Using mremap() to merge two identical pages into one physical page

Question

I have a C code where I know that the content of the page pointed to by void *p1 is the same as the content pointed to by page void *p2. p1 and p2 were dynamically allocated. My question is can I use remap() to let these two pages point to the same physical page instead of having two identical physical pages?

Edit: I am trying to change the virtual to physical mapping in the page table of this process so that p1 and p2 point to the same physical address. I do not want to make p1 and p2 to point to the same thing virtually.

Answer 1

If you are trying to map multiple virtual memory addresses to a single physical address, using the linux page scheme, that isn't what mremap() is for. mremap is for moving (remapping) an existing region, and if you use it to map to a specific newaddress, any old mappings to that address become invalid (per the man page). http://man7.org/linux/man-pages/man2/mremap.2.html

See the emphasized section...

MREMAP_FIXED (since Linux 2.3.31) This flag serves a similar purpose to the MAP_FIXED flag of mmap(2). If this flag is specified, then mremap() accepts a fifth argument, void *new_address, which specifies a page- aligned address to which the mapping must be moved. Any previous mapping at the address range specified by new_address and new_size is unmapped. If MREMAP_FIXED is specified, then MREMAP_MAYMOVE must also be specified.

If you are simply trying to merge the storage of 2 identical data structures, you would't need mremap() to point 2 "pages" to the same identical page, you'd need to point the 2 different data structure pointers to the same page and free the redundant page.

If the content is the same, you'd need to convert any pointers that are pointing to p2 to addresses into p1.

Even using mremap properly requires you to take care of your own pointer housekeeping, it doesn't magically do that for you; if you fail to do that, after the remap you may have dangling pointers.

PS: Its been years since I did kernel programming, so I might be wrong or out of date in my next statement, but I think you will need to use kernel calls (ie. kernel module / driver level calls) to get to the physical mappings because mmap() and mremap() are user-land calls and work within the virtual address space. The "page mapping" is done at the kernel level, outside of user space.