ext4: Use rbtrees to manage PAs instead of inode i_prealloc_list

Currently, the kernel uses i_prealloc_list to hold all the inode
preallocations. This is known to cause degradation in performance in
workloads which perform large number of sparse writes on a single file.
This is mainly because functions like ext4_mb_normalize_request() and
ext4_mb_use_preallocated() iterate over this complete list, resulting in
slowdowns when large number of PAs are present.

Patch 27bc446e

 partially fixed this by enforcing a limit of 512 for
the inode preallocation list and adding logic to continually trim the
list if it grows above the threshold, however our testing revealed that
a hardcoded value is not suitable for all kinds of workloads.

To optimize this, add an rbtree to the inode and hold the inode
preallocations in this rbtree. This will make iterating over inode PAs
faster and scale much better than a linked list. Additionally, we also
had to remove the LRU logic that was added during trimming of the list
(in ext4_mb_release_context()) as it will add extra overhead in rbtree.
The discards now happen in the lowest-logical-offset-first order.

** Locking notes **

With the introduction of rbtree to maintain inode PAs, we can't use RCU
to walk the tree for searching since it can result in partial traversals
which might miss some nodes(or entire subtrees) while discards happen
in parallel (which happens under a lock).  Hence this patch converts the
ei->i_prealloc_lock spin_lock to rw_lock.

Almost all the codepaths that read/modify the PA rbtrees are protected
by the higher level inode->i_data_sem (except
ext4_mb_discard_group_preallocations() and ext4_clear_inode()) IIUC, the
only place we need lock protection is when one thread is reading
"searching" the PA rbtree (earlier protected under rcu_read_lock()) and
another is "deleting" the PAs in ext4_mb_discard_group_preallocations()
function (which iterates all the PAs using the grp->bb_prealloc_list and
deletes PAs from the tree without taking any inode lock (i_data_sem)).

So, this patch converts all rcu_read_lock/unlock() paths for inode list
PA to use read_lock() and all places where we were using
ei->i_prealloc_lock spinlock will now be using write_lock().

Note that this makes the fast path (searching of the right PA e.g.
ext4_mb_use_preallocated() or ext4_mb_normalize_request()), now use
read_lock() instead of rcu_read_lock/unlock().  Ths also will now block
due to slow discard path (ext4_mb_discard_group_preallocations()) which
uses write_lock().

But this is not as bad as it looks. This is because -

1. The slow path only occurs when the normal allocation failed and we
   can say that we are low on disk space.  One can argue this scenario
   won't be much frequent.

2. ext4_mb_discard_group_preallocations(), locks and unlocks the rwlock
   for deleting every individual PA.  This gives enough opportunity for
   the fast path to acquire the read_lock for searching the PA inode
   list.

Suggested-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Signed-off-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/4137bce8f6948fedd8bae134dabae24acfe699c6.1679731817.git.ojaswin@linux.ibm.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>