[RFC PATCH 1/1] ext4: Try to better reuse recently freed space

[Lukas Czerner <lczerner@redhat.com>]

Currently if the block allocator can not find the goal to allocate we
would use global goal for stream allocation. However the global goal
(s_mb_last_group and s_mb_last_start) will move further every time such
allocation appears and never move backwards.

This causes several problems in certain scenarios:

- the goal will move further and further preventing us from reusing
  space which might have been freed since then. This is ok from the file
  system point of view because we will reuse that space eventually,
  however we're allocating block from slower parts of the spinning disk
  even though it might not be necessary.
- The above also causes more serious problem for example for thinly
  provisioned storage (sparse images backed storage as well), because
  instead of reusing blocks which are already provisioned we would try
  to use new blocks. This would unnecessarily drain storage free blocks
  pool.
- This will also cause blocks to be allocated further from the given
  goal than it's necessary. Consider for example truncating, or removing
  and rewriting the file in the loop. This workload will never reuse
  freed blocks until we continually claim and free all the block in the
  file system.

Note that file systems like xfs, ext3, or btrfs does not have this
problem. This is simply caused by the notion of global pool.

Fix this by changing the global goal to be goal per inode. This will
allow us to invalidate the goal every time the inode has been truncated,
or newly created, so in those cases we would try to use the proper more
specific goal which is based on inode position.

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
---
 fs/ext4/ext4.h    |  7 ++++---
 fs/ext4/inode.c   |  8 ++++++++
 fs/ext4/mballoc.c | 20 ++++++++------------
 3 files changed, 20 insertions(+), 15 deletions(-)

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index 6ed348d..4dffa92 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -917,6 +917,10 @@ struct ext4_inode_info {
 
 	/* Precomputed uuid+inum+igen checksum for seeding inode checksums */
 	__u32 i_csum_seed;
+
+	/* where last allocation was done - for stream allocation */
+	unsigned long i_last_group;
+	unsigned long i_last_start;
 };
 
 /*
@@ -1242,9 +1246,6 @@ struct ext4_sb_info {
 	unsigned int s_mb_order2_reqs;
 	unsigned int s_mb_group_prealloc;
 	unsigned int s_max_dir_size_kb;
-	/* where last allocation was done - for stream allocation */
-	unsigned long s_mb_last_group;
-	unsigned long s_mb_last_start;
 
 	/* stats for buddy allocator */
 	atomic_t s_bal_reqs;	/* number of reqs with len > 1 */
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 0188e65..07d0434 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -3702,6 +3702,10 @@ void ext4_truncate(struct inode *inode)
 	else
 		ext4_ind_truncate(handle, inode);
 
+	/* Invalidate last allocation counters */
+	ei->i_last_group = UINT_MAX;
+	ei->i_last_start = UINT_MAX;
+
 	up_write(&ei->i_data_sem);
 
 	if (IS_SYNC(inode))
@@ -4060,6 +4064,10 @@ struct inode *ext4_iget(struct super_block *sb, unsigned long ino)
 	inode->i_generation = le32_to_cpu(raw_inode->i_generation);
 	ei->i_block_group = iloc.block_group;
 	ei->i_last_alloc_group = ~0;
+
+	/* Invalidate last allocation counters */
+	ei->i_last_group = UINT_MAX;
+	ei->i_last_start = UINT_MAX;
 	/*
 	 * NOTE! The in-memory inode i_data array is in little-endian order
 	 * even on big-endian machines: we do NOT byteswap the block numbers!
diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index a9ff5e5..6c23666 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -1591,7 +1591,6 @@ static int mb_mark_used(struct ext4_buddy *e4b, struct ext4_free_extent *ex)
 static void ext4_mb_use_best_found(struct ext4_allocation_context *ac,
 					struct ext4_buddy *e4b)
 {
-	struct ext4_sb_info *sbi = EXT4_SB(ac->ac_sb);
 	int ret;
 
 	BUG_ON(ac->ac_b_ex.fe_group != e4b->bd_group);
@@ -1622,10 +1621,8 @@ static void ext4_mb_use_best_found(struct ext4_allocation_context *ac,
 	get_page(ac->ac_buddy_page);
 	/* store last allocated for subsequent stream allocation */
 	if (ac->ac_flags & EXT4_MB_STREAM_ALLOC) {
-		spin_lock(&sbi->s_md_lock);
-		sbi->s_mb_last_group = ac->ac_f_ex.fe_group;
-		sbi->s_mb_last_start = ac->ac_f_ex.fe_start;
-		spin_unlock(&sbi->s_md_lock);
+		EXT4_I(ac->ac_inode)->i_last_group = ac->ac_f_ex.fe_group;
+		EXT4_I(ac->ac_inode)->i_last_start = ac->ac_f_ex.fe_start;
 	}
 }
 
@@ -2080,13 +2077,12 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
 			ac->ac_2order = i - 1;
 	}
 
-	/* if stream allocation is enabled, use global goal */
-	if (ac->ac_flags & EXT4_MB_STREAM_ALLOC) {
-		/* TBD: may be hot point */
-		spin_lock(&sbi->s_md_lock);
-		ac->ac_g_ex.fe_group = sbi->s_mb_last_group;
-		ac->ac_g_ex.fe_start = sbi->s_mb_last_start;
-		spin_unlock(&sbi->s_md_lock);
+	/* if stream allocation is enabled and per inode goal is
+	 * set, use it */
+	if ((ac->ac_flags & EXT4_MB_STREAM_ALLOC) &&
+	   (EXT4_I(ac->ac_inode)->i_last_start != UINT_MAX)) {
+		ac->ac_g_ex.fe_group = EXT4_I(ac->ac_inode)->i_last_group;
+		ac->ac_g_ex.fe_start = EXT4_I(ac->ac_inode)->i_last_start;
 	}
 
 	/* Let's just scan groups to find more-less suitable blocks */
-- 
1.8.3.1