[PATCH v5 3/3] man/man2/mremap.2: describe previously undocumented shrink behaviour

[Lorenzo Stoakes]

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@xxxxxxxxxx>
---
 man/man2/mremap.2 | 31 +++++++++++++++++++++++++++++--
 1 file changed, 29 insertions(+), 2 deletions(-)

diff --git a/man/man2/mremap.2 b/man/man2/mremap.2
index 6d14bf627..53d4eda29 100644
--- a/man/man2/mremap.2
+++ b/man/man2/mremap.2
@@ -47,8 +47,35 @@ The
 .B MREMAP_DONTUNMAP
 flag may also be specified.
 .P
-If the operation is not
-simply moving mappings,
+Equally, 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.
-- 
2.50.1