In the scenario of writing sparse files, the Per-inode prealloc list may
be very long, resulting in high overhead for ext4_mb_use_preallocated().
To circumvent this problem, we limit the maximum length of per-inode
prealloc list to 512 and allow users to modify it.

Signed-off-by: Chunguang Xu <redacted>
---
 fs/ext4/ext4.h        |  3 ++-
 fs/ext4/extents.c     | 10 ++++-----
 fs/ext4/file.c        |  2 +-
 fs/ext4/indirect.c    |  2 +-
 fs/ext4/inode.c       |  6 +++---
 fs/ext4/ioctl.c       |  2 +-
 fs/ext4/mballoc.c     | 57 +++++++++++++++++++++++++++++++++++++++++++++++----
 fs/ext4/mballoc.h     |  4 ++++
 fs/ext4/move_extent.c |  4 ++--
 fs/ext4/super.c       |  2 +-
 fs/ext4/sysfs.c       |  2 ++
 11 files changed, 75 insertions(+), 19 deletions(-)

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index 42f5060..68e0ebe 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h

@@ -1501,6 +1501,7 @@ struct ext4_sb_info {

     unsigned int s_mb_stats;
     unsigned int s_mb_order2_reqs;
     unsigned int s_mb_group_prealloc;
+    unsigned int s_mb_max_inode_prealloc;
     unsigned int s_max_dir_size_kb;
     /* where last allocation was done - for stream allocation */
     unsigned long s_mb_last_group;

@@ -2651,7 +2652,7 @@ extern int ext4_init_inode_table(struct super_block *sb,

 extern ext4_fsblk_t ext4_mb_new_blocks(handle_t *,
                 struct ext4_allocation_request *, int *);
 extern int ext4_mb_reserve_blocks(struct super_block *, int);
-extern void ext4_discard_preallocations(struct inode *);
+extern void ext4_discard_preallocations(struct inode *, unsigned int);
 extern int __init ext4_init_mballoc(void);
 extern void ext4_exit_mballoc(void);
 extern void ext4_free_blocks(handle_t *handle, struct inode *inode,

diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index 221f240..a40f928 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c

@@ -100,7 +100,7 @@ static int ext4_ext_trunc_restart_fn(struct inode *inode, int *dropped)

      * i_mutex. So we can safely drop the i_data_sem here.
      */
     BUG_ON(EXT4_JOURNAL(inode) == NULL);
-    ext4_discard_preallocations(inode);
+    ext4_discard_preallocations(inode, 0);
     up_write(&EXT4_I(inode)->i_data_sem);
     *dropped = 1;
     return 0;

@@ -4272,7 +4272,7 @@ int ext4_ext_map_blocks(handle_t *handle, struct inode *inode,

              * not a good idea to call discard here directly,
              * but otherwise we'd need to call it every free().
              */
-            ext4_discard_preallocations(inode);
+            ext4_discard_preallocations(inode, 0);
             if (flags & EXT4_GET_BLOCKS_DELALLOC_RESERVE)
                 fb_flags = EXT4_FREE_BLOCKS_NO_QUOT_UPDATE;
             ext4_free_blocks(handle, inode, NULL, newblock,

@@ -5299,7 +5299,7 @@ static int ext4_collapse_range(struct inode *inode, loff_t offset, loff_t len)

     }
 
     down_write(&EXT4_I(inode)->i_data_sem);
-    ext4_discard_preallocations(inode);
+    ext4_discard_preallocations(inode, 0);
 
     ret = ext4_es_remove_extent(inode, punch_start,
                     EXT_MAX_BLOCKS - punch_start);

@@ -5313,7 +5313,7 @@ static int ext4_collapse_range(struct inode *inode, loff_t offset, loff_t len)

         up_write(&EXT4_I(inode)->i_data_sem);
         goto out_stop;
     }
-    ext4_discard_preallocations(inode);
+    ext4_discard_preallocations(inode, 0);
 
     ret = ext4_ext_shift_extents(inode, handle, punch_stop,
                      punch_stop - punch_start, SHIFT_LEFT);

