Lightweight guard region support has been added to Linux 6.13, which adds MADV_GUARD_INSTALL and MADV_GUARD_REMOVE flags to the madvise() system call. Therefore, update the manpage for madvise() and describe these operations. Reviewed-by: Jann Horn <jannh@xxxxxxxxxx> Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx> --- v5: * Prefer 'replaced' to 'cleared' when discussing existing mappings that have MADV_GUARD_INSTALL applied to them, as suggested by Vlastimil. * Included small changes Alejandro applied to patch. v4: * Reference function chapters as per Alejandro. * Minor rewording as per Alejandro. https://lore.kernel.org/all/20241205104125.67518-1-lorenzo.stoakes@xxxxxxxxxx v3: * Don't describe SIGSEGV as a fatal signal as per Jann. https://lore.kernel.org/all/20241202165829.72121-1-lorenzo.stoakes@xxxxxxxxxx v2: * Updated to use semantic newlines as suggested by Alejandro. * Avoided emboldening parens as suggested by Alejandro. * One very minor grammatical fix. https://lore.kernel.org/all/20241129155943.85215-1-lorenzo.stoakes@xxxxxxxxxx v1: https://lore.kernel.org/all/20241129093205.8664-1-lorenzo.stoakes@xxxxxxxxxx man/man2/madvise.2 | 103 +++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 103 insertions(+) diff --git a/man/man2/madvise.2 b/man/man2/madvise.2 index 0dd481d21..c9b4722db 100644 --- a/man/man2/madvise.2 +++ b/man/man2/madvise.2 @@ -676,6 +676,101 @@ or secret memory regions created using Note that with .BR MADV_POPULATE_WRITE , the process can be killed at any moment when the system runs out of memory. +.TP +.BR MADV_GUARD_INSTALL " (since Linux 6.13)" +Install a lightweight guard region +into the range specified by +.I addr +and +.IR size , +causing any read or write in the range to result in a +.B SIGSEGV +signal being raised. +.IP +If the region maps memory pages +those mappings will be replaced +as part of the operation, +though if +.B MADV_GUARD_INSTALL +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. +An +.B EINVAL +error is returned if it is attempted on any other kind of mapping. +.IP +This operation is more efficient than mapping a new region of memory +.BR PROT_NONE , +as it does not require the establishment of new mappings. +Instead, +regions of an existing mapping +simply have their page tables +manipulated to establish the desired behavior. +No additional memory is used. +.IP +Lightweight guard regions remain on fork +(except for any parts which have had +.B MADV_WIPEONFORK +applied to them), +and are not removed by +.BR MADV_DONTNEED , +.BR MADV_FREE , +.BR MADV_PAGEOUT , +or +.BR MADV_COLD . +.IP +Attempting to +.BR mlock (2) +lightweight guard regions will fail, +as will +.B MADV_POPULATE_READ +or +.BR MADV_POPULATE_WRITE . +.IP +If the mapping has its attributes changed, +or is split or partially unmapped, +any existing guard regions remain in place +(except if they are unmapped). +.IP +If a mapping is moved using +.BR mremap (2), +lightweight guard regions are moved with it. +.IP +Lightweight guard regions are removed when unmapped, +on process teardown, +or when the +.B MADV_GUARD_REMOVE +operation is applied to them. +.TP +.BR MADV_GUARD_REMOVE " (since Linux 6.13)" +Remove any lightweight guard regions +which exist in the range specified by +.I addr +and +.IR size . +.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, +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. +.IP +If any transparent huge pages are encountered in the operation, +they are left in place. .SH RETURN VALUE On success, .BR madvise () @@ -794,6 +889,14 @@ or or secret memory regions created using .BR memfd_secret(2) . .TP +.B EINVAL +.I advice +is +.B MADV_GUARD_INSTALL +or +.BR MADV_GUARD_REMOVE , +but the specified address range contains an unsupported mapping. +.TP .B EIO (for .BR MADV_WILLNEED ) -- 2.47.1
