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

[Michal Hocko <mhocko@suse.com>]

On Mon 06-01-25 21:59:07, Yafang Shao wrote:
> On Mon, Jan 6, 2025 at 8:28 PM Michal Hocko <mhocko@suse.com> wrote:
[...]
> > The purpose of mlock syscall is to _guarantee_ memory to be resident
> > (never swapped out). There might be additional constrains to prevent
> > from mlock succeeding - e.g. rlimit or if memcg aims to control amount
> > of the mlocked memory but those failures need to be explicitly
> > communicated via syscall failure.
> 
> Returning an error code like EBUSY to userspace is straightforward
> when attempting to mlock a page that is charged to a different memcg.

EAGAIN is already documented error failure when some pages cannot be
mlocked, but yes this would be acceptable solution. The question is how
to handle mlockall(MCL_FUTURE) resp. mlock(MLOCK_ONFAULT). I didn't give
those much time but I can see some comlications there. Either we fail
those on shared resources which could lead to pre-mature failures or we
need to somehow record lock ownership and enforce it during the fault
(charge).
-- 
Michal Hocko
SUSE Labs