Messages in this thread | | | From | kamezawa.hiroyu@jp ... | Date | Sat, 5 Jul 2008 17:16:22 +0900 (JST) | Subject | Re: Re: [PATCH] memcg: handle shmem's swap cache (Was 2.6.26-rc8-mm1 |
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----- Original Message ----- >Date: Sat, 05 Jul 2008 12:19:11 +0530 >From: Balbir Singh <balbir@linux.vnet.ibm.com> >To: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> >CC: Andrew Morton <akpm@linux-foundation.org>, linux-kernel@vger.kernel.org, > linux-mm@kvack.org, "hugh@veritas.com" <hugh@veritas.com>, > "nishimura@mxp.nes.nec.co.jp" <nishimura@mxp.nes.nec.co.jp>, > "yamamoto@valinux.co.jp" <yamamoto@valinux.co.jp> >Subject: Re: [PATCH] memcg: handle shmem's swap cache (Was 2.6.26-rc8-mm1 > > >KAMEZAWA Hiroyuki wrote: >> On Sat, 05 Jul 2008 11:11:10 +0530 >> Balbir Singh <balbir@linux.vnet.ibm.com> wrote: >> >>> KAMEZAWA Hiroyuki wrote: >>>> My swapcache accounting under memcg patch failed to catch tmpfs(shmem)'s one. >>>> Can I test this under -mm tree ? >>>> (If -mm is busy, I'm not in hurry.) >>>> This patch works well in my box. >>>> = >>>> SwapCache handling fix. >>>> >>>> shmem's swapcache behavior is a little different from anonymous's one and >>>> memcg failed to handle it. This patch tries to fix it. >>>> >>>> After this: >>>> >>>> Any page marked as SwapCache is not uncharged. (delelte_from_swap_cache() >>>> delete the SwapCache flag.) >>>> >>>> To check a shmem-page-cache is alive or not we use >>>> page->mapping && !PageAnon(page) instead of >>>> pc->flags & PAGE_CGROUP_FLAG_CACHE. >>>> >>>> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> >>> Though I am not opposed to this, I do sit up and think if keeping the refe rence >>> count around could avoid this complexity and from my point, the maintenanc e >>> overhead of this logic/code (I fear there might be more special cases :( ) >> >> yes, to me. but we have to fix.. >> >> But I don't like old code's refcnt handling which does >> - increment >> - does this increment was really neccesary ? >> No? ok, decrement it again. >> >> This was much more complex to me than current code. >> At first, what I have to say is "this is a fix against handle-swapcache patch not against remove-refcnt"
This complex comes from handle-swapcache. (But it's necessary.)
> >That can be redone -- the moment a page is used by a path, refcnt (increment) >it. Undo the same when the page is no longer in use. > >I expect > >rmap path to increment/decrement it on mapping >radix-tree (cache's) to do the same > > >Using a kref we should be able to get this logic right - no? > no What the old code does was
- a page is added to rmap (mapcount 0->1) +1 - a page is removed from rmap (mapcount ->0) -1 - a page is added to radix-tree (+1) - a page is removed from radix-tree (-1)
All information is recorded in struct page because it exists for. Then, why duplicates information ? It's usually bad habit.
>> And old ones will needs the check at treating swap-cache. (it couldn't but if we want) >> >>> The trade-off is complexity versus the overhead of reference counting. >>> >> refcnt was also very complex ;) > >I think that is easier to simply, instead of adding the complex checks we hav e >right now. refcnt is easier to prove as working correct than the checks.
About swap-cache, refcnt is just obstacle because you can't handle add-to-swapcache by refcnt.
If you want to add refcnt (or some code) for "debug", I have no objection.
Thanks, -Kame
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