[PATCH mmotm] memcg: fix for documentation (Re: [BUGFIX][PATCH mmotm] memcg fix swap accounting leak (v3))

[Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>]

Sorry for late reply.

> @@ -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.
>  
I've verified that charges will result in valid values by tracing source code
in all of these cases, but in case of (B) I don't think commit_charge_swapin
does memsw-- because PageSwapCache has been cleared already. swap_free does
memsw-- in this case.

I attached a fix patch.

Thanks,
Daisuke Nishimura.

===
From: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>

fix for documentation.

Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
---
 Documentation/controllers/memcg_test.txt |    6 +++---
 1 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/Documentation/controllers/memcg_test.txt b/Documentation/controllers/memcg_test.txt
index 3c1458a..08d4d3e 100644
--- a/Documentation/controllers/memcg_test.txt
+++ b/Documentation/controllers/memcg_test.txt
@@ -139,10 +139,10 @@ Under below explanation, we assume CONFIG_MEM_RES_CTRL_SWAP=y.
           ===========================================
           (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
+          (5)         0/-1     0/ 0     -1/-1    0/ 0
+          (6)          -       0/-1       -      0/-1
           ===========================================
-       Result         1/ 1     1/1       1/ 1    1/ 1
+       Result         1/ 1     1/ 1      1/ 1    1/ 1
 
        In any cases, charges to this page should be 1/ 1.
 
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