man/man2/madvise.2: Update MADV_GUARD_INSTALL, MADV_GUARD_REMOVE for Linux 6.15

Lightweight guard region support has been extended in Linux 6.15,
permitting the use of these features for file-backed and read-only
mappings.

Update the description for these operations in the madvise manpage to
describe the changed behaviour.

Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Message-ID: <20250423183105.116978-1-lorenzo.stoakes@oracle.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>

diff --git a/man/man2/madvise.2 b/man/man2/madvise.2
index bd2b90b7adcaad705b02c9753b31fa04bfeb4e0e..be1ba17aeb0efc991dd969bc1ae31756c98c0030 100644 (file)
--- a/man/man2/madvise.2
+++ b/man/man2/madvise.2
@@ -697,9 +697,24 @@ is applied to regions
 containing pre-existing lightweight guard regions,
 they are left in place.
 .IP
-This operation is supported
-only for writable anonymous private mappings
-which have not been mlock'd.
+Prior to Linux 6.15,
+this operation was supported
+only for writable anonymous private mappings.
+Since Linux 6.15,
+both anonymous and file-backed mappings are supported,
+including read-only mappings.
+.IP
+The mapping must not be mlock'd,
+map hugetlb ranges,
+nor contain special mappings.
+For example,
+mappings marked with kernel-internal flags such as
+.B VM_PFNMAP
+or
+.BR VM_IO ,
+or secret memory regions created using
+.BR memfd_secret (2).
+.IP
 An
 .B EINVAL
 error is returned if it is attempted on any other kind of mapping.
@@ -756,19 +771,23 @@ and
 .IP
 All mappings in the range
 other than lightweight guard regions
-are left in place
-(including mlock'd mappings).
-The operation is,
-however,
-valid only for writable anonymous private mappings,
+are left in place.
+The operation is supported on those mappings
+permitted by
+.B MADV_GUARD_INSTALL
+in addition to mlock()'d mappings,
 returning an
 .B EINVAL
 error otherwise.
 .IP
 When lightweight guard regions are removed,
 they act as empty regions of the containing mapping.
-Since only writable anonymous private mappings are supported,
-they therefore become zero-fill-on-demand pages.
+Therefore,
+anonymous private mappings become
+zero-fill-on-demand pages,
+and file-backed mappings are repopulated with the
+memory contents from the up-to-date contents of the
+underlying mapped file.
 .IP
 If any transparent huge pages are encountered in the operation,
 they are left in place.