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| -rw-r--r-- | Documentation/vm/unevictable-lru.txt | 117 |
1 files changed, 49 insertions, 68 deletions
diff --git a/Documentation/vm/unevictable-lru.txt b/Documentation/vm/unevictable-lru.txt index e14718572476..fdd84cb8d511 100644 --- a/Documentation/vm/unevictable-lru.txt +++ b/Documentation/vm/unevictable-lru.txt @@ -1,37 +1,13 @@ - ============================== - UNEVICTABLE LRU INFRASTRUCTURE - ============================== - -======== -CONTENTS -======== - - (*) The Unevictable LRU - - - The unevictable page list. - - Memory control group interaction. - - Marking address spaces unevictable. - - Detecting Unevictable Pages. - - vmscan's handling of unevictable pages. - - (*) mlock()'d pages. - - - History. - - Basic management. - - mlock()/mlockall() system call handling. - - Filtering special vmas. - - munlock()/munlockall() system call handling. - - Migrating mlocked pages. - - Compacting mlocked pages. - - mmap(MAP_LOCKED) system call handling. - - munmap()/exit()/exec() system call handling. - - try_to_unmap(). - - try_to_munlock() reverse map scan. - - Page reclaim in shrink_*_list(). +.. _unevictable_lru: +============================== +Unevictable LRU Infrastructure +============================== -============ -INTRODUCTION +.. contents:: :local: + + +Introduction ============ This document describes the Linux memory manager's "Unevictable LRU" @@ -46,8 +22,8 @@ details - the "what does it do?" - by reading the code. One hopes that the descriptions below add value by provide the answer to "why does it do that?". -=================== -THE UNEVICTABLE LRU + +The Unevictable LRU =================== The Unevictable LRU facility adds an additional LRU list to track unevictable @@ -66,17 +42,17 @@ completely unresponsive. The unevictable list addresses the following classes of unevictable pages: - (*) Those owned by ramfs. + * Those owned by ramfs. - (*) Those mapped into SHM_LOCK'd shared memory regions. + * Those mapped into SHM_LOCK'd shared memory regions. - (*) Those mapped into VM_LOCKED [mlock()ed] VMAs. + * Those mapped into VM_LOCKED [mlock()ed] VMAs. The infrastructure may also be able to handle other conditions that make pages unevictable, either by definition or by circumstance, in the future. -THE UNEVICTABLE PAGE LIST +The Unevictable Page List ------------------------- The Unevictable LRU infrastructure consists of an additional, per-zone, LRU list @@ -118,7 +94,7 @@ the unevictable list when one task has the page isolated from the LRU and other tasks are changing the "evictability" state of the page. -MEMORY CONTROL GROUP INTERACTION +Memory Control Group Interaction -------------------------------- The unevictable LRU facility interacts with the memory control group [aka @@ -144,7 +120,9 @@ effects: the control group to thrash or to OOM-kill tasks. -MARKING ADDRESS SPACES UNEVICTABLE +.. _mark_addr_space_unevict: + +Marking Address Spaces Unevictable ---------------------------------- For facilities such as ramfs none of the pages attached to the address space @@ -152,15 +130,15 @@ may be evicted. To prevent eviction of any such pages, the AS_UNEVICTABLE address space flag is provided, and this can be manipulated by a filesystem using a number of wrapper functions: - (*) void mapping_set_unevictable(struct address_space *mapping); + * ``void mapping_set_unevictable(struct address_space *mapping);`` Mark the address space as being completely unevictable. - (*) void mapping_clear_unevictable(struct address_space *mapping); + * ``void mapping_clear_unevictable(struct address_space *mapping);`` Mark the address space as being evictable. - (*) int mapping_unevictable(struct address_space *mapping); + * ``int mapping_unevictable(struct address_space *mapping);`` Query the address space, and return true if it is completely unevictable. @@ -177,12 +155,13 @@ These are currently used in two places in the kernel: ensure they're in memory. -DETECTING UNEVICTABLE PAGES +Detecting Unevictable Pages --------------------------- The function page_evictable() in vmscan.c determines whether a page is -evictable or not using the query function outlined above [see section "Marking -address spaces unevictable"] to check the AS_UNEVICTABLE flag. +evictable or not using the query function outlined above [see section +:ref:`Marking address spaces unevictable <mark_addr_space_unevict>`] +to check the AS_UNEVICTABLE flag. For address spaces that are so marked after being populated (as SHM regions might be), the lock action (eg: SHM_LOCK) can be lazy, and need not populate @@ -202,7 +181,7 @@ flag, PG_mlocked (as wrapped by PageMlocked()), which is set when a page is faulted into a VM_LOCKED vma, or found in a vma being VM_LOCKED. -VMSCAN'S HANDLING OF UNEVICTABLE PAGES +Vmscan's Handling of Unevictable Pages -------------------------------------- If unevictable pages are culled in the fault path, or moved to the unevictable @@ -233,8 +212,7 @@ extra evictabilty checks should not occur in the majority of calls to putback_lru_page(). -============= -MLOCKED PAGES +MLOCKED Pages ============= The unevictable page list is also useful for mlock(), in