRIVATE</code> mapping there could do the trick.</p> <p>Copy-on-write mechanics are part of the Linux VM, there has to be a way to make use of them without creating a new process.</p> <p><strong>As a note:</strong> I have found the appropriate mechanics in the Mach VM.</p> <p>The following code compiles on my OS X 10.7.5 and has the expected behaviour: <code>Darwin 11.4.2 Darwin Kernel Version 11.4.2: Thu Aug 23 16:25:48 PDT 2012; root:xnu-1699.32.7~1/RELEASE_X86_64 x86_64 i386</code></p> <p><code>gcc version 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2336.11.00)</code></p> <pre><code>#include <sys/mman.h> #include <stdlib.h> #include <stdio.h> #include <unistd.h> #ifdef __MACH__ #include <mach/mach.h> #endif int main() { mach_port_t this_task = mach_task_self(); struct { size_t rss; size_t vms; void * a1; void * a2; char p1; char p2; } results[3]; size_t length = sysconf(_SC_PAGE_SIZE); vm_address_t first_address; kern_return_t result = vm_allocate(this_task, &first_address, length, VM_FLAGS_ANYWHERE); if ( result != ERR_SUCCESS ) { fprintf(stderr, "Error allocating initial 0x%zu memory.\n", length); return -1; } char * first_address_p = first_address; char * mirror_address_p; *first_address_p = 'a'; struct task_basic_info t_info; mach_msg_type_number_t t_info_count = TASK_BASIC_INFO_COUNT; task_info(this_task, TASK_BASIC_INFO, (task_info_t)&t_info, &t_info_count); task_info(this_task, TASK_BASIC_INFO, (task_info_t)&t_info, &t_info_count); results[0].rss = t_info.resident_size; results[0].vms = t_info.virtual_size; results[0].a1 = first_address_p; results[0].p1 = *first_address_p; vm_address_t mirrorAddress; vm_prot_t cur_prot, max_prot; result = vm_remap(this_task, &mirrorAddress, // mirror target length, // size of mirror 0, // auto alignment 1, // remap anywhere this_task, // same task first_address, // mirror source 1, // Copy &cur_prot, // unused protection struct &max_prot, // unused protection struct VM_INHERIT_COPY); if ( result != ERR_SUCCESS ) { perror("vm_remap"); fprintf(stderr, "Error remapping pages.\n"); return -1; } mirror_address_p = mirrorAddress; task_info(this_task, TASK_BASIC_INFO, (task_info_t)&t_info, &t_info_count); results[1].rss = t_info.resident_size; results[1].vms = t_info.virtual_size; results[1].a1 = first_address_p; results[1].p1 = *first_address_p; results[1].a2 = mirror_address_p; results[1].p2 = *mirror_address_p; *mirror_address_p = 'b'; task_info(this_task, TASK_BASIC_INFO, (task_info_t)&t_info, &t_info_count); results[2].rss = t_info.resident_size; results[2].vms = t_info.virtual_size; results[2].a1 = first_address_p; results[2].p1 = *first_address_p; results[2].a2 = mirror_address_p; results[2].p2 = *mirror_address_p; printf("Allocated one page of memory and wrote to it.\n"); printf("*%p = '%c'\nRSS: %zu\tVMS: %zu\n",results[0].a1, results[0].p1, results[0].rss, results[0].vms); printf("Cloned that page copy-on-write.\n"); printf("*%p = '%c'\n*%p = '%c'\nRSS: %zu\tVMS: %zu\n",results[1].a1, results[1].p1,results[1].a2, results[1].p2, results[1].rss, results[1].vms); printf("Wrote to the new cloned page.\n"); printf("*%p = '%c'\n*%p = '%c'\nRSS: %zu\tVMS: %zu\n",results[2].a1, results[2].p1,results[2].a2, results[2].p2, results[2].rss, results[2].vms); return 0; } </code></pre> <p>I want the same effect in Linux.</p>

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