@@ -5445,7 +5445,7 @@ static int ext4_insert_range(struct inode *inode, loff_t offset, loff_t len)

         goto out_stop;
 
     down_write(&EXT4_I(inode)->i_data_sem);
-    ext4_discard_preallocations(inode);
+    ext4_discard_preallocations(inode, 0);
 
     path = ext4_find_extent(inode, offset_lblk, NULL, 0);
     if (IS_ERR(path)) {

diff --git a/fs/ext4/file.c b/fs/ext4/file.c
index 2a01e31..e3ab8ea 100644
--- a/fs/ext4/file.c
+++ b/fs/ext4/file.c

@@ -148,7 +148,7 @@ static int ext4_release_file(struct inode *inode, struct file *filp)

                 !EXT4_I(inode)->i_reserved_data_blocks)
     {
         down_write(&EXT4_I(inode)->i_data_sem);
-        ext4_discard_preallocations(inode);
+        ext4_discard_preallocations(inode, 0);
         up_write(&EXT4_I(inode)->i_data_sem);
     }
     if (is_dx(inode) && filp->private_data)

diff --git a/fs/ext4/indirect.c b/fs/ext4/indirect.c
index be2b66e..ec6b930 100644
--- a/fs/ext4/indirect.c
+++ b/fs/ext4/indirect.c

@@ -696,7 +696,7 @@ static int ext4_ind_trunc_restart_fn(handle_t *handle, struct inode *inode,

      * i_mutex. So we can safely drop the i_data_sem here.
      */
     BUG_ON(EXT4_JOURNAL(inode) == NULL);
-    ext4_discard_preallocations(inode);
+    ext4_discard_preallocations(inode, 0);
     up_write(&EXT4_I(inode)->i_data_sem);
     *dropped = 1;
     return 0;

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 10dd470..bb9e1cd 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c

@@ -383,7 +383,7 @@ void ext4_da_update_reserve_space(struct inode *inode,

      */
     if ((ei->i_reserved_data_blocks == 0) &&
         !inode_is_open_for_write(inode))
-        ext4_discard_preallocations(inode);
+        ext4_discard_preallocations(inode, 0);
 }
 
 static int __check_block_validity(struct inode *inode, const char *func,

@@ -4056,7 +4056,7 @@ int ext4_punch_hole(struct inode *inode, loff_t offset, loff_t length)

     if (stop_block > first_block) {
 
         down_write(&EXT4_I(inode)->i_data_sem);
-        ext4_discard_preallocations(inode);
+        ext4_discard_preallocations(inode, 0);
 
         ret = ext4_es_remove_extent(inode, first_block,
                         stop_block - first_block);

@@ -4211,7 +4211,7 @@ int ext4_truncate(struct inode *inode)

 
     down_write(&EXT4_I(inode)->i_data_sem);
 
-    ext4_discard_preallocations(inode);
+    ext4_discard_preallocations(inode, 0);
 
     if (ext4_test_inode_flag(inode, EXT4_INODE_EXTENTS))
         err = ext4_ext_truncate(handle, inode);

diff --git a/fs/ext4/ioctl.c b/fs/ext4/ioctl.c
index 999cf6a..a5fcc23 100644
--- a/fs/ext4/ioctl.c
+++ b/fs/ext4/ioctl.c

@@ -202,7 +202,7 @@ static long swap_inode_boot_loader(struct super_block *sb,

     reset_inode_seed(inode);
     reset_inode_seed(inode_bl);
 
-    ext4_discard_preallocations(inode);
+    ext4_discard_preallocations(inode, 0);
 
     err = ext4_mark_inode_dirty(handle, inode);
     if (err < 0) {

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index 4f21f34..28a139f 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c

@@ -2736,6 +2736,7 @@ int ext4_mb_init(struct super_block *sb)

     sbi->s_mb_stats = MB_DEFAULT_STATS;
     sbi->s_mb_stream_request = MB_DEFAULT_STREAM_THRESHOLD;
     sbi->s_mb_order2_reqs = MB_DEFAULT_ORDER2_REQS;
+    sbi->s_mb_max_inode_prealloc = MB_DEFAULT_MAX_INODE_PREALLOC;
     /*
      * The default group preallocation is 512, which for 4k block
      * sizes translates to 2 megabytes.  However for bigalloc file

