Merge branch 'akpm' (patches from Andrew)

#

# Feature name:          pmdp_splitting_flush

#         Kconfig:       __HAVE_ARCH_PMDP_SPLITTING_FLUSH

#         description:   arch supports the pmdp_splitting_flush() VM API

#

    -----------------------

    |         arch |status|

    -----------------------

    |       alpha: | TODO |

    |         arc: | TODO |

    |         arm: |  ok  |

    |       arm64: |  ok  |

    |       avr32: | TODO |

    |    blackfin: | TODO |

    |         c6x: | TODO |

    |        cris: | TODO |

    |         frv: | TODO |

    |       h8300: | TODO |

    |     hexagon: | TODO |

    |        ia64: | TODO |

    |        m32r: | TODO |

    |        m68k: | TODO |

    |       metag: | TODO |

    |  microblaze: | TODO |

    |        mips: |  ok  |

    |     mn10300: | TODO |

    |       nios2: | TODO |

    |    openrisc: | TODO |

    |      parisc: | TODO |

    |     powerpc: |  ok  |

    |        s390: |  ok  |

    |       score: | TODO |

    |          sh: | TODO |

    |       sparc: | TODO |

    |        tile: | TODO |

    |          um: | TODO |

    |   unicore32: | TODO |

    |         x86: |  ok  |

    |      xtensa: | TODO |

    -----------------------

	Passed by reference.

Command from struct task_struct

	%pT	ls

	For printing executable name excluding path from struct

	task_struct.

	Passed by reference.

If you add other %p extensions, please extend lib/test_printf.c with

one or more test cases, if at all feasible.

== Design ==

- "graceful fallback": mm components which don't have transparent

  hugepage knowledge fall back to breaking a transparent hugepage and

  working on the regular pages and their respective regular pmd/pte

  mappings

- "graceful fallback": mm components which don't have transparent hugepage

  knowledge fall back to breaking huge pmd mapping into table of ptes and,

  if necessary, split a transparent hugepage. Therefore these components

  can continue working on the regular pages or regular pte mappings.

- if a hugepage allocation fails because of memory fragmentation,

  regular pages should be gracefully allocated instead and mixed in

	of pages that should be collapsed into one huge page but failed

	the allocation.

thp_split is incremented every time a huge page is split into base

thp_split_page is incremented every time a huge page is split into base

	pages. This can happen for a variety of reasons but a common

	reason is that a huge page is old and is being reclaimed.

	This action implies splitting all PMD the page mapped with.

thp_split_page_failed is is incremented if kernel fails to split huge

	page. This can happen if the page was pinned by somebody.

thp_split_pmd is incremented every time a PMD split into table of PTEs.

	This can happen, for instance, when application calls mprotect() or

	munmap() on part of huge page. It doesn't split huge page, only

	page table entry.

thp_zero_page_alloc is incremented every time a huge zero page is

	successfully allocated. It includes allocations which where

if any driver is going to mangle over the page structure of the tail

page (like for checking page->mapping or other bits that are relevant

for the head page and not the tail page), it should be updated to jump

to check head page instead (while serializing properly against

split_huge_page() to avoid the head and tail pages to disappear from

under it, see the futex code to see an example of that, hugetlbfs also

needed special handling in futex code for similar reasons).

to check head page instead. Taking reference on any head/tail page would

prevent page from being split by anyone.

NOTE: these aren't new constraints to the GUP API, and they match the

same constrains that applies to hugetlbfs too, so any driver capable

== Graceful fallback ==

Code walking pagetables but unware about huge pmds can simply call

split_huge_page_pmd(vma, addr, pmd) where the pmd is the one returned by

split_huge_pmd(vma, pmd, addr) where the pmd is the one returned by

pmd_offset. It's trivial to make the code transparent hugepage aware

by just grepping for "pmd_offset" and adding split_huge_page_pmd where

by just grepping for "pmd_offset" and adding split_huge_pmd where

missing after pmd_offset returns the pmd. Thanks to the graceful

fallback design, with a one liner change, you can avoid to write

hundred if not thousand of lines of complex code to make your code

If you're not walking pagetables but you run into a physical hugepage

but you can't handle it natively in your code, you can split it by

calling split_huge_page(page). This is what the Linux VM does before

it tries to swapout the hugepage for example.

it tries to swapout the hugepage for example. split_huge_page() can fail

if the page is pinned and you must handle this correctly.

Example to make mremap.c transparent hugepage aware with a one liner

change:

		return NULL;

	pmd = pmd_offset(pud, addr);

+	split_huge_page_pmd(vma, addr, pmd);

+	split_huge_pmd(vma, pmd, addr);

	if (pmd_none_or_clear_bad(pmd))

		return NULL;

== Locking in hugepage aware code ==

We want as much code as possible hugepage aware, as calling

split_huge_page() or split_huge_page_pmd() has a cost.

split_huge_page() or split_huge_pmd() has a cost.

To make pagetable walks huge pmd aware, all you need to do is to call

pmd_trans_huge() on the pmd returned by pmd_offset. You must hold the

takes the mmap_sem in write mode in addition to the anon_vma lock). If

pmd_trans_huge returns false, you just fallback in the old code

paths. If instead pmd_trans_huge returns true, you have to take the

mm->page_table_lock and re-run pmd_trans_huge. Taking the

page_table_lock will prevent the huge pmd to be converted into a

regular pmd from under you (split_huge_page can run in parallel to the

page table lock (pmd_lock()) and re-run pmd_trans_huge. Taking the

page table lock will prevent the huge pmd to be converted into a

regular pmd from under you (split_huge_pmd can run in parallel to the

pagetable walk). If the second pmd_trans_huge returns false, you

should just drop the page_table_lock and fallback to the old code as

before. Otherwise you should run pmd_trans_splitting on the pmd. In

case pmd_trans_splitting returns true, it means split_huge_page is

already in the middle of splitting the page. So if pmd_trans_splitting

returns true it's enough to drop the page_table_lock and call

wait_split_huge_page and then fallback the old code paths. You are

guaranteed by the time wait_split_huge_page returns, the pmd isn't

huge anymore. If pmd_trans_splitting returns false, you can proceed to

process the huge pmd and the hugepage natively. Once finished you can

drop the page_table_lock.

