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NFS: Avoid quadratic search when freeing delegations.
There are three places that walk all delegation for an nfs_client and restart whenever they find something interesting - potentially resulting in a quadratic search: If there are 10,000 uninteresting delegations followed by 10,000 interesting one, then the code skips over 100,000,000 delegations, which can take a noticeable amount of time. Of these nfs_delegation_reap_unclaimed() and nfs_reap_expired_delegations() are only called during unusual events: a server reboots or reports expired delegations, probably due to a network partition. Optimizing these is not particularly important. The third, nfs_client_return_marked_delegations(), is called periodically via nfs_expire_unreferenced_delegations(). It could cause periodic problems on a busy server. New delegations are added to the end of the list, so if there are 10,000 open files with delegations, and 10,000 more recently opened files that received delegations but are now closed, then nfs_client_return_marked_delegations() can take seconds to skip over the 10,000 open files 10,000 times. That is a waste of time. The avoid this waste a place-holder (an inode) is kept when locks are dropped, so that the place can usually be found again after taking rcu_readlock(). This place holder ensure that we find the right starting point in the list of nfs_servers, and makes is probable that we find the right starting point in the list of delegations. We might need to occasionally restart at the head of that list. It might be possible that the place_holder inode could lose its delegation separately, and then get a new one using the same (freed and then reallocated) 'struct nfs_delegation'. Were this to happen, the new delegation would be at the end of the list and we would miss returning some other delegations. This would have the effect of unnecessarily delaying the return of some unused delegations until the next time this function is called - typically 90 seconds later. As this is not a correctness issue and is vanishingly unlikely to happen, it does not seem worth addressing. Signed-off-by:NeilBrown <neilb@suse.com> Signed-off-by: