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    SubjectRe: [PATCH 5/7] dax: lock based on slot instead of [mapping, index]
    On Wed 17-08-16 14:25:56, Ross Zwisler wrote:
    > On Tue, Aug 16, 2016 at 11:28:16AM +0200, Jan Kara wrote:
    > > On Mon 15-08-16 13:09:16, Ross Zwisler wrote:
    > > > DAX radix tree locking currently locks entries based on the unique
    > > > combination of the 'mapping' pointer and the pgoff_t 'index' for the entry.
    > > > This works for PTEs, but as we move to PMDs we will need to have all the
    > > > offsets within the range covered by the PMD to map to the same bit lock.
    > > > To accomplish this, lock based on the 'slot' pointer in the radix tree
    > > > instead of [mapping, index].
    > >
    > > I'm not convinced this is safe. What makes the slot pointer still valid
    > > after you drop tree_lock? At least radix_tree_shrink() or
    > > radix_tree_expand() could move your slot without letting the waiter know
    > > and he would be never woken.
    > >
    > > Honza
    >
    > Yep, you're right, thanks for catching that.
    >
    > Given that we can't rely on 'slot' being stable, my next idea is to use a
    > combination of [mapping, index], but tweak 'index' so that it's always the
    > beginning of the entry. So for 4k entries we'd leave it alone, but for 2MiB
    > entries we'd mask it down to the appropriate 2MiB barrier.
    >
    > Let me hack on that for a bit, unless you've a better idea.

    No, that's what I'd do as well.

    Honza
    --
    Jan Kara <jack@suse.com>
    SUSE Labs, CR

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