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Date: Mon, 15 Dec 2008 14:07:26 +0530
From: Balbir Singh <balbir@...ux.vnet.ibm.com>
To: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc: Hugh Dickins <hugh@...itas.com>,
Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
"linux-mm@...ck.org" <linux-mm@...ck.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
Subject: Re: [BUGFIX][PATCH mmotm] memcg fix swap accounting leak (v3)
* KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> [2008-12-15 16:07:51]:
>
> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
>
> Fix swapin charge operation of memcg.
>
> Now, memcg has hooks to swap-out operation and checks SwapCache is really
> unused or not. That check depends on contents of struct page.
> I.e. If PageAnon(page) && page_mapped(page), the page is recoginized as
> still-in-use.
>
> Now, reuse_swap_page() calles delete_from_swap_cache() before establishment
> of any rmap. Then, in followinig sequence
>
> (Page fault with WRITE)
> try_charge() (charge += PAGESIZE)
> commit_charge() (Check page_cgroup is used or not..)
> reuse_swap_page()
> -> delete_from_swapcache()
> -> mem_cgroup_uncharge_swapcache() (charge -= PAGESIZE)
> ......
> New charge is uncharged soon....
> To avoid this, move commit_charge() after page_mapcount() goes up to 1.
> By this,
>
> try_charge() (usage += PAGESIZE)
> reuse_swap_page() (may usage -= PAGESIZE if PCG_USED is set)
> commit_charge() (If page_cgroup is not marked as PCG_USED,
> add new charge.)
> Accounting will be correct.
>
> Changelog (v2) -> (v3)
> - fixed invalid charge to swp_entry==0.
> - updated documentation.
> Changelog (v1) -> (v2)
> - fixed comment.
>
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> ---
> Documentation/controllers/memcg_test.txt | 41 +++++++++++++++++++++++++++----
> mm/memcontrol.c | 7 +++--
> mm/memory.c | 11 ++++----
> 3 files changed, 46 insertions(+), 13 deletions(-)
>
> Index: mmotm-2.6.28-Dec12/mm/memory.c
> ===================================================================
> --- mmotm-2.6.28-Dec12.orig/mm/memory.c
> +++ mmotm-2.6.28-Dec12/mm/memory.c
> @@ -2428,22 +2428,23 @@ static int do_swap_page(struct mm_struct
> * while the page is counted on swap but not yet in mapcount i.e.
> * before page_add_anon_rmap() and swap_free(); try_to_free_swap()
> * must be called after the swap_free(), or it will never succeed.
> - * And mem_cgroup_commit_charge_swapin(), which uses the swp_entry
> - * in page->private, must be called before reuse_swap_page(),
> - * which may delete_from_swap_cache().
> + * Because delete_from_swap_page() may be called by reuse_swap_page(),
> + * mem_cgroup_commit_charge_swapin() may not be able to find swp_entry
> + * in page->private. In this case, a record in swap_cgroup is silently
> + * discarded at swap_free().
> */
>
> - mem_cgroup_commit_charge_swapin(page, ptr);
> inc_mm_counter(mm, anon_rss);
> pte = mk_pte(page, vma->vm_page_prot);
> if (write_access && reuse_swap_page(page)) {
> pte = maybe_mkwrite(pte_mkdirty(pte), vma);
> write_access = 0;
> }
> -
Removal of unassociated lines, not sure if that is a good practice.
> flush_icache_page(vma, page);
> set_pte_at(mm, address, page_table, pte);
> page_add_anon_rmap(page, vma, address);
> + /* It's better to call commit-charge after rmap is established */
> + mem_cgroup_commit_charge_swapin(page, ptr);
>
Yes, it does make sense
> swap_free(entry);
> if (vm_swap_full() || (vma->vm_flags & VM_LOCKED) || PageMlocked(page))
> Index: mmotm-2.6.28-Dec12/Documentation/controllers/memcg_test.txt
> ===================================================================
> --- mmotm-2.6.28-Dec12.orig/Documentation/controllers/memcg_test.txt
> +++ mmotm-2.6.28-Dec12/Documentation/controllers/memcg_test.txt
> @@ -1,6 +1,6 @@
> Memory Resource Controller(Memcg) Implementation Memo.
