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authorDavid Rientjes <rientjes@google.com>2015-04-15 16:14:26 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2015-04-15 16:35:19 -0700
commit80d6b94bd69a7a49b52bf503ef6a841f43cf5bbb (patch)
treeae701bac5add788e58774bb009f3a335e072eee2 /Documentation
parentae7efa507dee4813ec4bcdadebeec191e247d0c9 (diff)
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mm, doc: cleanup and clarify munmap behavior for hugetlb memory
munmap(2) of hugetlb memory requires a length that is hugepage aligned, otherwise it may fail. Add this to the documentation. This also cleans up the documentation and separates it into logical units: one part refers to MAP_HUGETLB and another part refers to requirements for shared memory segments. Signed-off-by: David Rientjes <rientjes@google.com> Cc: Jonathan Corbet <corbet@lwn.net> Cc: Davide Libenzi <davidel@xmailserver.org> Cc: Luiz Capitulino <lcapitulino@redhat.com> Cc: Shuah Khan <shuahkh@osg.samsung.com> Acked-by: Hugh Dickins <hughd@google.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Joern Engel <joern@logfs.org> Cc: Jianguo Wu <wujianguo@huawei.com> Cc: Eric B Munson <emunson@akamai.com> Cc: Michael Ellerman <mpe@ellerman.id.au> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'Documentation')
-rw-r--r--Documentation/vm/hugetlbpage.txt24
1 files changed, 16 insertions, 8 deletions
diff --git a/Documentation/vm/hugetlbpage.txt b/Documentation/vm/hugetlbpage.txt
index b32b9cd50c0e..030977fb8d2d 100644
--- a/Documentation/vm/hugetlbpage.txt
+++ b/Documentation/vm/hugetlbpage.txt
@@ -302,15 +302,23 @@ file systems, write system calls are not.
Regular chown, chgrp, and chmod commands (with right permissions) could be
used to change the file attributes on hugetlbfs.
-Also, it is important to note that no such mount command is required if the
+Also, it is important to note that no such mount command is required if
applications are going to use only shmat/shmget system calls or mmap with
-MAP_HUGETLB. Users who wish to use hugetlb page via shared memory segment
-should be a member of a supplementary group and system admin needs to
-configure that gid into /proc/sys/vm/hugetlb_shm_group. It is possible for
-same or different applications to use any combination of mmaps and shm*
-calls, though the mount of filesystem will be required for using mmap calls
-without MAP_HUGETLB. For an example of how to use mmap with MAP_HUGETLB see
-map_hugetlb.c.
+MAP_HUGETLB. For an example of how to use mmap with MAP_HUGETLB see map_hugetlb
+below.
+
+Users who wish to use hugetlb memory via shared memory segment should be a
+member of a supplementary group and system admin needs to configure that gid
+into /proc/sys/vm/hugetlb_shm_group. It is possible for same or different
+applications to use any combination of mmaps and shm* calls, though the mount of
+filesystem will be required for using mmap calls without MAP_HUGETLB.
+
+Syscalls that operate on memory backed by hugetlb pages only have their lengths
+aligned to the native page size of the processor; they will normally fail with
+errno set to EINVAL or exclude hugetlb pages that extend beyond the length if
+not hugepage aligned. For example, munmap(2) will fail if memory is backed by
+a hugetlb page and the length is smaller than the hugepage size.
+
Examples
========