Menu
I'm confused, does mmap allocate an entire page of memory (regardless of size specified), or does it just allocate the size you request? Really, I'm curious about what happens on subsequent calls to mmap -- would a second call allocate a new page (even if both calls use an amount under the page size) or would it allocate a block adjacent to the previous call?
Same thing for mprotect - does that protect the entire page, or just the part specified?
908
Quoated: In contrast address returned by mmap is only guaranteed to be page-aligned.
The mmap() system call can also be used to allocate memory (an anonymous mapping). A key point here is that the mapped pages are not actually brought into physical memory until they are referenced; thus mmap() can be used to implement lazy loading of pages into memory (demand paging).
mmap works by manipulating your process's page table, a data structure your CPU uses to map address spaces. The CPU will translate "virtual" addresses to "physical" ones, and does so according to the page table set up by your kernel. When you access the mapped memory for the first time, your CPU generates a page fault.
Malloc generally functions in most of the memory management process. In the event the program requires additional memory, this is borrowed from the OS. Mmap on the other hand makes use of a context switch that converts into kernel land.
If the length argument is not a page size multiple it will be rounded up to page size multiple.
As a consequence, the answer to your question is yes mmap() virtually allocates only entire pages.
Regarding mprotect() the man page clearly answer to your question:
mprotect() changes protection for the calling process's memory page(s) containing any part of the address range in the interval [addr, addr+len-1]. addr must be aligned to a page boundary.
22
But that is not because of mmap per se, it is because the kernel can't really do anything different. Memory is organized in pages, and the MMU "thinks" in terms of pages, so there is no way (no sane, reasonable way anyway) to allocate half a page and give the other half to someone else.
How would one e.g. prevent process 2 from stealing confidential data from process 1 if they each have allocated half a page? The memory protection system doesn't work that way, it would be impossible to prevent that from happening.
mmap mandates that length be non-zero, or it will fail. Other than that, it has no requirements on the input parameters (apart from contradicting flags), but of course an implementation is always allowed to have the call fail for other reasons, at its discretion ("implementation" here means for example "Linux").
The effective address of the mapping (which will be returned by a successful call to mmap) is an implementation-defined function of the address hint. Practically, this means rounding the hint down to the previous page (usually 4096 bytes) boundary and rounding the length up to the next page boundary.
Different versions of Linux behave differently on some address ranges, for example prior to version 2.6, hints below mmap_min_addr would fail with EINVAL whereas it now rounds the address up so it is valid.
Source: POSIX
123
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With
