docs/vm: unevictable-lru.txt: convert to ReST format
Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com> Signed-off-by: Jonathan Corbet <corbet@lwn.net>hifive-unleashed-5.1
Mike Rapoport
Jonathan Corbet
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==============================
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UNEVICTABLE LRU INFRASTRUCTURE
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==============================
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.. _unevictable_lru:
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========
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CONTENTS
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========
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==============================
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Unevictable LRU Infrastructure
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==============================
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(*) The Unevictable LRU
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- The unevictable page list.
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- Memory control group interaction.
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- Marking address spaces unevictable.
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- Detecting Unevictable Pages.
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- vmscan's handling of unevictable pages.
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(*) mlock()'d pages.
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- History.
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- Basic management.
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- mlock()/mlockall() system call handling.
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- Filtering special vmas.
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- munlock()/munlockall() system call handling.
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- Migrating mlocked pages.
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- Compacting mlocked pages.
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- mmap(MAP_LOCKED) system call handling.
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- munmap()/exit()/exec() system call handling.
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- try_to_unmap().
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- try_to_munlock() reverse map scan.
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- Page reclaim in shrink_*_list().
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.. contents:: :local:
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============
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INTRODUCTION
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Introduction
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============
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This document describes the Linux memory manager's "Unevictable LRU"
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@ -46,8 +22,8 @@ details - the "what does it do?" - by reading the code. One hopes that the
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descriptions below add value by provide the answer to "why does it do that?".
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===================
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THE UNEVICTABLE LRU
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The Unevictable LRU
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===================
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The Unevictable LRU facility adds an additional LRU list to track unevictable
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@ -66,17 +42,17 @@ completely unresponsive.
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The unevictable list addresses the following classes of unevictable pages:
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(*) Those owned by ramfs.
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* Those owned by ramfs.
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(*) Those mapped into SHM_LOCK'd shared memory regions.
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* Those mapped into SHM_LOCK'd shared memory regions.
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(*) Those mapped into VM_LOCKED [mlock()ed] VMAs.
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* Those mapped into VM_LOCKED [mlock()ed] VMAs.
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The infrastructure may also be able to handle other conditions that make pages
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unevictable, either by definition or by circumstance, in the future.
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THE UNEVICTABLE PAGE LIST
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The Unevictable Page List
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-------------------------
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The Unevictable LRU infrastructure consists of an additional, per-zone, LRU list
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@ -118,7 +94,7 @@ the unevictable list when one task has the page isolated from the LRU and other
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tasks are changing the "evictability" state of the page.
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MEMORY CONTROL GROUP INTERACTION
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Memory Control Group Interaction
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--------------------------------
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The unevictable LRU facility interacts with the memory control group [aka
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@ -144,7 +120,9 @@ effects:
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the control group to thrash or to OOM-kill tasks.
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MARKING ADDRESS SPACES UNEVICTABLE
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.. _mark_addr_space_unevict:
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Marking Address Spaces Unevictable
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----------------------------------
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For facilities such as ramfs none of the pages attached to the address space
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@ -152,15 +130,15 @@ may be evicted. To prevent eviction of any such pages, the AS_UNEVICTABLE
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address space flag is provided, and this can be manipulated by a filesystem
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using a number of wrapper functions:
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(*) void mapping_set_unevictable(struct address_space *mapping);
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* ``void mapping_set_unevictable(struct address_space *mapping);``
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Mark the address space as being completely unevictable.
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(*) void mapping_clear_unevictable(struct address_space *mapping);
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* ``void mapping_clear_unevictable(struct address_space *mapping);``
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Mark the address space as being evictable.
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(*) int mapping_unevictable(struct address_space *mapping);
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* ``int mapping_unevictable(struct address_space *mapping);``
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Query the address space, and return true if it is completely
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unevictable.
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@ -177,12 +155,13 @@ These are currently used in two places in the kernel:
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ensure they're in memory.
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DETECTING UNEVICTABLE PAGES
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Detecting Unevictable Pages
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---------------------------
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The function page_evictable() in vmscan.c determines whether a page is
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evictable or not using the query function outlined above [see section "Marking
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address spaces unevictable"] to check the AS_UNEVICTABLE flag.
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evictable or not using the query function outlined above [see section
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:ref:`Marking address spaces unevictable <mark_addr_space_unevict>`]
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to check the AS_UNEVICTABLE flag.
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For address spaces that are so marked after being populated (as SHM regions
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might be), the lock action (eg: SHM_LOCK) can be lazy, and need not populate
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@ -202,7 +181,7 @@ flag, PG_mlocked (as wrapped by PageMlocked()), which is set when a page is
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faulted into a VM_LOCKED vma, or found in a vma being VM_LOCKED.
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VMSCAN'S HANDLING OF UNEVICTABLE PAGES
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Vmscan's Handling of Unevictable Pages
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--------------------------------------
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If unevictable pages are culled in the fault path, or moved to the unevictable
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@ -233,8 +212,7 @@ extra evictabilty checks should not occur in the majority of calls to
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putback_lru_page().
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=============
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MLOCKED PAGES
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MLOCKED Pages
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=============
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The unevictable page list is also useful for mlock(), in addition to ramfs and
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@ -242,7 +220,7 @@ SYSV SHM. Note that mlock() is only available in CONFIG_MMU=y situations; in
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NOMMU situations, all mappings are effectively mlocked.
