diff options
Diffstat (limited to 'Documentation/mm/page_migration.rst')
| -rw-r--r-- | Documentation/mm/page_migration.rst | 72 |
1 files changed, 42 insertions, 30 deletions
diff --git a/Documentation/mm/page_migration.rst b/Documentation/mm/page_migration.rst index 11493bad7112..34602b254aa6 100644 --- a/Documentation/mm/page_migration.rst +++ b/Documentation/mm/page_migration.rst @@ -1,5 +1,3 @@ -.. _page_migration: - ============== Page migration ============== @@ -9,8 +7,8 @@ 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. -Also see :ref:`Heterogeneous Memory Management (HMM) <hmm>` -for migrating pages to or from device private memory. +Also see Documentation/mm/hmm.rst 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 @@ -65,32 +63,31 @@ and then a low level description of how the low level details work. In kernel use of migrate_pages() ================================ -1. Remove pages from the LRU. +1. Remove folios from the LRU. - 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 - so that it cannot vanish while the page migration occurs. + Lists of folios to be migrated are generated by scanning over + folios and moving them into lists. This is done by + calling folio_isolate_lru(). + Calling folio_isolate_lru() increases the references to the folio + so that it cannot vanish while the folio migration occurs. It also prevents the swapper or other scans from encountering - the page. + the folio. -2. We need to have a function of type new_page_t that can be +2. We need to have a function of type new_folio_t that can be passed to migrate_pages(). This function should figure out - how to allocate the correct new page given the old page. + how to allocate the correct new folio given the old folio. 3. The migrate_pages() function is called which attempts to do the migration. It will call the function to allocate - the new page for each page that is considered for - moving. + the new folio for each folio that is considered for moving. How migrate_pages() works ========================= -migrate_pages() does several passes over its list of pages. A page is moved -if all references to a page are removable at the time. The page has -already been removed from the LRU via isolate_lru_page() and the refcount -is increased so that the page cannot be freed while page migration occurs. +migrate_pages() does several passes over its list of folios. A folio is moved +if all references to a folio are removable at the time. The folio has +already been removed from the LRU via folio_isolate_lru() and the refcount +is increased so that the folio cannot be freed while folio migration occurs. Steps: @@ -149,18 +146,33 @@ Steps: 18. The new page is moved to the LRU and can be scanned by the swapper, etc. again. -Non-LRU page migration -====================== +movable_ops page migration +========================== + +Selected typed, non-folio pages (e.g., pages inflated in a memory balloon, +zsmalloc pages) can be migrated using the movable_ops migration framework. + +The "struct movable_operations" provide callbacks specific to a page type +for isolating, migrating and un-isolating (putback) these pages. + +Once a page is indicated as having movable_ops, that condition must not +change until the page was freed back to the buddy. This includes not +changing/clearing the page type and not changing/clearing the +PG_movable_ops page flag. + +Arbitrary drivers cannot currently make use of this framework, as it +requires: -Although migration originally aimed for reducing the latency of memory -accesses for NUMA, compaction also uses migration to create high-order -pages. For compaction purposes, it is also useful to be able to move -non-LRU pages, such as zsmalloc and virtio-balloon pages. +(a) a page type +(b) indicating them as possibly having movable_ops in page_has_movable_ops() + based on the page type +(c) returning the movable_ops from page_movable_ops() based on the page + type +(d) not reusing the PG_movable_ops and PG_movable_ops_isolated page flags + for other purposes -If a driver wants to make its pages movable, it should define a struct -movable_operations. It then needs to call __SetPageMovable() on each -page that it may be able to move. This uses the ``page->mapping`` field, -so this field is not available for the driver to use for other purposes. +For example, balloon drivers can make use of this framework through the +balloon-compaction infrastructure residing in the core kernel. Monitoring Migration ===================== @@ -183,7 +195,7 @@ The following events (counters) can be used to monitor page migration. 4. THP_MIGRATION_FAIL: A THP could not be migrated nor it could be split. 5. THP_MIGRATION_SPLIT: A THP was migrated, but not as such: first, the THP had - to be split. After splitting, a migration retry was used for it's sub-pages. + to be split. After splitting, a migration retry was used for its sub-pages. THP_MIGRATION_* events also update the appropriate PGMIGRATE_SUCCESS or PGMIGRATE_FAIL events. For example, a THP migration failure will cause both |