Re: [PATCH] fix leak of swap accounting as stale swap cache undermemcg

[KAMEZAWA Hiroyuki]

On Tue, 28 Apr 2009 10:09:30 +0900
nishimura@xxxxxxxxxxxxxxxxx wrote:

> > On Mon, 27 Apr 2009 21:08:56 +0900
> > Daisuke Nishimura <d-nishimura@xxxxxxxxxxxxxxxxx> wrote:
> > 
> >> > Index: mmotm-2.6.30-Apr24/mm/vmscan.c
> >> > ===================================================================
> >> > --- mmotm-2.6.30-Apr24.orig/mm/vmscan.c
> >> > +++ mmotm-2.6.30-Apr24/mm/vmscan.c
> >> > @@ -661,6 +661,9 @@ static unsigned long shrink_page_list(st
> >> >  		if (PageAnon(page) && !PageSwapCache(page)) {
> >> >  			if (!(sc->gfp_mask & __GFP_IO))
> >> >  				goto keep_locked;
> >> > +			/* avoid making more stale swap caches */
> >> > +			if (memcg_stale_swap_congestion())
> >> > +				goto keep_locked;
> >> >  			if (!add_to_swap(page))
> >> >  				goto activate_locked;
> >> >  			may_enter_fs = 1;
> >> > 
> >> Well, as I mentioned before(http://marc.info/?l=linux-kernel&m=124066623510867&w=2),
> >> this cannot avoid type-2(set !PageCgroupUsed by the owner process via
> >> page_remove_rmap()->mem_cgroup_uncharge_page() before being added to swap cache).
> >> If these swap caches go through shrink_page_list() without beeing freed
> >> for some reason, these swap caches doesn't go back to memcg's LRU.
> >> 
> >> Type-2 doesn't pressure memsw.usage, but you can see it by plotting
> >> "grep SwapCached /proc/meminfo".
> >> 
> >> And I don't think it's a good idea to add memcg_stale_swap_congestion() here.
> >> This means less possibility to reclaim pages.
> >> 
> > Hmm. maybe adding congestion_wait() ?
> > 
> I don't think no hook before add_to_swap() is needed.
> 
> >> Do you dislike the patch I attached in the above mail ?
> >> 
> > I doubt whether the patch covers all type-2 case.
> > 
> hmm, I didn't see any leak anymore when I tested the patch.
> 

At first, your patch
==
 		if (PageAnon(page) && !PageSwapCache(page)) {
 			if (!(sc->gfp_mask & __GFP_IO))
 				goto keep_locked;
-			/* avoid making more stale swap caches */
-			if (memcg_stale_swap_congestion())
-				goto keep_locked;
 			if (!add_to_swap(page))
 				goto activate_locked;
+			/*
+			 * The owner process might have uncharged the page
+			 * (by page_remove_rmap()) before it has been added
+			 * to swap cache.
+			 * Check it here to avoid making it stale.
+			 */
+			if (memcg_free_unused_swapcache(page))
+				goto keep_locked;
 			may_enter_fs = 1;
 		}
==
Should be
==

	if (PageAnon(page) && !PageSwapCache(page)) {
		... // don't modify here
	}
	if (PageAnon(page) && PageSwapCache(page) && !page_mapped(page)) {
		if (try_to_free_page(page)) // or memcg_free_unused_swapcache()
			goto free_it;
	}
==
I think.

And we need hook to free_swap_and_cache() for handling PageWriteback() case.


> But because of machine time limit, I could only test for about 3 hours.
> (I had seen some leak at that point before applying my patch)
> I'll test for longer time if possible.
> 
Sigh, my work time is also limited for these months ;(

> > I'll consider again and post v3.
> > But I'll go into a series of holidays, so, may not come back until May/6.
> > 
> It's the same for me :)
> 
Enjoy good holidays :)

Thanks,
-Kame




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