On Fri, Mar 04, 2022 at 12:19:12PM -0500, Johannes Weiner wrote:
MADV_DONTNEED historically rejects mlocked ranges, but with MLOCK_ONFAULT and MCL_ONFAULT allowing to mlock without populating, there are valid use cases for depopulating locked ranges as well.
Users mlock memory to protect secrets. There are allocators for secure buffers that want in-use memory generally mlocked, but cleared and invalidated memory to give up the physical pages. This could be done with explicit munlock -> mlock calls on free -> alloc of course, but that adds two unnecessary syscalls, heavy mmap_sem write locks, vma splits and re-merges - only to get rid of the backing pages.
Users also mlockall(MCL_ONFAULT) to suppress sustained paging, but are okay with on-demand initial population. It seems valid to selectively free some memory during the lifetime of such a process, without having to mess with its overall policy.
Why add a separate flag? Isn't this a pretty niche usecase?
- MADV_DONTNEED has been bailing on locked vmas forever. It's at least conceivable that someone, somewhere is relying on mlock to protect data from perhaps broader invalidation calls. Changing this behavior now could lead to quiet data corruption.
- It also clarifies expectations around MADV_FREE and maybe MADV_REMOVE. It avoids the situation where one quietly behaves different than the others. MADV_FREE_LOCKED can be added later.
- The combination of mlock() and madvise() in the first place is probably niche. But where it happens, I'd say that dropping pages from a locked region once they don't contain secrets or won't page anymore is much saner than relying on mlock to protect memory from speculative or errant invalidation calls. It's just that we can't change the default behavior because of the two previous points.
Given that, an explicit new flag seems to make the most sense.
Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx> Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Reviewed-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
