Re: [RFC][PATCH 14/14]memcg: mem+swap accounting

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Wed, 3 Sep 2008 15:23:14 +0900
Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> wrote:

> On Tue, 2 Sep 2008 20:40:53 +0900, KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:
> > maybe like this.
> > swap_cgroup_set_account()
> >    -> mem_cgroup_uncharge()
> >         -> the page is mapped ..no uncharge here.
> >               -> then, res.page, res.swaps is not changed.
> >                  -> we should mark swap account as false.
> > 
> > Anyway, thank you! I'll consider this again.
> > 
> I add some debug code to show why the uncharge fails there.
> 
> It showed that the cause of uncharge failure at swapoff was that
> it tried to free a mapped page, as you said.
> 
> I think this can happen in the sequence below:
> 
>   try_to_unuse()
>     unuse_mm()
>       ...
>       unuse_pte()
>         mem_cgroup_charge()
>           swap_cgroup_delete_account()
>             - swap_cgroup->count is turned off.
>         page_add_anon_rmap()
>           - map page.
>     ...
>     delete_from_swap_cache()
>       swap_cgroup_delete_swapcache()
>         - turns swap_cgroup->count on again.
>         - tries to uncharge a mapped page.
> 
> And I think deleting a swapcache of a mapped page can also happen
> in other sequences(e.g. do_swap_page()->remove_exclusive_swap_cache()).
> 
> OTOH, as for shmem/tmpfs, swap_cgroup_delete_swapcache() tries to
> uncharge a page which is on radix tree(that's why I saw over-uncharging),
> because shmem_getpage() calls add_to_page_cache() before
> delete_from_swap_cache().
> 
> 
> So, I think current implementation should be changed anyway.
> 
Thank you for your investigation. I'll refine logic.

I think I have to tune lockless** series first. It's almost done.
mem+swap patch is also maintained but my usual 8cpu host is
still under maintaince. (and the newest mmtom doesn't work on 2cpu my machine.)
So, please be patient for a while if no updates from me.

Of course, if you have your own, please post. I think kmap_atomic()
logic in my patch is also benefial to your original version.

Thanks,
-Kame

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