> -Last Updated: 2008/12/10
> -Base Kernel Version: based on 2.6.28-rc7-mm.
> +Last Updated: 2008/12/15
> +Base Kernel Version: based on 2.6.28-rc8-mm.
>
> Because VM is getting complex (one of reasons is memcg...), memcg's behavior
> is complex. This is a document for memcg's internal behavior.
> @@ -111,9 +111,40 @@ Under below explanation, we assume CONFI
> (b) If the SwapCache has been mapped by processes, it has been
> charged already.
>
> - In case (a), we charge it. In case (b), we don't charge it.
> - (But racy state between (a) and (b) exists. We do check it.)
> - At charging, a charge recorded in swap_cgroup is moved to page_cgroup.
> + This swap-in is one of the most complicated work. In do_swap_page(),
> + following events occur when pte is unchanged.
> +
> + (1) the page (SwapCache) is looked up.
> + (2) lock_page()
> + (3) try_charge_swapin()
> + (4) reuse_swap_page() (may call delete_swap_cache())
> + (5) commit_charge_swapin()
> + (6) swap_free().
> +
> + Considering following situation for example.
> +
> + (A) The page has not been charged before (2) and reuse_swap_page()
> + doesn't call delete_from_swap_cache().
> + (B) The page has not been charged before (2) and reuse_swap_page()
> + calls delete_from_swap_cache().
> + (C) The page has been charged before (2) and reuse_swap_page() doesn't
> + call delete_from_swap_cache().
> + (D) The page has been charged before (2) and reuse_swap_page() calls
> + delete_from_swap_cache().
> +
> + memory.usage/memsw.usage changes to this page/swp_entry will be
> + Case (A) (B) (C) (D)
> + Event
> + Before (2) 0/ 1 0/ 1 1/ 1 1/ 1
> + ===========================================
> + (3) +1/+1 +1/+1 +1/+1 +1/+1
> + (4) - 0/ 0 - -1/ 0
> + (5) 0/ 1 0/-1 -1/-1 0/ 0
> + (6) - - - 0/-1
> + ===========================================
> + Result 1/ 1 1/1 1/ 1 1/ 1
> +
> + In any cases, charges to this page should be 1/ 1.
>
The documentation patch failed to apply for me
> 4.2 Swap-out.
> At swap-out, typical state transition is below.
> Index: mmotm-2.6.28-Dec12/mm/memcontrol.c
> ===================================================================
> --- mmotm-2.6.28-Dec12.orig/mm/memcontrol.c
> +++ mmotm-2.6.28-Dec12/mm/memcontrol.c
> @@ -1139,10 +1139,11 @@ void mem_cgroup_commit_charge_swapin(str
> /*
> * Now swap is on-memory. This means this page may be
> * counted both as mem and swap....double count.
> - * Fix it by uncharging from memsw. This SwapCache is stable
> - * because we're still under lock_page().
> + * Fix it by uncharging from memsw. Basically, this SwapCache is stable
> + * under lock_page(). But in do_swap_page()::memory.c, reuse_swap_page()
> + * may call delete_from_swap_cache() before reach here.
> */
> - if (do_swap_account) {
> + if (do_swap_account && PageSwapCache(page)) {
> swp_entry_t ent = {.val = page_private(page)};
> struct mem_cgroup *memcg;
> memcg = swap_cgroup_record(ent, NULL);
>
>
Looks good to me
Acked-by: Balbir Singh <balbir@...ux.vnet.ibm.com>
Tested-by: Balbir Singh <balbir@...ux.vnet.ibm.com>
--
Balbir
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