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    SubjectRe: [PATCH 05/10] vmscan: Synchrounous lumpy reclaim use lock_page() instead trylock_page()
    Date
    > On Thu, Sep 09, 2010 at 01:13:22PM +0900, KOSAKI Motohiro wrote:
    > > > On Thu, 9 Sep 2010 12:04:48 +0900
    > > > KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:
    > > >
    > > > > On Mon, 6 Sep 2010 11:47:28 +0100
    > > > > Mel Gorman <mel@csn.ul.ie> wrote:
    > > > >
    > > > > > From: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
    > > > > >
    > > > > > With synchrounous lumpy reclaim, there is no reason to give up to reclaim
    > > > > > pages even if page is locked. This patch uses lock_page() instead of
    > > > > > trylock_page() in this case.
    > > > > >
    > > > > > Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
    > > > > > Signed-off-by: Mel Gorman <mel@csn.ul.ie>
    > > > >
    > > > > Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
    > > > >
    > > > Ah......but can't this change cause dead lock ??
    > >
    > > Yes, this patch is purely crappy. please drop. I guess I was poisoned
    > > by poisonous mushroom of Mario Bros.
    > >
    >
    > Lets be clear on what the exact dead lock conditions are. The ones I had
    > thought about when I felt this patch was ok were;
    >
    > o We are not holding the LRU lock (or any lock, we just called cond_resched())
    > o We do not have another page locked because we cannot lock multiple pages
    > o Kswapd will never be in LUMPY_MODE_SYNC so it is not getting blocked
    > o lock_page() itself is not allocating anything that we could recurse on

    True, all.

    >
    > One potential dead lock would be if the direct reclaimer held a page
    > lock and ended up here but is that situation even allowed?

    example,

    __do_fault()
    {
    (snip)
    if (unlikely(!(ret & VM_FAULT_LOCKED)))
    lock_page(vmf.page);
    else
    VM_BUG_ON(!PageLocked(vmf.page));

    /*
    * Should we do an early C-O-W break?
    */
    page = vmf.page;
    if (flags & FAULT_FLAG_WRITE) {
    if (!(vma->vm_flags & VM_SHARED)) {
    anon = 1;
    if (unlikely(anon_vma_prepare(vma))) {
    ret = VM_FAULT_OOM;
    goto out;
    }
    page = alloc_page_vma(GFP_HIGHUSER_MOVABLE,
    vma, address);


    Afaik, detailed rule is,

    o kswapd can call lock_page() because they never take page lock outside vmscan
    o if try_lock() is successed, we can call lock_page_nosync() against its page after unlock.
    because the task have gurantee of no lock taken.
    o otherwise, direct reclaimer can't call lock_page(). the task may have a lock already.

    I think.


    > I did not
    > think of an obvious example of when this would happen. Similarly,
    > deadlock situations with mmap_sem shouldn't happen unless multiple page
    > locks are being taken.
    >
    > (prepares to feel foolish)
    >
    > What did I miss?







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