man/man2/mremap.2: Describe previously undocumented shrink behaviour

There is pre-existing logic that appears to be undocumented for an
mremap() shrink operation, where it turns out that the usual 'input
range must span a single mapping' requirement no longer applies.

In fact, it turns out that the input range specified by [old_address,
old_address + old_size) may span any number of mappings.

If shrinking in-place (that is, neither the MREMAP_FIXED nor
MREMAP_DONTUNMAP flags are specified), then the new span may also span
any number of VMAs - [old_address, old_address + new_size).

If shrinking and moving, the range specified by [old_address,
old_address + new_size) must span a single VMA.

There must be at least one VMA contained within the [old_address,
old_address + old_size) range, and old_address must be within the range
of a VMA.

Explicitly document this.

Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Message-ID: <ab2264d8c29d103d400c028f0417cada002ffc11.1754924278.git.lorenzo.stoakes@oracle.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>

diff --git a/man/man2/mremap.2 b/man/man2/mremap.2
index 65b4d5f58d4d75c5c80ecbda7ae64c856b74c6ec..2bdb5799cbe84cf58866b0ac9ce06f2f244e362f 100644 (file)
--- a/man/man2/mremap.2
+++ b/man/man2/mremap.2
@@ -47,8 +47,37 @@ The
 .B MREMAP_DONTUNMAP
 flag may also be specified.
 .P
-If the operation is not
-simply moving mappings,
+Similarly,
+if the operation performs a shrink,
+that is,
+if
+.I old_size
+is greater than
+.IR new_size ,
+then
+.I old_size
+may also span multiple mappings,
+which do not have to be
+adjacent to one another.
+If this shrink is performed
+in-place,
+that is,
+neither
+.BR MREMAP_FIXED ,
+nor
+.B MREMAP_DONTUNMAP
+are specified,
+.I new_size
+may also span multiple VMAs.
+However, if the range is moved,
+then
+.I new_size
+must span only a single mapping.
+.P
+If the operation is neither a
+.B MREMAP_FIXED
+move
+nor a shrink,
 then
 .I old_size
 must span only a single mapping.