From 9b012a29a300ea780d919205906d00d15cc6286e Mon Sep 17 00:00:00 2001 From: Michal Hocko Date: Wed, 24 Jun 2015 16:57:50 -0700 Subject: [PATCH] Documentation/vm/unevictable-lru.txt: clarify MAP_LOCKED behavior There is a very subtle difference between mmap()+mlock() vs mmap(MAP_LOCKED) semantic. The former one fails if the population of the area fails while the later one doesn't. This basically means that mmap(MAPLOCKED) areas might see major fault after mmap syscall returns which is not the case for mlock. mmap man page has already been altered but Documentation/vm/unevictable-lru.txt deserves a clarification as well. Signed-off-by: Michal Hocko Reported-by: David Rientjes Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- Documentation/vm/unevictable-lru.txt | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-) diff --git a/Documentation/vm/unevictable-lru.txt b/Documentation/vm/unevictable-lru.txt index 3be0bfc4738d..32ee3a67dba2 100644 --- a/Documentation/vm/unevictable-lru.txt +++ b/Documentation/vm/unevictable-lru.txt @@ -467,7 +467,13 @@ mmap(MAP_LOCKED) SYSTEM CALL HANDLING In addition the mlock()/mlockall() system calls, an application can request that a region of memory be mlocked supplying the MAP_LOCKED flag to the mmap() -call. Furthermore, any mmap() call or brk() call that expands the heap by a +call. There is one important and subtle difference here, though. mmap() + mlock() +will fail if the range cannot be faulted in (e.g. because mm_populate fails) +and returns with ENOMEM while mmap(MAP_LOCKED) will not fail. The mmaped +area will still have properties of the locked area - aka. pages will not get +swapped out - but major page faults to fault memory in might still happen. + +Furthermore, any mmap() call or brk() call that expands the heap by a task that has previously called mlockall() with the MCL_FUTURE flag will result in the newly mapped memory being mlocked. Before the unevictable/mlock changes, the kernel simply called make_pages_present() to allocate pages and