I'm allocating memory using mmap Linux syscall.

        mov     x5, 0                   ; offset is zero
        mov     x4, -1                  ; no file descriptor
        mov     x3, 0x22                ; MAP_PRIVATE   MAP_ANONYMOUS
        mov     x2, 3                   ; PROT_READ   PROT_WRITE
        mov     x1, 4096                ; initial region size in bytes 
        mov     x0, 0                   ; let Linux choose region address
        mov     x8, 222                 ; mmap
        svc     0

Is it possible to increase the size of allocated memory region preserving its start address and contents? How to do it properly?

CodePudding user response：

If there's free virtual address space behind your original region, just create an additional mmap-ed region right behind the original one, using the MAP_FIXED flag and identical permissions. If the page size of both regions are identical, they'll be coalesced into single mapping.

CodePudding user response：

On Linux, use the mremap(2) Linux-specific system call without MREMAP_MAYMOVE to extend the existing mapping, without considering the option of remapping those physical pages to a different virtual address where there's enough room.

It will return an error if some other mapping already exists for the pages you want to grow into. (Unlike mmap(MAP_FIXED) which will silently replace those mappings.)

With just portable POSIX calls, mmap() with a non-NULL hint address = right after you existing mapping, but without MAP_FIXED; it will pick that address if the pages are free (and as @datenwolf says, merge with the earlier mapping into one long extent). Otherwise it will pick somewhere else. (Then you have to munmap that mapping that ended up not where you wanted it.)

But if you're writing in asm, portability is irrelevant; other OSes will have different call numbers and maybe ABIs, so just look up __NR_mremap in asm/unistd.h, and get the flags patterns from sys/mman.h.