Messages in this thread | | | Date | Tue, 28 Apr 2009 11:38:00 +0900 | From | nishimura@mxp ... | Subject | Re: [PATCH] fix leak of swap accounting as stale swap cache under memcg |
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> On Tue, 28 Apr 2009 10:09:30 +0900 > nishimura@mxp.nes.nec.co.jp wrote: > >> > On Mon, 27 Apr 2009 21:08:56 +0900 >> > Daisuke Nishimura <d-nishimura@mtf.biglobe.ne.jp> 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. > It may work too.
But if the page is on swap cache already at the point of page_remove_rmap() -> mem_cgroup_uncharge_page, the page is not uncharged. So, it can be freed in memcg's LRU scanning in the long run by shrink_page_list()->pageout()->swap_writepage()->try_to_free_swap().
I added the hook there just because I wanted to clarify what the problematic case is.
And I don't think "goto free_it" is good. It calls free_hot_cold_page(), but some process (like swapoff) might have got the swap cache already and be waiting for the lock of the page.
> And we need hook to free_swap_and_cache() for handling PageWriteback() case. > Ah, You're right.
Thanks, Daisuke Nishimura.
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