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    SubjectRe: [PATCH] memcg: fix stale swap cache leak v5
    On Thu, 30 Apr 2009 15:12:52 +0530
    Balbir Singh <balbir@linux.vnet.ibm.com> wrote:

    > * KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> [2009-04-30 18:04:26]:
    >
    > > On Thu, 30 Apr 2009 16:35:39 +0900
    > > KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:
    > >
    > > > On Thu, 30 Apr 2009 16:16:27 +0900
    > > > KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:
    > > >
    > > > > This is v5 but all codes are rewritten.
    > > > >
    > > > > After this patch, when memcg is used,
    > > > > 1. page's swapcount is checked after I/O (without locks). If the page is
    > > > > stale swap cache, freeing routine will be scheduled.
    > > > > 2. vmscan.c calls try_to_free_swap() when __remove_mapping() fails.
    > > > >
    > > > > Works well for me. no extra resources and no races.
    > > > >
    > > > > Because my office will be closed until May/7, I'll not be able to make a
    > > > > response. Posting this for showing what I think of now.
    > > > >
    > > > I found a hole immediately after posted this...sorry. plz ignore this patch/
    > > > see you again in the next month.
    > > >
    > > I'm now wondering to disable "swapin readahed" completely when memcg is used...
    > > Then, half of the problems will go away immediately.
    > > And it's not so bad to try to free swapcache if swap writeback ends. Then, another
    > > half will go away...
    > >
    >
    > Could you clarify? Will memcg not account for swapin readahead pages?
    >
    swapin-readahead pages are _not_ accounted now. (And I think _never_)
    But has race and leak swp_entry account until global LRU runs.

    "Don't do swapin-readahead, at all" will remove following race completely.
    ==
    CPU0 CPU1
    free_swap_and_cache()
    read_swapcache_async()
    ==
    swp_entry to be freed will not be read-in.

    I think there will no performance regression in _usual_ case even if no readahead.
    But has no number yet.


    Thanks,
    -Kame





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