Advanced Operating Systems
COMP9242 2015/S2 |
UNSW
CRICOS Provider Number: 00098G |
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M3: A pagerUse your memory manager from M2 to write a simple pager that supports on-demand memory mapping of an application and an allocate on demand memory heap. Current Implementation The current implementation simply pre-maps in the pages for
the single binary. Any actual VM page faults will trigger an
assert() which halts the system, under the assumption something
has gone wrong. The executable itself is mapped in with untracked
frames (i.e. we leak them), and finally, the current sos
application A small malloc() intro'malloc' is the standard library function to allocate memory in C. In our system, malloc is provided by the musl libc library. malloc manages memory from a bigger memory pool. In the SOS code you are provided, malloc uses memory from a pre-allocated array in the initial data section as the memory pool. This pool is fixed in size, and malloc fails when the pool is exhausted. See the diagram below for an approximate picture of the memory layout. The code musl uses to allocate memory from the static region for SOS is in apps/sos/src/sys/sys_morecore.c SOS applications use an independent implementation that
uses a pre-allocated memory pool in the application's data section
(as shown in the middle of the diagram). The code musl libc uses in
applications is in One of the tasks of this milestone is to leverage virtual memory to create a dynamically allocated memory region for malloc's use, usually termed the heap, as shown at the bottom of the diagram. The dynamic region is allocated on-demand via VM faults, and can be expanded in range dynamically by requesting that SOS increase the brk point. In this milestone, you will modify the application-level
morecore routines in Malloc and musl libc peculiaritiesThe
The MilestoneIn this milestone you will:
Design alternativesProbably the main thing that you should consider here is the layout of your processes address space. Some things you will want to consider is where you place various parts of memory such as the stack, heap and code segments. You may also have some other regions in your process address space, one of these is the IPC Buffer. You should also think about if you want to make different ranges of the address space have different permissions, eg: you may want to make code read-only to prevent bugs, or have a guard page at the end of your stack to prevent overflow. While not needed for this milestone, you should think about what book keeping is required to delete an address space and free all the resources associated with it. AssessmentThe main demonstration here will be to show a user process running with a high stack pointer (> 0x20000000). You should also demonstrate a user process using malloc() from a heap. #define NPAGES 27 /* called from pt_test */ static void do_pt_test( char *buf ) { int i; /* set */ for(i = 0; i < NPAGES; i ++) buf[i * 4096] = i; /* check */ for(i = 0; i < NPAGES; i ++) assert(buf[i * 4096] == i); } static void pt_test( void ) { /* need a decent sized stack */ char buf1[NPAGES * 4096], *buf2 = NULL; /* check the stack is above phys mem */ assert((void *) buf1 > (void *) 0x20000000); /* stack test */ do_pt_test(buf1); /* heap test */ buf2 = malloc(NPAGES * 4096); assert(buf2); do_pt_test(buf2); free(buf2); } You should also be able to explain to the tutor how your code works and any design decisions you took. Last modified: 28 Jul 2015. |