== compound_lock, get_user_pages and put_page ==

should just drop the page table lock and fallback to the old code as

before. Otherwise you can proceed to process the huge pmd and the

hugepage natively. Once finished you can drop the page table lock.

== Refcounts and transparent huge pages ==

Refcounting on THP is mostly consistent with refcounting on other compound

pages:

  - get_page()/put_page() and GUP operate in head page's ->_count.

  - ->_count in tail pages is always zero: get_page_unless_zero() never

    succeed on tail pages.

  - map/unmap of the pages with PTE entry increment/decrement ->_mapcount

    on relevant sub-page of the compound page.

  - map/unmap of the whole compound page accounted in compound_mapcount

    (stored in first tail page).

PageDoubleMap() indicates that ->_mapcount in all subpages is offset up by one.

This additional reference is required to get race-free detection of unmap of

subpages when we have them mapped with both PMDs and PTEs.

This is optimization required to lower overhead of per-subpage mapcount

tracking. The alternative is alter ->_mapcount in all subpages on each

map/unmap of the whole compound page.

We set PG_double_map when a PMD of the page got split for the first time,

but still have PMD mapping. The addtional references go away with last

compound_mapcount.

split_huge_page internally has to distribute the refcounts in the head

page to the tail pages before clearing all PG_head/tail bits from the

page structures. It can do that easily for refcounts taken by huge pmd

mappings. But the GUI API as created by hugetlbfs (that returns head

and tail pages if running get_user_pages on an address backed by any

hugepage), requires the refcount to be accounted on the tail pages and

not only in the head pages, if we want to be able to run

split_huge_page while there are gup pins established on any tail

page. Failure to be able to run split_huge_page if there's any gup pin

on any tail page, would mean having to split all hugepages upfront in

get_user_pages which is unacceptable as too many gup users are

performance critical and they must work natively on hugepages like

they work natively on hugetlbfs already (hugetlbfs is simpler because

hugetlbfs pages cannot be split so there wouldn't be requirement of

accounting the pins on the tail pages for hugetlbfs). If we wouldn't

account the gup refcounts on the tail pages during gup, we won't know

anymore which tail page is pinned by gup and which is not while we run

split_huge_page. But we still have to add the gup pin to the head page

too, to know when we can free the compound page in case it's never

split during its lifetime. That requires changing not just

get_page, but put_page as well so that when put_page runs on a tail

page (and only on a tail page) it will find its respective head page,

and then it will decrease the head page refcount in addition to the

tail page refcount. To obtain a head page reliably and to decrease its

refcount without race conditions, put_page has to serialize against

__split_huge_page_refcount using a special per-page lock called

compound_lock.

page to the tail pages before clearing all PG_head/tail bits from the page

structures. It can be done easily for refcounts taken by page table

entries. But we don't have enough information on how to distribute any

additional pins (i.e. from get_user_pages). split_huge_page() fails any

requests to split pinned huge page: it expects page count to be equal to

sum of mapcount of all sub-pages plus one (split_huge_page caller must

have reference for head page).

split_huge_page uses migration entries to stabilize page->_count and

page->_mapcount.

We safe against physical memory scanners too: the only legitimate way

scanner can get reference to a page is get_page_unless_zero().

All tail pages has zero ->_count until atomic_add(). It prevent scanner

from geting reference to tail page up to the point. After the atomic_add()

we don't care about ->_count value.  We already known how many references

with should uncharge from head page.

For head page get_page_unless_zero() will succeed and we don't mind. It's

clear where reference should go after split: it will stay on head page.

Note that split_huge_pmd() doesn't have any limitation on refcounting:

pmd can be split at any point and never fails.

== Partial unmap and deferred_split_huge_page() ==

Unmapping part of THP (with munmap() or other way) is not going to free

memory immediately. Instead, we detect that a subpage of THP is not in use

in page_remove_rmap() and queue the THP for splitting if memory pressure

comes. Splitting will free up unused subpages.

Splitting the page right away is not an option due to locking context in

the place where we can detect partial unmap. It's also might be

counterproductive since in many cases partial unmap unmap happens during

exit(2) if an THP crosses VMA boundary.

Function deferred_split_huge_page() is used to queue page for splitting.

The splitting itself will happen when we get memory pressure via shrinker

interface.

#define MADV_WILLNEED	3		/* will need these pages */

#define	MADV_SPACEAVAIL	5		/* ensure resources are available */

#define MADV_DONTNEED	6		/* don't need these pages */

#define MADV_FREE	7		/* free pages only if memory pressure */

/* common/generic parameters */

#define MADV_FREE	8		/* free pages only if memory pressure */

#define MADV_REMOVE	9		/* remove these pages & resources */

#define MADV_DONTFORK	10		/* don't inherit across fork */

#define MADV_DOFORK	11		/* do inherit across fork */

	def_bool y

	select STACKTRACE

config HAVE_LATENCYTOP_SUPPORT

	def_bool y

config HAVE_ARCH_TRANSPARENT_HUGEPAGE

	def_bool y

	depends on ARC_MMU_V4