Re: [RFC PATCH 0/2] memcg: add nomlock to avoid folios beling mlocked in a memcg

[Michal Hocko <mhocko@suse.com>]

On Sun 15-12-24 15:34:13, Yafang Shao wrote:
> Implementation Options
> ----------------------
> 
> - Solution A: Allow file caches on the unevictable list to become
>   reclaimable. 
>   This approach would require significant refactoring of the page reclaim
>   logic.
> 
> - Solution B: Prevent file caches from being moved to the unevictable list
>   during mlock and ignore the VM_LOCKED flag during page reclaim.
>   This is a more straightforward solution and is the one we have chosen.
>   If the file caches are reclaimed from the download-proxy's memcg and
>   subsequently accessed by tasks in the application’s memcg, a filemap
>   fault will occur. A new file cache will be faulted in, charged to the
>   application’s memcg, and locked there.

Both options are silently breaking userspace because a non failing mlock
doesn't give guarantees it is supposed to AFAICS. So unless I am missing
something really importanant I do not think this is an acceptable memcg
extension.
-- 
Michal Hocko
SUSE Labs