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mremap.2: Document MREMAP_DONTUNMAP 

Signed-off-by: Brian Geffon <bgeffon@google.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Brian Geffon 2020-04-15 09:49:49 -07:00 committed by Michael Kerrisk
parent cb76d6e5fe
commit 8debc4f595
1 changed files with 49 additions and 0 deletions
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man2/mremap.2
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@ -130,6 +130,22 @@ If
is specified, then is specified, then
.B MREMAP_MAYMOVE .B MREMAP_MAYMOVE
must also be specified. 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 .PP
If the memory segment specified by If the memory segment specified by
.I old_address .I old_address
@ -177,6 +193,8 @@ a value other than
.B MREMAP_MAYMOVE .B MREMAP_MAYMOVE
or or
.B MREMAP_FIXED .B MREMAP_FIXED
or
.B MREMAP_DONTUNMAP
was specified in was specified in
.IR flags ; .IR flags ;
.IP * .IP *
@ -198,9 +216,22 @@ and
.IR old_size ; .IR old_size ;
.IP * .IP *
.B MREMAP_FIXED .B MREMAP_FIXED
or
.B MREMAP_DONTUNMAP
was specified without also specifying was specified without also specifying
.BR MREMAP_MAYMOVE ; .BR MREMAP_MAYMOVE ;
.IP * .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 \fIold_size\fP was zero and \fIold_address\fP does not refer to a
shareable mapping (but see BUGS); shareable mapping (but see BUGS);
.IP * .IP *
@ -210,10 +241,20 @@ flag was not specified.
.RE .RE
.TP .TP
.B ENOMEM .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 The memory area cannot be expanded at the current virtual address, and the
.B MREMAP_MAYMOVE .B MREMAP_MAYMOVE
flag is not set in \fIflags\fP. flag is not set in \fIflags\fP.
Or, there is not enough (virtual) memory available. 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 .SH CONFORMING TO
This call is Linux-specific, and should not be used in programs This call is Linux-specific, and should not be used in programs
intended to be portable. intended to be portable.
@ -239,6 +280,14 @@ call will make a best effort to populate the new area but will not fail
with with
.B ENOMEM .B ENOMEM
if the area cannot be populated. 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 .SH BUGS
Before Linux 4.14, Before Linux 4.14,
if if