Re: [RFC PATCH] mmap.2: clarify MAP_LOCKED semantic (was: Re: Should mmap MAP_LOCKED fail if mm_poppulate fails?)

[Michal Hocko]

On Wed 29-04-15 17:28:54, David Rientjes wrote:
[...]
> The wording of this begs the question on the behavior of 
> MAP_LOCKED | MAP_POPULATE since this same man page specifies that 
> accesses to memory mapped with MAP_POPULATE will not block on page faults 
> later.

Interesting. I haven't thought of this combination. The wording of
MAP_POPULATE is too strong and it really might suggest that no future
major faults will happen. And that is simply not true.
---
diff --git a/man2/mmap.2 b/man2/mmap.2
index 1486be2e96b3..c51d3f241ff9 100644
--- a/man2/mmap.2
+++ b/man2/mmap.2
@@ -284,7 +284,7 @@ private writable mappings.
 .BR MAP_POPULATE " (since Linux 2.5.46)"
 Populate (prefault) page tables for a mapping.
 For a file mapping, this causes read-ahead on the file.
-Later accesses to the mapping will not be blocked by page faults.
+This will help to reduce blocking on the page faults later.
 .BR MAP_POPULATE
 is supported for private mappings only since Linux 2.6.23.
 .TP
 
> I think Documentation/vm/unevictable-lru.txt would benefit from an update 
> under the mmap(MAP_LOCKED) section where all this can be laid out and 
> perhaps reference it from the man page?

Sure, what about the following:
---
diff --git a/Documentation/vm/unevictable-lru.txt b/Documentation/vm/unevictable-lru.txt
index 3be0bfc4738d..9106f50781ac 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 are
+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

-- 
Michal Hocko
SUSE Labs
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