mm/doc: editorial pass on page migration

Add Sphinx reference links to HMM and CPUSETS, and numerous small
editorial changes to make the page_migration.rst document more readable.

Signed-off-by: Ralph Campbell <rcampbell@nvidia.com>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/20200902225247.15213-1-rcampbell@nvidia.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

2 files changed, 85 insertions, 81 deletions

diff --git a/Documentation/vm/hmm.rst b/Documentation/vm/hmm.rst
index 6f9e000757fa..7453acc1ea4f 100644
--- a/Documentation/vm/hmm.rst
+++ b/Documentation/vm/hmm.rst
@@ -1,4 +1,4 @@
-.. hmm:
+.. _hmm:
 
 =====================================
 Heterogeneous Memory Management (HMM)
diff --git a/Documentation/vm/page_migration.rst b/Documentation/vm/page_migration.rst
index 68883ac485fa..91a98a6b43bb 100644
--- a/Documentation/vm/page_migration.rst
+++ b/Documentation/vm/page_migration.rst
@@ -4,25 +4,28 @@
 Page migration
 ==============
 
-Page migration allows the moving of the physical location of pages between
-nodes in a numa system while the process is running. This means that the
+Page migration allows moving the physical location of pages between
+nodes in a NUMA system while the process is running. This means that the
 virtual addresses that the process sees do not change. However, the
 system rearranges the physical location of those pages.
 
-The main intend of page migration is to reduce the latency of memory access
+Also see :ref:`Heterogeneous Memory Management (HMM) <hmm>`
+for migrating pages to or from device private memory.
+
+The main intent of page migration is to reduce the latency of memory accesses
 by moving pages near to the processor where the process accessing that memory
 is running.
 
 Page migration allows a process to manually relocate the node on which its
 pages are located through the MF_MOVE and MF_MOVE_ALL options while setting
-a new memory policy via mbind(). The pages of process can also be relocated
+a new memory policy via mbind(). The pages of a process can also be relocated
 from another process using the sys_migrate_pages() function call. The
-migrate_pages function call takes two sets of nodes and moves pages of a
+migrate_pages() function call takes two sets of nodes and moves pages of a
 process that are located on the from nodes to the destination nodes.
 Page migration functions are provided by the numactl package by Andi Kleen
 (a version later than 0.9.3 is required. Get it from
-ftp://oss.sgi.com/www/projects/libnuma/download/). numactl provides libnuma
-which provides an interface similar to other numa functionality for page
+https://github.com/numactl/numactl.git). numactl provides libnuma
+which provides an interface similar to other NUMA functionality for page
 migration.  cat ``/proc/<pid>/numa_maps`` allows an easy review of where the
 pages of a process are located. See also the numa_maps documentation in the
 proc(5) man page.
@@ -30,19 +33,19 @@ proc(5) man page.
 Manual migration is useful if for example the scheduler has relocated
 a process to a processor on a distant node. A batch scheduler or an
 administrator may detect the situation and move the pages of the process
-nearer to the new processor. The kernel itself does only provide
+nearer to the new processor. The kernel itself only provides
 manual page migration support. Automatic page migration may be implemented
 through user space processes that move pages. A special function call
 "move_pages" allows the moving of individual pages within a process.
-A NUMA profiler may f.e. obtain a log showing frequent off node
+For example, A NUMA profiler may obtain a log showing frequent off-node
 accesses and may use the result to move pages to more advantageous
 locations.
 
 Larger installations usually partition the system using cpusets into
 sections of nodes. Paul Jackson has equipped cpusets with the ability to
 move pages when a task is moved to another cpuset (See
-Documentation/admin-guide/cgroup-v1/cpusets.rst).
-Cpusets allows the automation of process locality. If a task is moved to
+:ref:`CPUSETS <cpusets>`).
+Cpusets allow the automation of process locality. If a task is moved to
 a new cpuset then also all its pages are moved with it so that the
 performance of the process does not sink dramatically. Also the pages
 of processes in a cpuset are moved if the allowed memory nodes of a
@@ -67,9 +70,9 @@ In kernel use of migrate_pages()
    Lists of pages to be migrated are generated by scanning over
    pages and moving them into lists. This is done by
    calling isolate_lru_page().
-   Calling isolate_lru_page increases the references to the page
+   Calling isolate_lru_page() increases the references to the page
    so that it cannot vanish while the page migration occurs.
-   It also prevents the swapper or other scans to encounter
+   It also prevents the swapper or other scans from encountering
    the page.
 
 2. We need to have a function of type new_page_t that can be
@@ -91,23 +94,24 @@ is increased so that the page cannot be freed while page migration occurs.
 
 Steps:
 
-1. Lock the page to be migrated
+1. Lock the page to be migrated.
 
 2. Ensure that writeback is complete.
 
 3. Lock the new page that we want to move to. It is locked so that accesses to
-   this (not yet uptodate) page immediately lock while the move is in progress.
+   this (not yet uptodate) page immediately block while the move is in progress.
 
