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    SubjectRe: [PATCH] Memory management livelock
    On Thu, 2 Oct 2008, Andrew Morton wrote:

    > On Fri, 3 Oct 2008 13:47:21 +1000 Nick Piggin <nickpiggin@yahoo.com.au> wrote:
    >
    > > > I expect there's no solution which avoids blocking the writers at some
    > > > stage.
    > >
    > > See my other email. Something roughly like this would do the trick
    > > (hey, it actually boots and runs and does fix the problem too).
    >
    > It needs exclusion to protect all those temp tags. Is do_fsync()'s
    > i_mutex sufficient? It's qute unobvious (and unmaintainable?) that all
    > the callers of this stuff are running under that lock.

    That filemap_fdatawrite and filemap_fdatawait in fsync() aren't really
    called under i_mutex (see do_fsync).

    So the possible solutions are:

    1. Add jiffies when the page was diried and wroteback to struct page
    + no impact on locking and concurrency
    - increases the structure by 8 bytes

    2. Stop the writers when the starvation happens (what I did)
    + doesn't do any locking if the livelock doesn't happen
    - locks writers when the livelock happens (I think it's not really serious
    --- because very few people complained about the livelock, very few people
    will see performance degradation from blocking the writers).

    3. Add another bit to radix tree (what Nick did)
    + doesn't ever block writers
    - unconditionally takes the lock on fsync path and serializates concurrent
    syncs/fsyncs. Probably low overhead too ... or I don't know, is there any
    possible situation when more processes execute sync() in parallel and user
    would see degradations if those syncs were serialized?

    Mikulas


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