@@ -4103,7 +4104,7 @@ static void ext4_mb_new_preallocation(struct ext4_allocation_context *ac)

  *
  * FIXME!! Make sure it is valid at all the call sites
  */
-void ext4_discard_preallocations(struct inode *inode)
+void ext4_discard_preallocations(struct inode *inode, unsigned int needed)
 {
     struct ext4_inode_info *ei = EXT4_I(inode);
     struct super_block *sb = inode->i_sb;

@@ -4121,15 +4122,18 @@ void ext4_discard_preallocations(struct inode *inode)

 
     mb_debug(sb, "discard preallocation for inode %lu\n",
          inode->i_ino);
-    trace_ext4_discard_preallocations(inode);
+    trace_ext4_discard_preallocations(inode,  needed);
 
     INIT_LIST_HEAD(&list);
 
+    if (needed == 0)
+        needed = UINT_MAX;
+
 repeat:
     /* first, collect all pa's in the inode */
     spin_lock(&ei->i_prealloc_lock);
-    while (!list_empty(&ei->i_prealloc_list)) {
-        pa = list_entry(ei->i_prealloc_list.next,
+    while (!list_empty(&ei->i_prealloc_list) && needed) {
+        pa = list_entry(ei->i_prealloc_list.prev,
                 struct ext4_prealloc_space, pa_inode_list);
         BUG_ON(pa->pa_obj_lock != &ei->i_prealloc_lock);
         spin_lock(&pa->pa_lock);

@@ -4150,6 +4154,7 @@ void ext4_discard_preallocations(struct inode *inode)

             spin_unlock(&pa->pa_lock);
             list_del_rcu(&pa->pa_inode_list);
             list_add(&pa->u.pa_tmp_list, &list);
+            needed--;
             continue;
         }
 

@@ -4549,10 +4554,42 @@ static void ext4_mb_add_n_trim(struct ext4_allocation_context *ac)

 }
 
 /*
+ * if per-inode prealloc list is too long, trim some PA
+ */
+static void
+ext4_mb_trim_inode_pa(struct inode *inode)
+{
+    struct ext4_inode_info *ei = EXT4_I(inode);
+    struct ext4_sb_info *sbi = EXT4_SB(inode->i_sb);
+    struct ext4_prealloc_space *pa;
+    int count = 0, delta;
+
+    rcu_read_lock();
+    list_for_each_entry_rcu(pa, &ei->i_prealloc_list, pa_inode_list) {
+        spin_lock(&pa->pa_lock);
+        if (pa->pa_deleted) {
+            spin_unlock(&pa->pa_lock);
+            continue;
+        }
+        count++;
+        spin_unlock(&pa->pa_lock);
+    }
+    rcu_read_unlock();
+
+    delta = (sbi->s_mb_max_inode_prealloc >> 2) + 1;
+    if (count > sbi->s_mb_max_inode_prealloc + delta) {
+        count -= sbi->s_mb_max_inode_prealloc;
+        ext4_discard_preallocations(inode, count);
+    }
+}
+
+/*
  * release all resource we used in allocation
  */
 static int ext4_mb_release_context(struct ext4_allocation_context *ac)
 {
+    struct inode *inode = ac->ac_inode;
+    struct ext4_inode_info *ei = EXT4_I(inode);
     struct ext4_sb_info *sbi = EXT4_SB(ac->ac_sb);
     struct ext4_prealloc_space *pa = ac->ac_pa;
     if (pa) {

@@ -4578,6 +4615,17 @@ static int ext4_mb_release_context(struct ext4_allocation_context *ac)

                 ext4_mb_add_n_trim(ac);
             }
         }
+
+        if (pa->pa_type == MB_INODE_PA) {
+            /*
+             * treat per-inode prealloc list as a lru list, then try
+             * to trim the least recently used PA.
+             */
+            spin_lock(pa->pa_obj_lock);
+            list_move(&ei->i_prealloc_list, &pa->pa_inode_list);
+            spin_unlock(pa->pa_obj_lock);
+        }
+
         ext4_mb_put_pa(ac, ac->ac_sb, pa);
     }
     if (ac->ac_bitmap_page)