 4. All the page table references to the page are converted to migration
    entries. This decreases the mapcount of a page. If the resulting
    mapcount is not zero then we do not migrate the page. All user space
-   processes that attempt to access the page will now wait on the page lock.
+   processes that attempt to access the page will now wait on the page lock
+   or wait for the migration page table entry to be removed.
 
 5. The i_pages lock is taken. This will cause all processes trying
    to access the page via the mapping to block on the spinlock.
 
-6. The refcount of the page is examined and we back out if references remain
-   otherwise we know that we are the only one referencing this page.
+6. The refcount of the page is examined and we back out if references remain.
+   Otherwise, we know that we are the only one referencing this page.
 
 7. The radix tree is checked and if it does not contain the pointer to this
    page then we back out because someone else modified the radix tree.
@@ -134,124 +138,124 @@ Steps:
 
 15. Queued up writeback on the new page is triggered.
 
-16. If migration entries were page then replace them with real ptes. Doing
-    so will enable access for user space processes not already waiting for
-    the page lock.
+16. If migration entries were inserted into the page table, then replace them
+    with real ptes. Doing so will enable access for user space processes not
+    already waiting for the page lock.
 
-19. The page locks are dropped from the old and new page.
+17. The page locks are dropped from the old and new page.
     Processes waiting on the page lock will redo their page faults
     and will reach the new page.
 
-20. The new page is moved to the LRU and can be scanned by the swapper
-    etc again.
+18. The new page is moved to the LRU and can be scanned by the swapper,
+    etc. again.
 
 Non-LRU page migration
 ======================
 
-Although original migration aimed for reducing the latency of memory access
-for NUMA, compaction who want to create high-order page is also main customer.
+Although migration originally aimed for reducing the latency of memory accesses
+for NUMA, compaction also uses migration to create high-order pages.
 
 Current problem of the implementation is that it is designed to migrate only
-*LRU* pages. However, there are potential non-lru pages which can be migrated
+*LRU* pages. However, there are potential non-LRU pages which can be migrated
 in drivers, for example, zsmalloc, virtio-balloon pages.
 
 For virtio-balloon pages, some parts of migration code path have been hooked
 up and added virtio-balloon specific functions to intercept migration logics.
 It's too specific to a driver so other drivers who want to make their pages
-movable would have to add own specific hooks in migration path.
+movable would have to add their own specific hooks in the migration path.
 
-To overclome the problem, VM supports non-LRU page migration which provides
+To overcome the problem, VM supports non-LRU page migration which provides
 generic functions for non-LRU movable pages without driver specific hooks
-migration path.
+in the migration path.
 
-If a driver want to make own pages movable, it should define three functions
+If a driver wants to make its pages movable, it should define three functions
 which are function pointers of struct address_space_operations.
 
 1. ``bool (*isolate_page) (struct page *page, isolate_mode_t mode);``
 
-   What VM expects on isolate_page function of driver is to return *true*
-   if driver isolates page successfully. On returing true, VM marks the page
+   What VM expects from isolate_page() function of driver is to return *true*
+   if driver isolates the page successfully. On returning true, VM marks the page
    as PG_isolated so concurrent isolation in several CPUs skip the page
    for isolation. If a driver cannot isolate the page, it should return *false*.
 
    Once page is successfully isolated, VM uses page.lru fields so driver
-   shouldn't expect to preserve values in that fields.
+   shouldn't expect to preserve values in those fields.
 
 2. ``int (*migratepage) (struct address_space *mapping,``
 |	``struct page *newpage, struct page *oldpage, enum migrate_mode);``
 
-   After isolation, VM calls migratepage of driver with isolated page.
-   The function of migratepage is to move content of the old page to new page
+   After isolation, VM calls migratepage() of driver with the isolated page.
+   The function of migratepage() is to move the contents of the old page to the
+   new page
    and set up fields of struct page newpage. Keep in mind that you should
    indicate to the VM the oldpage is no longer movable via __ClearPageMovable()
-   under page_lock if you migrated the oldpage successfully and returns
+   under page_lock if you migrated the oldpage successfully and returned
    MIGRATEPAGE_SUCCESS. If driver cannot migrate the page at the moment, driver
    can return -EAGAIN. On -EAGAIN, VM will retry page migration in a short time
-   because VM interprets -EAGAIN as "temporal migration failure". On returning
-   any error except -EAGAIN, VM will give up the page migration without retrying
-   in this time.
+   because VM interprets -EAGAIN as "temporary migration failure". On returning
+   any error except -EAGAIN, VM will give up the page migration without
+   retrying.
 
-   Driver shouldn't touch page.lru field VM using in the functions.
+   Driver shouldn't touch the page.lru field while in the migratepage() function.
 
 3. ``void (*putback_page)(struct page *);``
 
-   If migration fails on isolated page, VM should return the isolated page
-   to the driver so VM calls driver's putback_page with migration failed page.
-   In this function, driver should put the isolated page back to the own data
+   If migration fails on the isolated page, VM should return the isolated page
+   to the driver so VM calls the driver's putback_page() with the isolated page.
+   In this function, the driver should put the isolated page back into its own data
    structure.
 
