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    SubjectRe: [PATCH 6/7] hugetlb: hugepage migration core
    On Tue, Jul 06, 2010 at 09:13:37AM +0200, Andi Kleen wrote:
    > On Tue, Jul 06, 2010 at 12:33:42PM +0900, Naoya Horiguchi wrote:
    > > > There's more code that handles LRU in this file. Do they all handle huge pages
    > > > correctly?
    > > >
    > > > I also noticed we do not always lock all sub pages in the huge page. Now if
    > > > IO happens it will lock on subpages, not the head page. But this code
    > > > handles all subpages as a unit. Could this cause locking problems?
    > > > Perhaps it would be safer to lock all sub pages always? Or would
    > > > need to audit other page users to make sure they always lock on the head
    > > > and do the same here.
    > > >
    > > > Hmm page reference counts may have the same issue?
    > >
    > > If we try to implement paging out of hugepage in the future, we need to
    > > solve all these problems straightforwardly. But at least for now we can
    > > skirt them by not touching LRU code for hugepage extension.
    > We need the page lock to avoid migrating pages that are currently
    > under IO. This can happen even without swapping when the process
    > manually starts IO.

    I see. I understood we should work on locking problem in now.
    I digged and learned hugepage IO can happen in direct IO from/to
    hugepage or coredump of hugepage user.

    We can resolve race between memory failure and IO by checking
    page lock and writeback flag, right?

    BTW I surveyed direct IO code, but page lock seems not to be taken.
    Am I missing something?
    (Before determining whether we lock all subpages or only headpage,
    I want to clarify how current code for non-hugepage resolves this problem.)

    Naoya Horiguchi

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