I’m trying to get my head around the two files mentioned in the title.
I’ve looked up what the bits are; however, I’m failing to understand how to extract useful info from them (or I’m simply approaching it the wrong way).
Let me explain: The pagemaps is a rather newer “feature” pseudo file that contains the physical frame information of virtual pages assigned to a current [pid]. That is, given a virtual page that starts at address x, say ‘vas’ for virtual address start, i can index the pagemap file using vas to get the 64bits of the mapped physical page frame. These bits contain info about that virtual page.
However, when I extract the bits and do a bit of shifting I’m getting lost with what I’m seeing.
The bits are represented as follows: 0-54 is the page frame number, 55-60 is the page shift, 63rd bit is the present bit, there are other bits of little interest to me.
After I do a bit of mapping using vas addresses from /proc/[pid]/maps, it seems that just about every process’ page is swapped, i.e. the 63rd bit is always a zero. 🙁
I guess the question would be, how should I go about effectively using pagemaps to get the equivalent physical address of the address given by /proc/[pid]/maps
To be fair, I’ve posted a similar question but the approach was a bit different a few days earlier.
If anyone can shed some light on this matter I would be greatly appreciative.
===EDIT===
To address the comment below:
I’m reading a line from /proc/[pid]/maps and the lines look like:
00400000-00401000 r-xp 00000000 08:01 8915461 /home/janjust/my_programs/shared_mem
7ffffef1b000-7ffffef3c000 rw-p 00000000 00:00 0 [stack]
Then I’m extracting the number of virtual pages it touches and indexing a binary file /proc/[pid]/pagemaps , and for each virtual page I can extract the physical page it is assigned to.
The output looks like:
00400000-00401000 r-xp 00000000 08:01 8915461 /home/janjust/my_programs/shared_mem
num_pages: 1
: 86000000001464C6
One physical address for each virtual page in the virtual range.
The code for reading the line and extracting the physical address is:
74 /* process /proc/pid/maps, by line*/
75 while(fgets(line, 256, in_map) != NULL){
76 unsigned long vas;
77 unsigned long vae;
78 int num_pages;
79
80 //print line
81 printf("%s", line);
82
83 /*scan for the virtual addresses*/
84 n = sscanf(line, "%lX-%lX", &vas, &vae);
85 if(n != 2){
86 printf("Involid line read from %s\n",maps);
87 continue;
88 }
89
90 num_pages = (vae - vas) / PAGE_SIZE;
91 printf("num_pages: %d\n", num_pages);
92
93 if(num_pages > 0){
94 long index = (vas / PAGE_SIZE) * sizeof(unsigned long long);
95 off64_t o;
96 ssize_t t;
97
98 /* seek to index in pagemaps */
99 o = lseek64(pm, index, SEEK_SET);
100 if (o != index){
101 printf("Error seeking to o:%ld, index:%ld.\n", o, index);
102 }
103
104 /* map the virtual to physical page */
105 while(num_pages > 0){
106 unsigned long long pa;
107
108 /* Read a 64-bit word from each pagemap file... */
109 t = read(pm, &pa, sizeof(unsigned long long));
110 if(t < 0){
111 printf("Error reading file \"%s\" \n", page_map);
112 goto next_line;
113 }
114 printf(": %016llX\n", pa);
However, although I think I’m getting the right output, the index seems to be either a type mismatch or something else is going on:
The output say for instance for the [shared mem] line in maps gives a wrong index; yet I’m still able to scan through the binary file and get the physical page address.
The example of that output is below:
969 7f7f08d58000-7f7f08d59000 rw-s 00000000 00:04 0 /SYSV00003039 (deleted)
970 num_pages: 1
971 Error seeking to o:-1081840960, index:273796065984.
972 : 8600000000148267
Ok, now, lastly I should say that this is under a 64bit OS, and this problem doesn’t persist in a 32bit OS.
Oooh K, the index was correct but comparing off64_t o (8bytes) with long index was interpreting o wrong hence why I was getting that error.
Ha! this was a stupid mistake.
So adding the appropriate header took care of that.
Missing header :-/ sigh fixes the issue of comparing off64_t with a unsigned long.