CODEWITHSUNDEEP
memory-managementlinux-kernelmmap
arkadish
arkadish

Reputation: 355

Linux page allocation

In linux if malloc can't allocate data chunk from preallocated page. it uses mmap() or brk() to assign new pages. I wanted to clarify a few things :

  1. I don't understand the following statment , I thought that when I use mmap() or brk() the kernel maps me a whole page. but the allocator alocates me only what I asked for from that new page? In this case trying to access unallocated space (In the newly mapped page) will not cause a page fault? (I understand that its not recommended )

The libc allocator manages each page: slicing them into smaller blocks, assigning them to processes, freeing them, and so on. For example, if your program uses 4097 bytes total, you need to use two pages, even though in reality the allocator gives you somewhere between 4105 to 4109 bytes

  1. How does the allocator knows the VMA bounders ?(I assume no system call used) because the VMA struct that hold that information can only get accessed from kernel mode?

Upvotes: 0

Views: 1724

Answers (1)

P.T.
P.T.

Reputation: 25177

The system-level memory allocation (via mmap and brk) is all page-aligned and page-sized. This means if you use malloc (or other libc API that allocates memory) some small amount of memory (say 10 bytes), you are guaranteed that all the other bytes on that page of memory are readable without triggering a page fault.

Malloc and family do their own bookkeeping within the pages returned from the OS, so the mmap'd pages used by libc also contain a bunch of malloc metadata, in addition to whatever space you've allocated.

The libc allocator knows where everything is because it invokes the brk() and mmap() calls. If it invokes mmap() it passes in a size, and the kernel returns a start address. Then the libc allocator just stores those values in its metadata.

Doug Lea's malloc implementation is a very, very well documented memory allocator implementation and its comments will shed a lot of light on how allocators work in general: http://g.oswego.edu/dl/html/malloc.html

Upvotes: 2

Related Questions