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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>