Re: [PATCH 4/3] mm: drop MMF_OOM_SKIP from exit_mmap

[Hugh Dickins]

On Mon, 3 Jan 2022, Michal Hocko wrote:
> On Thu 30-12-21 09:29:40, Suren Baghdasaryan wrote:
> > On Thu, Dec 30, 2021 at 12:24 AM Michal Hocko <mhocko@xxxxxxxx> wrote:
> > >
> > > It would be really great to make unlock_range oom reaper aware IMHO.
> > 
> > What exactly do you envision? Say unlock_range() knows that it's
> > racing with __oom_reap_task_mm() and that calling follow_page() is
> > unsafe without locking, what should it do?
> 
> My original plan was to make the page lock conditional and use
> trylocking from the oom reaper (aka lockless context). It is OK to
> simply bail out and leave some mlocked memory behind if there is a
> contention on a specific page. The overall objective is to free as much
> memory as possible, not all of it.
> 
> IIRC Hugh was not a fan of this approach and he has mentioned that the
> lock might not be even really needed and that the area would benefit
> from a clean up rather than oom reaper specific hacks. I do tend to
> agree with that. I just never managed to find any spare time for that
> though and heavily mlocked oom victims tend to be really rare.

I forget when that was, and what I had in mind at that time.
But yes, by now I am very sure that munlocking needs a cleanup.

And I do have that cleanup (against a much older tree), but never
the time to rebase or post or shepherd it through N revisions.

It was 22 files changed, 464 insertions, 706 deletions:
which is too much to help with this immediate oom reaper question.

I'd better not drive this discussion further off-course; but it pains
me to see munlock_vma_pages obstructing, knowing there's a better way.

I wonder: what if I were to steal time and promise to post a
rebased series against 5.17-rc1 or rc2: not support it thereafter,
but might there be someone to pick it up and shepherd it through?
But there's no answer to that, without you seeing what it's like.

Hugh