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Documentation/vm/unevictable-lru.txt: clarify MAP_LOCKED behavior

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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 <mhocko@suse.cz>
Reported-by: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Michal Hocko 2015-06-24 16:57:50 -07:00 committed by Linus Torvalds
parent 22cc877b32
commit 9b012a29a3
1 changed files with 7 additions and 1 deletions
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Documentation/vm/unevictable-lru.txt
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@ -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