Re: [RFC PATCH] futex: Remove requirement for lock_page in get_futex_key

[Chris Mason]

Quoting Mel Gorman (2013-10-29 13:38:14)
> Thomas Gleixner and Peter Zijlstra discussed off-list that real-time users
> currently have a problem with the page lock being contended for unbounded
> periods of time during futex operations. The three of us discussed the
> possibiltity that the page lock is unnecessary in this case because we are
> not concerned with the usual races with reclaim and page cache updates. For
> anonymous pages, the associated futex object is the mm_struct which does
> not require the page lock. For inodes, we should be able to check under
> RCU read lock if the page mapping is still valid to take a reference to
> the inode.  This just leaves one rare race that requires the page lock
> in the slow path. This patch does not completely eliminate the page lock
> but it should reduce contention in the majority of cases.
> 
> Patch boots and futextest did not explode but I did no comparison
> performance tests. Thomas, do you have details of the workload that
> drove you to examine this problem? Alternatively, can you test it and
> see does it help you? I added Chris to the To list because he mentioned
> that some filesystems might already be doing tricks similar to this
> patch that are worth copying.

Unfortunately, all the special cases I see in the filesystems either
have an inode ref or are trylocking the page to safety.

XFS is a special case because they have their own inode cache, but by my
reading they are still using i_count and free by rcu.

The iput in here is a little tricky:

> 
> +               /* Should be impossible but lets be paranoid for now */
> +               if (WARN_ON(inode->i_mapping != mapping)) {
> +                       rcu_read_unlock();
> +                       iput(inode);
> +                       put_page(page_head);
> +                       goto again;
> +               }
> +

Once you call iput, you add the potential to call the filesystem unlink
operation if i_nlink had gone to zero.  This shouldn't be a problem
since you've dropped the rcu lock, but just for fun I'd move the
put_page up a line.

Or, change it to a BUG_ON instead, it really should be impossible.

-chris

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