[PATCH v6 16/24] erofs: introduce workstation for decompression

Gao Xiang
2019-08-02T12:55:41Z

This patch introduces another concept used by decompress
subsystem called 'workstation'. It can be seen as
a sparse array that stores pointers pointed to data
structures related to the corresponding physical clusters.

All lookups are protected by RCU read lock. Besides,
reference count and spin_lock are also introduced
to manage its lifetime and serialize all update
operations.

`workstation' is currently implemented on the in-kernel
radix tree approach for backward compatibility. With the
evolution of linux kernel, it will be migrated into
new XArray implementation in the future.

Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
---
fs/erofs/internal.h | 80 +++++++++++++++++++++
fs/erofs/super.c | 4 ++
fs/erofs/utils.c | 166 +++++++++++++++++++++++++++++++++++++++++++-
3 files changed, 248 insertions(+), 2 deletions(-)

diff --git a/fs/erofs/internal.h b/fs/erofs/internal.h
index adc82993c8e9..6f0384c3f07a 100644
--- a/fs/erofs/internal.h
+++ b/fs/erofs/internal.h
@@ -66,6 +66,9 @@ struct erofs_sb_info {
struct list_head list;
struct mutex umount_mutex;

+ /* the dedicated workstation for compression */
+ struct radix_tree_root workstn_tree;
+
unsigned int shrinker_run_no;
#endif /* CONFIG_EROFS_FS_ZIP */
u32 blocks;
@@ -151,6 +154,77 @@ static inline void *erofs_kmalloc(struct erofs_sb_info *sbi,
#define set_opt(sbi, option) ((sbi)->mount_opt |= EROFS_MOUNT_##option)
#define test_opt(sbi, option) ((sbi)->mount_opt & EROFS_MOUNT_##option)

+#ifdef CONFIG_EROFS_FS_ZIP
+#define EROFS_LOCKED_MAGIC (INT_MIN | 0xE0F510CCL)
+
+/* basic unit of the workstation of a super_block */
+struct erofs_workgroup {
+ /* the workgroup index in the workstation */
+ pgoff_t index;
+
+ /* overall workgroup reference count */
+ atomic_t refcount;
+};
+
+#if defined(CONFIG_SMP)
+static inline bool erofs_workgroup_try_to_freeze(struct erofs_workgroup *grp,
+ int val)
+{
+ preempt_disable();
+ if (val != atomic_cmpxchg(&grp->refcount, val, EROFS_LOCKED_MAGIC)) {
+ preempt_enable();
+ return false;
+ }
+ return true;
+}
+
+static inline void erofs_workgroup_unfreeze(struct erofs_workgroup *grp,
+ int orig_val)
+{
+ /*
+ * other observers should notice all modifications
+ * in the freezing period.
+ */
+ smp_mb();
+ atomic_set(&grp->refcount, orig_val);
+ preempt_enable();
+}
+
+static inline int erofs_wait_on_workgroup_freezed(struct erofs_workgroup *grp)
+{
+ return atomic_cond_read_relaxed(&grp->refcount,
+ VAL != EROFS_LOCKED_MAGIC);
+}
+#else
+static inline bool erofs_workgroup_try_to_freeze(struct erofs_workgroup *grp,
+ int val)
+{
+ preempt_disable();
+ /* no need to spin on UP platforms, let's just disable preemption. */
+ if (val != atomic_read(&grp->refcount)) {
+ preempt_enable();
+ return false;
+ }
+ return true;
+}
+
+static inline void erofs_workgroup_unfreeze(struct erofs_workgroup *grp,
+ int orig_val)
+{
+ preempt_enable();
+}
+
+static inline int erofs_wait_on_workgroup_freezed(struct erofs_workgroup *grp)
+{
+ int v = atomic_read(&grp->refcount);
+
+ /* workgroup is never freezed on uniprocessor systems */
+ DBG_BUGON(v == EROFS_LOCKED_MAGIC);
+ return v;
+}
+#endif /* !CONFIG_SMP */
+#endif /* !CONFIG_EROFS_FS_ZIP */
+
/* we strictly follow PAGE_SIZE and no buffer head yet */
#define LOG_BLOCK_SIZE PAGE_SHIFT

