Re: [PATCH] mm: madvise: MADV_DONTNEED_LOCKED

[Johannes Weiner <hannes@cmpxchg.org>]

On Thu, Mar 03, 2022 at 04:29:56PM -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@cmpxchg.org>

Just for context, I found this discussion back from 2018:

https://lkml.iu.edu/hypermail/linux/kernel/1806.1/00483.html

It seems to me that the usecase wasn't really in question, but people
weren't sure about the API, and then Jason found a workaround before
the discussion really concluded. I was asked internally about this
feature, so I'm submitting another patch in this direction, but with
more thoughts on why I chose to go with a new flag. Hopefully we can
work it out this time around :-)

Thanks