Re: [PATCH] memcg: allow exiting tasks to write back data to swap

[Rik van Riel <riel@surriel.com>]

On Thu, 2024-12-12 at 10:15 +1100, Balbir Singh wrote:
> On 12/12/24 02:53, Rik van Riel wrote:
> > 
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 7b3503d12aaf..03d77e93087e 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -5371,6 +5371,15 @@ bool
> > mem_cgroup_zswap_writeback_enabled(struct mem_cgroup *memcg)
> >  	if (!zswap_is_enabled())
> >  		return true;
> >  
> > +	/*
> > +	 * Always allow exiting tasks to push data to swap. A
> > process in
> > +	 * the middle of exit cannot get OOM killed, but may need
> > to push
> > +	 * uncompressible data to swap in order to get the cgroup
> > memory
> > +	 * use below the limit, and make progress with the exit.
> > +	 */
> > +	if ((current->flags & PF_EXITING) && memcg ==
> > mem_cgroup_from_task(current))
> > +		return true;
> > +
> >  	for (; memcg; memcg = parent_mem_cgroup(memcg))
> >  		if (!READ_ONCE(memcg->zswap_writeback))
> >  			return false;
> 
> Rik,
> 
> I am unable to understand the motivation here, so we want 
> mem_cgroup_zswap_writeback_enabled() to return true, it only
> returns false if a memcg in the hierarchy has zswap_writeback
> set to 0 (false). In my git-grep I can't seem to find how/why
> that may be the case. I can see memcg starts of with the value
> set to true, if CONFIG_ZSWAP is enabled.
> 
> Your changelog above makes sense, but I am unable to map it to
> the code changes.
> 

Wait, are you asking about the code that I'm
adding, or about the code that was already
there?

I want to add the code that allows zswap
writeback if the reclaiming task is exiting,
and in the same cgroup as the to be written
back memory.

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