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    SubjectRe: [PATCH 3/5 resend] VFS: Fix s_umount thaw/write deadlock
    On Thu, Dec 08, 2011 at 12:16:58AM +0100, Jan Kara wrote:
    > On Tue 06-12-11 06:35:44, Christoph Hellwig wrote:
    > > We make sure to not dirty any new inodes after the first phase of the
    > > freeze, so this should be a BUG_ON/WARN_ON.
    > This is not really true in presence of mmaped writes. To block mmaped
    > writes on a frozen filesystem, we need some synchronization between
    > page_mkwrite() and freezing code. Currently, to avoid any additional
    > locking overhead, we set page dirty and *then* check for filesystem being
    > frozen. Only this order can make sure either the page is written (and
    > write-protected) or the frozen check triggers and we wait... (see the
    > comment in block_page_mkwrite()). The nasty sideeffect of this is that
    > there can be dirty pages & inodes on a frozen filesystem. We are blocked in
    > the page fault of these pages so user cannot write any data to these pages
    > but still they are marked dirty.

    Can't we make the mmaps read-only during the freezing process?

    --
    Matthew Wilcox Intel Open Source Technology Centre
    "Bill, look, we understand that you're interested in selling us this
    operating system, but compare it to ours. We can't possibly take such
    a retrograde step."


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