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

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

On Tue, 10 Mar 2009 12:18:56 +0900
Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> wrote:

> On Tue, 10 Mar 2009 11:35:02 +0900, KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:
> > 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 ?
> > 
> I'm saying about PageSwapCache() case.
> IIUC, swapcache which has been unmapped but has not been removed from
> swapcache yet on swap-out path (so not uncharged yet) can go through
> this (D) path.
> 
Ok, then, lookup_swap_cgroup() can return NULL if swap_cgroup is cleared.
i.e. PageCgroupUsed(pc) == true.

Thank you for your continuous effort!

Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>



> 
> Thanks,
> Daisuke Nishimura.
> 

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