@@ -410,6 +484,12 @@ extern const struct file_operations erofs_dir_fops;

/* utils.c */
#ifdef CONFIG_EROFS_FS_ZIP
+int erofs_workgroup_put(struct erofs_workgroup *grp);
+struct erofs_workgroup *erofs_find_workgroup(struct super_block *sb,
+ pgoff_t index, bool *tag);
+int erofs_register_workgroup(struct super_block *sb,
+ struct erofs_workgroup *grp, bool tag);
+static inline void erofs_workgroup_free_rcu(struct erofs_workgroup *grp) {}
void erofs_shrinker_register(struct super_block *sb);
void erofs_shrinker_unregister(struct super_block *sb);
int __init erofs_init_shrinker(void);
diff --git a/fs/erofs/super.c b/fs/erofs/super.c
index 09992cc3b2fd..ea8d065068fa 100644
--- a/fs/erofs/super.c
+++ b/fs/erofs/super.c
@@ -338,6 +338,10 @@ static int erofs_fill_super(struct super_block *sb, void *data, int silent)
else
sb->s_flags &= ~SB_POSIXACL;

+#ifdef CONFIG_EROFS_FS_ZIP
+ INIT_RADIX_TREE(&sbi->workstn_tree, GFP_ATOMIC);
+#endif
+
/* get the root inode */
inode = erofs_iget(sb, ROOT_NID(sbi), true);
if (IS_ERR(inode))
diff --git a/fs/erofs/utils.c b/fs/erofs/utils.c
index cab7d77c4e59..628178261056 100644
--- a/fs/erofs/utils.c
+++ b/fs/erofs/utils.c
@@ -7,11 +7,173 @@
* Created by Gao Xiang <gaoxiang25@huawei.com>
*/
#include "internal.h"
+#include <linux/pagevec.h>

#ifdef CONFIG_EROFS_FS_ZIP
/* global shrink count (for all mounted EROFS instances) */
static atomic_long_t erofs_global_shrink_cnt;

