Virtualizable Compound Page Support V5

Allocations of larger pages are not reliable in Linux. If larger pages have to be allocated then one faces various choices of allowing graceful fallback or using vmalloc with a performance penalty due to the use of a page table. Virtualizable Compound Pages are a simple solution out of this dilemma.

A virtualizable compound allocation means that there will be first an attempt to satisfy the request with physically contiguous memory through a traditional compound page. If that is not possible then virtually contiguous memory will be used for the page.

This has two advantages:

1. Current uses of vmalloc can be converted to request for virtualizable compounds instead. In most cases physically contiguous memory can be used which avoids the vmalloc performance penalty. See f.e. the e1000 driver patch.

2. Uses of higher order allocations (stacks, buffers etc) can be converted to use virtualizable compounds instead. Physically contiguous memory will still be used for those higher order allocs in general but the system can degrade to the use of vmalloc should memory become heavily fragmented.

There is a compile time option to switch on fallback for testing purposes. Virtually mapped memory may behave differently and the CONFIG_VIRTUALIZE_ALWAYS option can be used ensure that the code is tested to deal with virtualized compound page.

This patchset contains first of all the core pieces to make virtualizable compound pages possible and then a set of example uses of virtualizable compound pages.

V4->V5
- Cleanup various portions
- Simplify code
- Complete documentation
- Limit the number of example uses.

V3->V4:
- Drop fallback for IA64 stack (arches with software tlb handlers could get into deep trouble if a tlb needs to be installed for the stack that is needed by the tlb fault handler).
- Drop ehash_lock vcompound patch.

V2->V3:
- Put the code into mm/vmalloc.c and leave the page allocator alone.
- Add a series of examples where virtual compound pages can be used.
- Diffed on top of the page flags and the vmalloc info patches already in mm.
- Simplify things by omitting some of the more complex code that used to be in there.

V1->V2
- Remove some cleanup patches and the SLUB patches from this set.
- Transparent vcompound support through page_address() and virt_to_head_page().
- Additional use cases.
- Factor the code better for an easier read
- Add configurable stack size.
- Follow up on various suggestions made for V1

RFC->V1
- Complete support for all compound functions for virtual compound pages (including the compound_nth_page() necessary for LBS mmap support)
- Fix various bugs
- Fix i386 build

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