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HISTORY
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History
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-------
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The "Unevictable mlocked Pages" infrastructure is based on work originally
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@ -263,7 +241,7 @@ replaced by walking the reverse map to determine whether any VM_LOCKED VMAs
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mapped the page. More on this below.
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BASIC MANAGEMENT
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Basic Management
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----------------
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mlocked pages - pages mapped into a VM_LOCKED VMA - are a class of unevictable
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@ -304,10 +282,10 @@ mlocked pages become unlocked and rescued from the unevictable list when:
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(4) before a page is COW'd in a VM_LOCKED VMA.
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mlock()/mlockall() SYSTEM CALL HANDLING
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mlock()/mlockall() System Call Handling
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---------------------------------------
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Both [do_]mlock() and [do_]mlockall() system call handlers call mlock_fixup()
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Both [do\_]mlock() and [do\_]mlockall() system call handlers call mlock_fixup()
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for each VMA in the range specified by the call. In the case of mlockall(),
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this is the entire active address space of the task. Note that mlock_fixup()
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is used for both mlocking and munlocking a range of memory. A call to mlock()
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@ -351,7 +329,7 @@ mlock_vma_page() is unable to isolate the page from the LRU, vmscan will handle
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it later if and when it attempts to reclaim the page.
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FILTERING SPECIAL VMAS
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Filtering Special VMAs
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----------------------
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mlock_fixup() filters several classes of "special" VMAs:
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@ -379,8 +357,9 @@ VM_LOCKED flag. Therefore, we won't have to deal with them later during
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munlock(), munmap() or task exit. Neither does mlock_fixup() account these
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VMAs against the task's "locked_vm".
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.. _munlock_munlockall_handling:
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munlock()/munlockall() SYSTEM CALL HANDLING
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munlock()/munlockall() System Call Handling
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-------------------------------------------
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The munlock() and munlockall() system calls are handled by the same functions -
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@ -426,7 +405,7 @@ This is fine, because we'll catch it later if and if vmscan tries to reclaim
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the page. This should be relatively rare.
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MIGRATING MLOCKED PAGES
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Migrating MLOCKED Pages
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-----------------------
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A page that is being migrated has been isolated from the LRU lists and is held
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@ -451,7 +430,7 @@ list because of a race between munlock and migration, page migration uses the
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putback_lru_page() function to add migrated pages back to the LRU.
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COMPACTING MLOCKED PAGES
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Compacting MLOCKED Pages
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------------------------
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The unevictable LRU can be scanned for compactable regions and the default
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@ -461,7 +440,7 @@ unevictable LRU is enabled, the work of compaction is mostly handled by
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the page migration code and the same work flow as described in MIGRATING
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MLOCKED PAGES will apply.
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MLOCKING TRANSPARENT HUGE PAGES
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MLOCKING Transparent Huge Pages
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-------------------------------
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A transparent huge page is represented by a single entry on an LRU list.
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@ -483,7 +462,7 @@ to unevictable LRU and the rest can be reclaimed.
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See also comment in follow_trans_huge_pmd().
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mmap(MAP_LOCKED) SYSTEM CALL HANDLING
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mmap(MAP_LOCKED) System Call Handling
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-------------------------------------
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In addition the mlock()/mlockall() system calls, an application can request
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@ -514,7 +493,7 @@ memory range accounted as locked_vm, as the protections could be changed later
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and pages allocated into that region.
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munmap()/exit()/exec() SYSTEM CALL HANDLING
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munmap()/exit()/exec() System Call Handling
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-------------------------------------------
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When unmapping an mlocked region of memory, whether by an explicit call to
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@ -568,16 +547,18 @@ munlock or munmap system calls, mm teardown (munlock_vma_pages_all), reclaim,
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holepunching, and truncation of file pages and their anonymous COWed pages.
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try_to_munlock() REVERSE MAP SCAN
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try_to_munlock() Reverse Map Scan
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---------------------------------
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[!] TODO/FIXME: a better name might be page_mlocked() - analogous to the
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page_referenced() reverse map walker.
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.. warning::
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[!] TODO/FIXME: a better name might be page_mlocked() - analogous to the
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page_referenced() reverse map walker.
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When munlock_vma_page() [see section "munlock()/munlockall() System Call
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Handling" above] tries to munlock a page, it needs to determine whether or not
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the page is mapped by any VM_LOCKED VMA without actually attempting to unmap
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all PTEs from the page. For this purpose, the unevictable/mlock infrastructure
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When munlock_vma_page() [see section :ref:`munlock()/munlockall() System Call
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Handling <munlock_munlockall_handling>` above] tries to munlock a
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page, it needs to determine whether or not the page is mapped by any
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VM_LOCKED VMA without actually attempting to unmap all PTEs from the
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page. For this purpose, the unevictable/mlock infrastructure
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introduced a variant of try_to_unmap() called try_to_munlock().
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try_to_munlock() calls the same functions as try_to_unmap() for anonymous and
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@ -595,7 +576,7 @@ large region or tearing down a large address space that has been mlocked via
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mlockall(), overall this is a fairly rare event.
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PAGE RECLAIM IN shrink_*_list()
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Page Reclaim in shrink_*_list()
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-------------------------------
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shrink_active_list() culls any obviously unevictable pages - i.e.
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