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    SubjectRe: [PATCH v2 3/3] NFS: Fix a memory leak in nfs_readdir
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    On Wed, 2010-12-01 at 15:31 -0800, Linus Torvalds wrote:
    > On Wed, Dec 1, 2010 at 2:38 PM, Andrew Morton <akpm@linux-foundation.org> wrote:
    > >
    > > OK, the stop_machine() plugs a lot of potential race-vs-module-unload
    > > things. But Trond is referring to races against vmscan inode reclaim,
    > > unmount, etc.
    >
    > So?
    >
    > A filesystem module cannot be unloaded while it's still mounted.
    >
    > And unmount doesn't succeed until all inodes are gone.
    >
    > And getting rid of an inode doesn't succeed until all pages associated
    > with it are gone.
    >
    > And getting rid of the pages involves locking them (whether in
    > truncate or vmscan) and removing them from all lists.
    >
    > Ergo: vmscan has a locked page leads to the filesystem being
    > guaranteed to not be unmounted. And that, in turn, guarantees that
    > the module won't be unloaded until the machine has gone through an
    > idle cycle.
    >
    > It really is that simple. There's nothing subtle there. The reason
    > spin_unlock(&mapping->tree_lock) is safe is exactly the above trivial
    > chain of dependencies. And it's also exactly why
    > mapping->a_ops->freepage() would also be safe.
    >
    > This is pretty much how all the module races are handled. Doing module
    > ref-counts per page (or per packet in flight for things like
    > networking) would be prohibitively expensive. There's no way we can
    > ever do that.

    Although the page is locked, it may no longer be visible to the lockless
    page lookup once the radix_tree_delete() completes in
    __remove_from_page_cache.
    Furthermore, if the same routine causes mapping->nr_pages to go to zero
    before iput_final() hits truncate_inode_pages(), then the latter exits
    immediately.

    Both these cases would appear to allow iput_final() to release the inode
    before vmscan gets round to unlocking the mapping->tree_lock since
    truncate_inode_pages() no longer thinks it has any work to do.

    --
    Trond Myklebust
    Linux NFS client maintainer

    NetApp
    Trond.Myklebust@netapp.com
    www.netapp.com



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