Re: [PATCH 2/5] oom reaper: handle mlocked pages

[Hugh Dickins]

On Tue, 8 Mar 2016, Michal Hocko wrote:
> On Mon 29-02-16 14:41:39, Michal Hocko wrote:
> > On Sun 28-02-16 19:19:11, Hugh Dickins wrote:
> > > On Tue, 23 Feb 2016, Michal Hocko wrote:
> > > > On Mon 22-02-16 17:36:07, David Rientjes wrote:
> > > > > 
> > > > > Are we concerned about munlock_vma_pages_all() taking lock_page() and 
> > > > > perhaps stalling forever, the same way it would stall in exit_mmap() for 
> > > > > VM_LOCKED vmas, if another thread has locked the same page and is doing an 
> > > > > allocation?
> > > > 
> > > > This is a good question. I have checked for that particular case
> > > > previously and managed to convinced myself that this is OK(ish).
> > > > munlock_vma_pages_range locks only THP pages to prevent from the
> > > > parallel split-up AFAICS.
> > > 
> > > I think you're mistaken on that: there is also the lock_page()
> > > on every page in Phase 2 of __munlock_pagevec().
> > 
> > Ohh, I have missed that one. Thanks for pointing it out!
> > 
> > [...]
> > > > Just for the reference this is what I came up with (just compile tested).
> > > 
> > > I tried something similar internally (on an earlier kernel).  Like
> > > you I've set that work aside for now, there were quicker ways to fix
> > > the issue at hand.  But it does continue to offend me that munlock
> > > demands all those page locks: so if you don't get back to it before me,
> > > I shall eventually.
> > > 
> > > I didn't understand why you complicated yours with the "enforce"
> > > arg to munlock_vma_pages_range(): why not just trylock in all cases?
> > 
> > Well, I have to confess that I am not really sure I understand all the
> > consequences of the locking here. It has always been subtle and weird
> > issues popping up from time to time. So I only wanted to have that
> > change limitted to the oom_reaper. So I would really appreciate if
> > somebody more knowledgeable had a look. We can drop the mlock patch for
> > now.
> 
> According to the rc7 announcement it seems we are approaching the merge
> window. Should we drop the patch for now or the risk of the lockup is
> too low to care about and keep it in for now as it might be already
> useful and change the munlock path to not depend on page locks later on?
> 
> I am OK with both ways.

You're asking about the Subject patch, "oom reaper: handle mlocked pages",
I presume.  Your Work-In-Progress mods to munlock_vma_pages_range() should
certainly be dropped for now, and revisited by one of us another time.

I vote for dropping "oom reaper: handle mlocked pages" for now too.
If I understand correctly, the purpose of the oom reaper is to free up
as much memory from the targeted task as possible, while avoiding getting
stuck on locks; in advance of the task actually exiting and doing the
freeing itself, but perhaps getting stuck on locks as it does so.

If that's a fair description, then it's inappropriate for the oom reaper
to call munlock_vma_pages_all(), with the risk of getting stuck on many
page locks; best leave that risk to the task when it exits as at present.
Of course we should come back to this later, fix munlock_vma_pages_range()
with trylocks (on the pages only? rmap mutexes also?), and then integrate
"oom reaper: handle mlocked pages".

(Or if we had the old mechanism for scanning unevictable lrus on demand,
perhaps simply not avoid the VM_LOCKED vmas in __oom_reap_vmas(), let
the clear_page_mlock() in page_remove_*rmap() handle all the singly
mapped and mlocked pages, and un-mlock the rest by scanning unevictables.)

Hugh