+#define __erofs_workgroup_get(grp) atomic_inc(&(grp)->refcount)
+#define __erofs_workgroup_put(grp) atomic_dec(&(grp)->refcount)
+
+static int erofs_workgroup_get(struct erofs_workgroup *grp)
+{
+ int o;
+
+repeat:
+ o = erofs_wait_on_workgroup_freezed(grp);
+ if (unlikely(o <= 0))
+ return -1;
+
+ if (unlikely(atomic_cmpxchg(&grp->refcount, o, o + 1) != o))
+ goto repeat;
+
+ /* decrease refcount paired by erofs_workgroup_put */
+ if (unlikely(o == 1))
+ atomic_long_dec(&erofs_global_shrink_cnt);
+ return 0;
+}
+
+struct erofs_workgroup *erofs_find_workgroup(struct super_block *sb,
+ pgoff_t index, bool *tag)
+{
+ struct erofs_sb_info *sbi = EROFS_SB(sb);
+ struct erofs_workgroup *grp;
+
+repeat:
+ rcu_read_lock();
+ grp = radix_tree_lookup(&sbi->workstn_tree, index);
+ if (grp) {
+ *tag = xa_pointer_tag(grp);
+ grp = xa_untag_pointer(grp);
+
+ if (erofs_workgroup_get(grp)) {
+ /* prefer to relax rcu read side */
+ rcu_read_unlock();
+ goto repeat;
+ }
+
+ DBG_BUGON(index != grp->index);
+ }
+ rcu_read_unlock();
+ return grp;
+}
+
+int erofs_register_workgroup(struct super_block *sb,
+ struct erofs_workgroup *grp,
+ bool tag)
+{
+ struct erofs_sb_info *sbi;
+ int err;
+
+ /* grp shouldn't be broken or used before */
+ if (unlikely(atomic_read(&grp->refcount) != 1)) {
+ DBG_BUGON(1);
+ return -EINVAL;
+ }
+
+ err = radix_tree_preload(GFP_NOFS);
+ if (err)
+ return err;
+
+ sbi = EROFS_SB(sb);
+ xa_lock(&sbi->workstn_tree);
+
+ grp = xa_tag_pointer(grp, tag);
+
+ /*
+ * Bump up reference count before making this workgroup
+ * visible to other users in order to avoid potential UAF
+ * without serialized by workstn_lock.
+ */
+ __erofs_workgroup_get(grp);
+
+ err = radix_tree_insert(&sbi->workstn_tree, grp->index, grp);
+ if (unlikely(err))
+ /*
+ * it's safe to decrease since the workgroup isn't visible
+ * and refcount >= 2 (cannot be freezed).
+ */
+ __erofs_workgroup_put(grp);
+
+ xa_unlock(&sbi->workstn_tree);
+ radix_tree_preload_end();
+ return err;
+}
+
+static void __erofs_workgroup_free(struct erofs_workgroup *grp)
+{
+ atomic_long_dec(&erofs_global_shrink_cnt);
+ erofs_workgroup_free_rcu(grp);
+}
+
+int erofs_workgroup_put(struct erofs_workgroup *grp)
+{
+ int count = atomic_dec_return(&grp->refcount);
+
+ if (count == 1)
+ atomic_long_inc(&erofs_global_shrink_cnt);
+ else if (!count)
+ __erofs_workgroup_free(grp);
+ return count;
+}
+
+/* for nocache case, no customized reclaim path at all */
+static bool erofs_try_to_release_workgroup(struct erofs_sb_info *sbi,
+ struct erofs_workgroup *grp,
+ bool cleanup)
+{
+ int cnt = atomic_read(&grp->refcount);
+
+ DBG_BUGON(cnt <= 0);
+ DBG_BUGON(cleanup && cnt != 1);
+
+ if (cnt > 1)
+ return false;
+
+ DBG_BUGON(xa_untag_pointer(radix_tree_delete(&sbi->workstn_tree,
+ grp->index)) != grp);
+
+ /* (rarely) could be grabbed again when freeing */
+ erofs_workgroup_put(grp);
+ return true;
+}
+
+static unsigned long erofs_shrink_workstation(struct erofs_sb_info *sbi,
+ unsigned long nr_shrink,
+ bool cleanup)
+{
+ pgoff_t first_index = 0;
+ void *batch[PAGEVEC_SIZE];
+ unsigned int freed = 0;
+
+ int i, found;
+repeat:
+ xa_lock(&sbi->workstn_tree);
+
+ found = radix_tree_gang_lookup(&sbi->workstn_tree,
+ batch, first_index, PAGEVEC_SIZE);
+
+ for (i = 0; i < found; ++i) {
+ struct erofs_workgroup *grp = xa_untag_pointer(batch[i]);
+
+ first_index = grp->index + 1;
+
+ /* try to shrink each valid workgroup */
+ if (!erofs_try_to_release_workgroup(sbi, grp, cleanup))
+ continue;
+
+ ++freed;
+ if (unlikely(!--nr_shrink))
+ break;
+ }
+ xa_unlock(&sbi->workstn_tree);
+
+ if (i && nr_shrink)
+ goto repeat;
+ return freed;
+}
+
/* protected by 'erofs_sb_list_lock' */
static unsigned int shrinker_run_no;

@@ -35,7 +197,7 @@ void erofs_shrinker_unregister(struct super_block *sb)
struct erofs_sb_info *const sbi = EROFS_SB(sb);

mutex_lock(&sbi->umount_mutex);
- /* will add shrink final handler here */
+ erofs_shrink_workstation(sbi, ~0UL, true);

spin_lock(&erofs_sb_list_lock);
list_del(&sbi->list);
@@ -84,7 +246,7 @@ static unsigned long erofs_shrink_scan(struct shrinker *shrink,
spin_unlock(&erofs_sb_list_lock);
sbi->shrinker_run_no = run_no;

- /* will add shrink handler here */
+ freed += erofs_shrink_workstation(sbi, nr, false);

spin_lock(&erofs_sb_list_lock);
/* Get the next list element before we move this one */
--
2.17.1