Re: [BUGFIX][PATCH] memcg: charge swapcache to proper memcg

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

On Tue, 10 Mar 2009 10:07:07 +0900
Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> wrote:

> From: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
> 
> memcg_test.txt says at 4.1:
> 
> 	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/ 0     -1/-1    0/ 0
>           (6)          -       0/-1       -      0/-1
>           ===========================================
>        Result         1/ 1     1/ 1      1/ 1    1/ 1
> 
>        In any cases, charges to this page should be 1/ 1.
> 
> In case of (D), mem_cgroup_try_get_from_swapcache() returns NULL
> (because lookup_swap_cgroup() returns NULL), so "+1/+1" at (3) means
> charges to the memcg("foo") to which the "current" belongs.

Hmm...in try_charge_swapin(), if !PageSwapCache(),
it seems no charges and returns NULL...(means commit will not occur.)
Could you clarify ?

Regards,
-Kame

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