docs/vm: Minor editorial changes in the THP and hugetlbfs

A huge page mapping or segment is either private or shared.  If private,

it is typically only available to a single address space (task).  If shared,

it can be mapped into multiple address spaces (tasks).  The location and

semantics of the reservation map is significantly different for two types

semantics of the reservation map is significantly different for the two types

of mappings.  Location differences are:

- For private mappings, the reservation map hangs off the the VMA structure.

- For private mappings, the reservation map hangs off the VMA structure.

  Specifically, vma->vm_private_data.  This reserve map is created at the

  time the mapping (mmap(MAP_PRIVATE)) is created.

- For shared mappings, the reservation map hangs off the inode.  Specifically,

				  struct vm_area_struct *vma,

				  vm_flags_t vm_flags)

The first thing hugetlb_reserve_pages() does is check for the NORESERVE

The first thing hugetlb_reserve_pages() does is check if the NORESERVE

flag was specified in either the shmget() or mmap() call.  If NORESERVE

was specified, then this routine returns immediately as no reservation

was specified, then this routine returns immediately as no reservations

are desired.

The arguments 'from' and 'to' are huge page indices into the mapping or

underlying file.  For shmget(), 'from' is always 0 and 'to' corresponds to

the length of the segment/mapping.  For mmap(), the offset argument could

be used to specify the offset into the underlying file.  In such a case

be used to specify the offset into the underlying file.  In such a case,

the 'from' and 'to' arguments have been adjusted by this offset.

One of the big differences between PRIVATE and SHARED mappings is the way

The reservation map is consulted to determine how many huge page reservations

are needed for the current mapping/segment.  For private mappings, this is

always the value (to - from).  However, for shared mappings it is possible that some reservations may already exist within the range (to - from).  See the

always the value (to - from).  However, for shared mappings it is possible that

some reservations may already exist within the range (to - from).  See the

section :ref:`Reservation Map Modifications <resv_map_modifications>`

for details on how this is accomplished.

If there were enough free huge pages and the global count resv_huge_pages

was adjusted, then the reservation map associated with the mapping is

modified to reflect the reservations.  In the case of a shared mapping, a

file_region will exist that includes the range 'from' 'to'.  For private

file_region will exist that includes the range 'from' - 'to'.  For private

mappings, no modifications are made to the reservation map as lack of an

entry indicates a reservation exists.

The routine vma_commit_reservation() is then called to adjust the reserve

map based on the consumption of the reservation.  In general, this involves

ensuring the page is represented within a file_region structure of the region

map.  For shared mappings where the the reservation was present, an entry

map.  For shared mappings where the reservation was present, an entry

in the reserve map already existed so no change is made.  However, if there

was no reservation in a shared mapping or this was a private mapping a new

entry must be created.

Transparent Hugepage Support

============================

This document describes design principles Transparent Hugepage (THP)

Support and its interaction with other parts of the memory management.

This document describes design principles for Transparent Hugepage (THP)

support and its interaction with other parts of the memory management

system.

Design principles

=================

get_user_pages and follow_page if run on a hugepage, will return the

head or tail pages as usual (exactly as they would do on

hugetlbfs). Most gup users will only care about the actual physical

hugetlbfs). Most GUP users will only care about the actual physical

address of the page and its temporary pinning to release after the I/O

is complete, so they won't ever notice the fact the page is huge. But

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. Taking reference on any head/tail page would

prevent page from being split by anyone.

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

prevent the 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

   same constraints that apply to hugetlbfs too, so any driver capable

   of handling GUP on hugetlbfs will also work fine on transparent

   hugepage backed mappings.

In case you can't handle compound pages if they're returned by

follow_page, the FOLL_SPLIT bit can be specified as parameter to

follow_page, the FOLL_SPLIT bit can be specified as a parameter to

follow_page, so that it will split the hugepages before returning

them.

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

hundreds if not thousands of lines of complex code to make your code

hugepage aware.

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

that you can't handle 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. split_huge_page() can fail

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

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

mmap_sem in read (or write) mode to be sure an huge pmd cannot be

mmap_sem in read (or write) mode to be sure a huge pmd cannot be

created from under you by khugepaged (khugepaged collapse_huge_page

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

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

page table lock will prevent the huge pmd being 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 can proceed to process the huge pmd and the

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

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 ->_refcount.

  - get_page()/put_page() and GUP operate on head page's ->_refcount.

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

    succeed on tail pages.

    succeeds 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

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

    (stored in first tail page). For file huge pages, we also increment

    ->_mapcount of all sub-pages in order to have race-free detection of

    last unmap of subpages.

PageDoubleMap() indicates that the page is *possibly* mapped with PTEs.

For anonymous pages PageDoubleMap() also indicates ->_mapcount in all

For anonymous pages, PageDoubleMap() also indicates ->_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

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

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

map/unmap of the whole compound page.

For anonymous pages, we set PG_double_map when a PMD of the page got split

for the first time, but still have PMD mapping. The additional references

go away with last compound_mapcount.

For anonymous pages, we set PG_double_map when a PMD of the page is split

for the first time, but still have a PMD mapping. The additional references

go away with the last compound_mapcount.

File pages get PG_double_map set on first map of the page with PTE and

goes away when the page gets evicted from page cache.

File pages get PG_double_map set on the first map of the page with PTE and

goes away when the page gets evicted from the page cache.

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 be done easily for refcounts taken by page table

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

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).

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

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

have a reference to the head page).

split_huge_page uses migration entries to stabilize page->_refcount and

page->_mapcount of anonymous pages. File pages just got unmapped.

page->_mapcount of anonymous pages. File pages just get unmapped.

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

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

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

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

All tail pages have zero ->_refcount until atomic_add(). This prevents the

scanner from getting a reference to the tail page up to that point. After the

atomic_add() we don't care about the ->_refcount value. We already known how

atomic_add() we don't care about the ->_refcount value. We already know how

many references should be uncharged from the 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.

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

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

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

pmd can be split at any point and never fails.

Partial unmap and deferred_split_huge_page()

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

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

counterproductive since in many cases partial unmap happens during exit(2) if

a THP crosses a VMA boundary.

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

The function deferred_split_huge_page() is used to queue a page for splitting.

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

interface.