Re: [patch 5/6] mm: memcg: enable memcg OOM killer only for user faults

[Johannes Weiner <hannes@cmpxchg.org>]

On Mon, Jul 29, 2013 at 03:18:36PM -0400, KOSAKI Motohiro wrote:
> (7/25/13 6:25 PM), Johannes Weiner wrote:
> > System calls and kernel faults (uaccess, gup) can handle an out of
> > memory situation gracefully and just return -ENOMEM.
> > 
> > Enable the memcg OOM killer only for user faults, where it's really
> > the only option available.
> > 
> > Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> > ---
> >   include/linux/memcontrol.h | 23 +++++++++++++++++++++++
> >   include/linux/sched.h      |  3 +++
> >   mm/filemap.c               | 11 ++++++++++-
> >   mm/memcontrol.c            |  2 +-
> >   mm/memory.c                | 40 ++++++++++++++++++++++++++++++----------
> >   5 files changed, 67 insertions(+), 12 deletions(-)
> > 
> > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> > index 7b4d9d7..9bb5eeb 100644
> > --- a/include/linux/memcontrol.h
> > +++ b/include/linux/memcontrol.h
> > @@ -125,6 +125,24 @@ extern void mem_cgroup_print_oom_info(struct mem_cgroup *memcg,
> >   extern void mem_cgroup_replace_page_cache(struct page *oldpage,
> >   					struct page *newpage);
> >   
> > +/**
> > + * mem_cgroup_xchg_may_oom - toggle the memcg OOM killer for a task
> > + * @p: task
> > + * @new: true to enable, false to disable
> > + *
> > + * Toggle whether a failed memcg charge should invoke the OOM killer
> > + * or just return -ENOMEM.  Returns the previous toggle state.
> > + */
> > +static inline bool mem_cgroup_xchg_may_oom(struct task_struct *p, bool new)
> > +{
> > +	bool old;
> > +
> > +	old = p->memcg_oom.may_oom;
> > +	p->memcg_oom.may_oom = new;
> > +
> > +	return old;
> > +}
> 
> The name of xchg strongly suggest the function use compare-swap op. So, it seems
> misleading name. I suggest just use "set_*" or something else. In linux kernel,
> many setter functions already return old value. Don't mind.

I renamed it to bool mem_cgroup_toggle_oom(bool onoff) when I
incorporated Michal's feedback, would you be okay with that?

> > diff --git a/include/linux/sched.h b/include/linux/sched.h
> > index fc09d21..4b3effc 100644
> > --- a/include/linux/sched.h
> > +++ b/include/linux/sched.h
> > @@ -1398,6 +1398,9 @@ struct task_struct {
> >   		unsigned long memsw_nr_pages; /* uncharged mem+swap usage */
> >   	} memcg_batch;
> >   	unsigned int memcg_kmem_skip_account;
> > +	struct memcg_oom_info {
> > +		unsigned int may_oom:1;
> > +	} memcg_oom;
> 
> This ":1" makes slower but doesn't diet any memory space, right? I suggest
> to use bool. If anybody need to diet in future, he may change it to bit field.
> That's ok, let's stop too early and questionable micro optimization.

It should sit in the same word as the memcg_kmem_skip_account, plus
I'm adding another bit in the next patch (in_memcg_oom), so we save
space.  It's also the OOM path, so anything but performance critical.

> > diff --git a/mm/filemap.c b/mm/filemap.c
> > index a6981fe..2932810 100644
> > --- a/mm/filemap.c
> > +++ b/mm/filemap.c
> > @@ -1617,6 +1617,7 @@ int filemap_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
> >   	struct file_ra_state *ra = &file->f_ra;
> >   	struct inode *inode = mapping->host;
> >   	pgoff_t offset = vmf->pgoff;
> > +	unsigned int may_oom;
> 
> Why don't you use bool? your mem_cgroup_xchg_may_oom() uses bool and it seems cleaner more.

Yup, forgot to convert it with the interface, I changed it to bool.

> > @@ -3851,6 +3843,34 @@ retry:
> >   	return handle_pte_fault(mm, vma, address, pte, pmd, flags);
> >   }
> >   
> > +int handle_mm_fault(struct mm_struct *mm, struct vm_area_struct *vma,
> > +		    unsigned long address, unsigned int flags)
> > +{
> > +	int ret;
> > +
> > +	__set_current_state(TASK_RUNNING);
> > +
> > +	count_vm_event(PGFAULT);
> > +	mem_cgroup_count_vm_event(mm, PGFAULT);
> > +
> > +	/* do counter updates before entering really critical section. */
> > +	check_sync_rss_stat(current);
> > +
> > +	/*
> > +	 * Enable the memcg OOM handling for faults triggered in user
> > +	 * space.  Kernel faults are handled more gracefully.
> > +	 */
> > +	if (flags & FAULT_FLAG_USER)
> > +		WARN_ON(mem_cgroup_xchg_may_oom(current, true) == true);
> 
> Please don't assume WARN_ON never erase any code. I'm not surprised if embedded
> guys replace WARN_ON with nop in future.

That would be really messed up.

But at the same time, the WARN_ON() obfuscates what's going on a
little bit, so putting it separately should make the code more
readable.  I'll change it.

Thanks for your input!

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