-4. non-lru movable page flags
+4. non-LRU movable page flags
 
-   There are two page flags for supporting non-lru movable page.
+   There are two page flags for supporting non-LRU movable page.
 
    * PG_movable
 
-     Driver should use the below function to make page movable under page_lock::
+     Driver should use the function below to make page movable under page_lock::
 
 	void __SetPageMovable(struct page *page, struct address_space *mapping)
 
      It needs argument of address_space for registering migration
      family functions which will be called by VM. Exactly speaking,
-     PG_movable is not a real flag of struct page. Rather than, VM
-     reuses page->mapping's lower bits to represent it.
+     PG_movable is not a real flag of struct page. Rather, VM
+     reuses the page->mapping's lower bits to represent it::
 
-::
 	#define PAGE_MAPPING_MOVABLE 0x2
 	page->mapping = page->mapping | PAGE_MAPPING_MOVABLE;
 
      so driver shouldn't access page->mapping directly. Instead, driver should
-     use page_mapping which mask off the low two bits of page->mapping under
-     page lock so it can get right struct address_space.
-
-     For testing of non-lru movable page, VM supports __PageMovable function.
-     However, it doesn't guarantee to identify non-lru movable page because
-     page->mapping field is unified with other variables in struct page.
-     As well, if driver releases the page after isolation by VM, page->mapping
-     doesn't have stable value although it has PAGE_MAPPING_MOVABLE
-     (Look at __ClearPageMovable). But __PageMovable is cheap to catch whether
-     page is LRU or non-lru movable once the page has been isolated. Because
-     LRU pages never can have PAGE_MAPPING_MOVABLE in page->mapping. It is also
-     good for just peeking to test non-lru movable pages before more expensive
-     checking with lock_page in pfn scanning to select victim.
-
-     For guaranteeing non-lru movable page, VM provides PageMovable function.
-     Unlike __PageMovable, PageMovable functions validates page->mapping and
-     mapping->a_ops->isolate_page under lock_page. The lock_page prevents sudden
-     destroying of page->mapping.
-
-     Driver using __SetPageMovable should clear the flag via __ClearMovablePage
-     under page_lock before the releasing the page.
+     use page_mapping() which masks off the low two bits of page->mapping under
+     page lock so it can get the right struct address_space.
+
+     For testing of non-LRU movable pages, VM supports __PageMovable() function.
+     However, it doesn't guarantee to identify non-LRU movable pages because
+     the page->mapping field is unified with other variables in struct page.
+     If the driver releases the page after isolation by VM, page->mapping
+     doesn't have a stable value although it has PAGE_MAPPING_MOVABLE set
+     (look at __ClearPageMovable). But __PageMovable() is cheap to call whether
+     page is LRU or non-LRU movable once the page has been isolated because LRU
+     pages can never have PAGE_MAPPING_MOVABLE set in page->mapping. It is also
+     good for just peeking to test non-LRU movable pages before more expensive
+     checking with lock_page() in pfn scanning to select a victim.
+
+     For guaranteeing non-LRU movable page, VM provides PageMovable() function.
+     Unlike __PageMovable(), PageMovable() validates page->mapping and
+     mapping->a_ops->isolate_page under lock_page(). The lock_page() prevents
+     sudden destroying of page->mapping.
+
+     Drivers using __SetPageMovable() should clear the flag via
+     __ClearMovablePage() under page_lock() before the releasing the page.
 
    * PG_isolated
 
      To prevent concurrent isolation among several CPUs, VM marks isolated page
-     as PG_isolated under lock_page. So if a CPU encounters PG_isolated non-lru
-     movable page, it can skip it. Driver doesn't need to manipulate the flag
-     because VM will set/clear it automatically. Keep in mind that if driver
-     sees PG_isolated page, it means the page have been isolated by VM so it
-     shouldn't touch page.lru field.
-     PG_isolated is alias with PG_reclaim flag so driver shouldn't use the flag
-     for own purpose.
+     as PG_isolated under lock_page(). So if a CPU encounters PG_isolated
+     non-LRU movable page, it can skip it. Driver doesn't need to manipulate the
+     flag because VM will set/clear it automatically. Keep in mind that if the
+     driver sees a PG_isolated page, it means the page has been isolated by the
+     VM so it shouldn't touch the page.lru field.
+     The PG_isolated flag is aliased with the PG_reclaim flag so drivers
+     shouldn't use PG_isolated for its own purposes.
 
 Monitoring Migration
 =====================
@@ -266,8 +270,8 @@ The following events (counters) can be used to monitor page migration.
    512.
 
 2. PGMIGRATE_FAIL: Normal page migration failure. Same counting rules as for
-   _SUCCESS, above: this will be increased by the number of subpages, if it was
-   a THP.
+   PGMIGRATE_SUCCESS, above: this will be increased by the number of subpages,
+   if it was a THP.
 
 3. THP_MIGRATION_SUCCESS: A THP was migrated without being split.