addition to ramfs and @@ -242,7 +220,7 @@ SYSV SHM. Note that mlock() is only available in CONFIG_MMU=y situations; in NOMMU situations, all mappings are effectively mlocked. -HISTORY +History ------- The "Unevictable mlocked Pages" infrastructure is based on work originally @@ -263,7 +241,7 @@ replaced by walking the reverse map to determine whether any VM_LOCKED VMAs mapped the page. More on this below. -BASIC MANAGEMENT +Basic Management ---------------- mlocked pages - pages mapped into a VM_LOCKED VMA - are a class of unevictable @@ -304,10 +282,10 @@ mlocked pages become unlocked and rescued from the unevictable list when: (4) before a page is COW'd in a VM_LOCKED VMA. -mlock()/mlockall() SYSTEM CALL HANDLING +mlock()/mlockall() System Call Handling --------------------------------------- -Both [do_]mlock() and [do_]mlockall() system call handlers call mlock_fixup() +Both [do\_]mlock() and [do\_]mlockall() system call handlers call mlock_fixup() for each VMA in the range specified by the call. In the case of mlockall(), this is the entire active address space of the task. Note that mlock_fixup() is used for both mlocking and munlocking a range of memory. A call to mlock() @@ -351,7 +329,7 @@ mlock_vma_page() is unable to isolate the page from the LRU, vmscan will handle it later if and when it attempts to reclaim the page. -FILTERING SPECIAL VMAS +Filtering Special VMAs ---------------------- mlock_fixup() filters several classes of "special" VMAs: @@ -379,8 +357,9 @@ VM_LOCKED flag. Therefore, we won't have to deal with them later during munlock(), munmap() or task exit. Neither does mlock_fixup() account these VMAs against the task's "locked_vm". +.. _munlock_munlockall_handling: -munlock()/munlockall() SYSTEM CALL HANDLING +munlock()/munlockall() System Call Handling ------------------------------------------- The munlock() and munlockall() system calls are handled by the same functions - @@ -426,7 +405,7 @@ This is fine, because we'll catch it later if and if vmscan tries to reclaim the page. This should be relatively rare. -MIGRATING MLOCKED PAGES +Migrating MLOCKED Pages ----------------------- A page that is being migrated has been isolated from the LRU lists and is held @@ -451,7 +430,7 @@ list because of a race between munlock and migration, page migration uses the putback_lru_page() function to add migrated pages back to the LRU. -COMPACTING MLOCKED PAGES +Compacting MLOCKED Pages ------------------------ The unevictable LRU can be scanned for compactable regions and the default @@ -461,7 +440,7 @@ unevictable LRU is enabled, the work of compaction is mostly handled by the page migration code and the same work flow as described in MIGRATING MLOCKED PAGES will apply. -MLOCKING TRANSPARENT HUGE PAGES +MLOCKING Transparent Huge Pages ------------------------------- A transparent huge page is represented by a single entry on an LRU list. @@ -483,7 +462,7 @@ to unevictable LRU and the rest can be reclaimed. See also comment in follow_trans_huge_pmd(). -mmap(MAP_LOCKED) SYSTEM CALL HANDLING +mmap(MAP_LOCKED) System Call Handling ------------------------------------- In addition the mlock()/mlockall() system calls, an application can request @@ -514,7 +493,7 @@ memory range accounted as locked_vm, as the protections could be changed later and pages allocated into that region. -munmap()/exit()/exec() SYSTEM CALL HANDLING +munmap()/exit()/exec() System Call Handling ------------------------------------------- When unmapping an mlocked region of memory, whether by an explicit call to @@ -568,16 +547,18 @@ munlock or munmap system calls, mm teardown (munlock_vma_pages_all), reclaim, holepunching, and truncation of file pages and their anonymous COWed pages. -try_to_munlock() REVERSE MAP SCAN +try_to_munlock() Reverse Map Scan --------------------------------- - [!] TODO/FIXME: a better name might be page_mlocked() - analogous to the - page_referenced() reverse map walker. +.. warning:: + [!] TODO/FIXME: a better name might be page_mlocked() - analogous to the + page_referenced() reverse map walker. -When munlock_vma_page() [see section "munlock()/munlockall() System Call -Handling" above] tries to munlock a page, it needs to determine whether or not -the page is mapped by any VM_LOCKED VMA without actually attempting to unmap -all PTEs from the page. For this purpose, the unevictable/mlock infrastructure +When munlock_vma_page() [see section :ref:`munlock()/munlockall() System Call +Handling <munlock_munlockall_handling>` above] tries to munlock a +page, it needs to determine whether or not the page is mapped by any +VM_LOCKED VMA without actually attempting to unmap all PTEs from the +page. For this purpose, the unevictable/mlock infrastructure introduced a variant of try_to_unmap() called try_to_munlock(). try_to_munlock() calls the same functions as try_to_unmap() for anonymous and @@ -595,7 +576,7 @@ large region or tearing down a large address space that has been mlocked via mlockall(), overall this is a fairly rare event. -PAGE RECLAIM IN shrink_*_list() +Page Reclaim in shrink_*_list() ------------------------------- shrink_active_list() culls any obviously unevictable pages - i.e. |