[PATCH v2] mremap.2: Add information for MREMAP_DONTUNMAP.

[Brian Geffon <bgeffon@google.com>]

Signed-off-by: Brian Geffon <bgeffon@google.com>
---
 man2/mremap.2 | 49 +++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 49 insertions(+)

diff --git a/man2/mremap.2 b/man2/mremap.2
index d73fb64fa..ff5939ff1 100644
--- a/man2/mremap.2
+++ b/man2/mremap.2
@@ -129,6 +129,22 @@ If
 is specified, then
 .B MREMAP_MAYMOVE
 must also be specified.
+.TP
+.BR MREMAP_DONTUNMAP " (since Linux 5.7)"
+.\" commit e346b3813067d4b17383f975f197a9aa28a3b077
+This flag which must be used in conjunction with
+.B MREMAP_MAYMOVE
+remaps a mapping to a new address and it does not unmap the mapping at
+.BR old_address .
+This flag can only be used with private anonymous mappings.
+Any access to the range specified at
+.BR old_address
+after completion will result in an anonymous page fault.
+The anonymous page fault will be handled by a
+.BR userfaultfd (2)
+if the range was previously registered on the mapping specified by
+.BR old_address .
+Otherwise, it will be zero filled by the kernel.
 .PP
 If the memory segment specified by
 .I old_address
@@ -176,6 +192,8 @@ a value other than
 .B MREMAP_MAYMOVE
 or
 .B MREMAP_FIXED
+or
+.B MREMAP_DONTUNMAP
 was specified in
 .IR flags ;
 .IP *
@@ -197,9 +215,22 @@ and
 .IR old_size ;
 .IP *
 .B MREMAP_FIXED
+or
+.B MREMAP_DONTUNMAP
 was specified without also specifying
 .BR MREMAP_MAYMOVE ;
 .IP *
+.B MREMAP_DONTUNMAP
+was specified with and
+.BR old_address
+that was not private anonymous;
+.IP *
+.B MREMAP_DONTUNMAP
+was specified and
+.BR old_size
+was not equal to
+.BR new_size ;
+.IP *
 \fIold_size\fP was zero and \fIold_address\fP does not refer to a
 shareable mapping (but see BUGS);
 .IP *
@@ -209,10 +240,20 @@ flag was not specified.
 .RE
 .TP
 .B ENOMEM
+Not enough memory was available to complete the operation.
+Possible causes are:
+.RS
+.IP * 3
 The memory area cannot be expanded at the current virtual address, and the
 .B MREMAP_MAYMOVE
 flag is not set in \fIflags\fP.
 Or, there is not enough (virtual) memory available.
+.IP *
+.B MREMAP_DONTUNMAP
+was used causing a new mapping to be created that would exceed the
+(virtual) memory available.
+Or, it would exceed the maximum number of allowed mappings.
+.RE
 .SH CONFORMING TO
 This call is Linux-specific, and should not be used in programs
 intended to be portable.
@@ -238,6 +279,14 @@ call will make a best effort to populate the new area but will not fail
 with
 .B ENOMEM
 if the area cannot be populated.
+.PP
+The
+.BR MREMAP_DONTUNMAP
+flag may be used to atomically move a mapping while leaving the source
+mapped.
+Possible applications for this behavior might be garbage collection or
+non-cooperative
+.BR userfaultfd (2) .
 .SH BUGS
 Before Linux 4.14,
 if
-- 
2.26.0.110.g2183baf09c-goog