fork() will remove the write PTE bit from the page table on each VMA which will be copied via COW. A such such, the memory is available but marked read only in the page table and will fault on write access. This renders the previous mlock() operation almost useless because in a multi threaded application the RT thread may block on mmap_sem while the thread with low priority is holding the mmap_sem (for instance because it is allocating memory which needs to be mapped in). There is actually nothing we can do to mitigate the outcome. We could add a warning to the kernel for people that are not yet aware of the updated documentation. Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> --- man2/mlock.2 | 14 ++++++++++++++ 1 file changed, 14 insertions(+) diff --git a/man2/mlock.2 b/man2/mlock.2 index e34bb3b4e045..27f80f6664ef 100644 --- a/man2/mlock.2 +++ b/man2/mlock.2 @@ -350,6 +350,20 @@ settings are not inherited by a child created via and are cleared during an .BR execve (2). +Note that +.BR fork (2) +will prepare the address space for a copy-on-write operation. The consequence +is that any write access that follows will cause a page fault which in turn may +cause high latencies for a real-time process. Therefore it is crucial not to +invoke +.BR fork (2) +after the +.BR mlockall () +or +.BR mlock () +operation not even from thread which runs at a low priority within a process +which also has a thread running at elevated priority. + The memory lock on an address range is automatically removed if the address range is unmapped via .BR munmap (2). -- 2.9.3 -- To unsubscribe from this list: send the line "unsubscribe linux-man" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