@@ -4587,6 +4635,7 @@ static int ext4_mb_release_context(struct ext4_allocation_context *ac)

     if (ac->ac_flags & EXT4_MB_HINT_GROUP_ALLOC)
         mutex_unlock(&ac->ac_lg->lg_mutex);
     ext4_mb_collect_stats(ac);
+    ext4_mb_trim_inode_pa(inode);
     return 0;
 }
 

diff --git a/fs/ext4/mballoc.h b/fs/ext4/mballoc.h
index 6b4d17c..e75b474 100644
--- a/fs/ext4/mballoc.h
+++ b/fs/ext4/mballoc.h

@@ -73,6 +73,10 @@

  */
 #define MB_DEFAULT_GROUP_PREALLOC    512
 
+/*
+ * maximum length of inode prealloc list
+ */
+#define MB_DEFAULT_MAX_INODE_PREALLOC    512
 
 struct ext4_free_data {
     /* this links the free block information from sb_info */

diff --git a/fs/ext4/move_extent.c b/fs/ext4/move_extent.c
index 1ed86fb..0d601b8 100644
--- a/fs/ext4/move_extent.c
+++ b/fs/ext4/move_extent.c

@@ -686,8 +686,8 @@

 
 out:
     if (*moved_len) {
-        ext4_discard_preallocations(orig_inode);
-        ext4_discard_preallocations(donor_inode);
+        ext4_discard_preallocations(orig_inode, 0);
+        ext4_discard_preallocations(donor_inode, 0);
     }
 
     ext4_ext_drop_refs(path);

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 330957e..8ce61f3 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c

@@ -1216,7 +1216,7 @@ void ext4_clear_inode(struct inode *inode)

 {
     invalidate_inode_buffers(inode);
     clear_inode(inode);
-    ext4_discard_preallocations(inode);
+    ext4_discard_preallocations(inode, 0);
     ext4_es_remove_extent(inode, 0, EXT_MAX_BLOCKS);
     dquot_drop(inode);
     if (EXT4_I(inode)->jinode) {

diff --git a/fs/ext4/sysfs.c b/fs/ext4/sysfs.c
index 6c9fc9e..92f04e9 100644
--- a/fs/ext4/sysfs.c
+++ b/fs/ext4/sysfs.c

@@ -215,6 +215,7 @@ static ssize_t journal_task_show(struct ext4_sb_info *sbi, char *buf)

 EXT4_RW_ATTR_SBI_UI(mb_order2_req, s_mb_order2_reqs);
 EXT4_RW_ATTR_SBI_UI(mb_stream_req, s_mb_stream_request);
 EXT4_RW_ATTR_SBI_UI(mb_group_prealloc, s_mb_group_prealloc);
+EXT4_RW_ATTR_SBI_UI(mb_max_inode_prealloc, s_mb_max_inode_prealloc);
 EXT4_RW_ATTR_SBI_UI(extent_max_zeroout_kb, s_extent_max_zeroout_kb);
 EXT4_ATTR(trigger_fs_error, 0200, trigger_test_error);
 EXT4_RW_ATTR_SBI_UI(err_ratelimit_interval_ms, s_err_ratelimit_state.interval);

@@ -257,6 +258,7 @@ static ssize_t journal_task_show(struct ext4_sb_info *sbi, char *buf)

     ATTR_LIST(mb_order2_req),
     ATTR_LIST(mb_stream_req),
     ATTR_LIST(mb_group_prealloc),
+    ATTR_LIST(mb_max_inode_prealloc),
     ATTR_LIST(max_writeback_mb_bump),
     ATTR_LIST(extent_max_zeroout_kb),
     ATTR_LIST(trigger_fs_error),
-- 
1.8.3.1