[PATCH] madvise MADV_DONTFORK/MADV_DOFORK

Currently, copy-on-write may change the physical address of a page even if the
user requested that the page is pinned in memory (either by mlock or by
get_user_pages).  This happens if the process forks meanwhile, and the parent
writes to that page.  As a result, the page is orphaned: in case of
get_user_pages, the application will never see any data hardware DMA's into
this page after the COW.  In case of mlock'd memory, the parent is not getting
the realtime/security benefits of mlock.

In particular, this affects the Infiniband modules which do DMA from and into
user pages all the time.

This patch adds madvise options to control whether memory range is inherited
across fork.  Useful e.g.  for when hardware is doing DMA from/into these
pages.  Could also be useful to an application wanting to speed up its forks
by cutting large areas out of consideration.

Signed-off-by: Michael S. Tsirkin <mst@mellanox.co.il>
Acked-by: Hugh Dickins <hugh@veritas.com>
Cc: Michael Kerrisk <mtk-manpages@gmx.net>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 2 insertions, 0 deletions

diff --git a/include/asm-arm/mman.h b/include/asm-arm/mman.h
index f0bebca2ac21..693ed859e632 100644
--- a/include/asm-arm/mman.h
+++ b/include/asm-arm/mman.h
@@ -36,6 +36,8 @@
 #define MADV_WILLNEED   0x3             /* pre-fault pages */
 #define MADV_DONTNEED   0x4             /* discard these pages */
 #define MADV_REMOVE     0x5             /* remove these pages & resources */
+#define MADV_DONTFORK   0x30            /* dont inherit across fork */
+#define MADV_DOFORK     0x31            /* do inherit across fork */
 
 /* compatibility flags */
 #define MAP_ANON        MAP_ANONYMOUS