Paging

Page faults

Special kind of exception, IV 14.

Types:

• invalid: access to invalid address (e.g. write to RO page)
• major (slower): valid, but page not in memory (e.g. file, swapped pages)
• minor (faster): valid, page in memory

Kernel page fault:

• invalid:
  • on kernel address: oops (panic/recovery)
  • on user address: checks (copy to/from user)
• major: kernel memory is paged out (not on Linux, e.g. in Windows)
• minor: lazy allocations (e.g. old vmalloc)

User page fault:

• could be anything

Page fault information:

• cr2 register has faulting virtual address
• error code on stack has:
  • bunch of reserved bits
  • P: PTE present bit set (so page present, maybe fault caused by some other problem)
  • W: write access
  • U: user access (so we were in user eecution)
  • R: PTE had reserved bits set
  • I: faulted while trying to fetch instruction

VMA:

• contiguous virtual region with given properties (bit flags)
• anonymous: no file
• stored in red-black tree (for efficiency) and linked list (for traversing in order)

Demand paging:

• new pages are not mapped onto physical memory
• on first access, process page faults
• kernel assigns page frame to process, creates PTE, resumes execution

VMA operations:

• mmap: maps new region in adress space
  • find fitting hole in memory map, creates and links new VMA (in simplest case)
• munmap: unmaps region in address space
  • unlinks and deletes VMA (in simplest case), removes mapped page frames, removes allocated page tables, flushes TLB
• mprotect: changes region’s protection bits
  • update VMA protection (in simplest case), update affected PTEs
• madvise: give kernel advice on region, like MADV_DONTNEED (unmap page frames), MADV_WILLNEED (read ahead page frames)
• find and split: when you work on a part of a VMA (might even split one into three parts)
• merge: merge any adjacent VMAs with same properties (flags)

Transparent Huge Pages (THP)

• kernel transparently allocates THP when possible
• map: allocate compound pages (physically contiguous pages) and map as THPs
• collapse: replace some regular pages with THP
• split: split THP into number of regular pages
• compact: memory compaction to allocate compound pages