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btrfs: convert snapshot/nocow exlcusion to drew lock
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This patch removes all haphazard code implementing nocow writers
exclusion from pending snapshot creation and switches to using the drew
lock to ensure this invariant still holds.

'Readers' are snapshot creators from create_snapshot and 'writers' are
nocow writers from buffered write path or btrfs_setsize. This locking
scheme allows for multiple snapshots to happen while any nocow writers
are blocked, since writes to page cache in the nocow path will make
snapshots inconsistent.

So for performance reasons we'd like to have the ability to run multiple
concurrent snapshots and also favors readers in this case. And in case
there aren't pending snapshots (which will be the majority of the cases)
we rely on the percpu's writers counter to avoid cacheline contention.

The main gain from using the drew lock is it's now a lot easier to
reason about the guarantees of the locking scheme and whether there is
some silent breakage lurking.

Signed-off-by: Nikolay Borisov <[email protected]>
Reviewed-by: David Sterba <[email protected]>
Signed-off-by: David Sterba <[email protected]>
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@lorddoskias @kdave
lorddoskias authored and kdave committed Mar 23, 2020
1 parent 2992df7 commit dcc3eb9
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Showing 6 changed files with 25 additions and 103 deletions.
10 changes: 3 additions & 7 deletions fs/btrfs/ctree.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -957,11 +957,6 @@ static inline struct btrfs_fs_info *btrfs_sb(struct super_block *sb)
return sb->s_fs_info;
}

struct btrfs_subvolume_writers {
struct percpu_counter counter;
wait_queue_head_t wait;
};

/*
* The state of btrfs root
*/
Expand Down Expand Up @@ -1133,8 +1128,9 @@ struct btrfs_root {
* root_item_lock.
*/
int dedupe_in_progress;
struct btrfs_subvolume_writers *subv_writers;
atomic_t will_be_snapshotted;
/* For exclusion of snapshot creation and nocow writes */
struct btrfs_drew_lock snapshot_lock;

atomic_t snapshot_force_cow;

/* For qgroup metadata reserved space */
Expand Down
45 changes: 10 additions & 35 deletions fs/btrfs/disk-io.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -1104,32 +1104,6 @@ void btrfs_clean_tree_block(struct extent_buffer *buf)
}
}

static struct btrfs_subvolume_writers *btrfs_alloc_subvolume_writers(void)
{
struct btrfs_subvolume_writers *writers;
int ret;

writers = kmalloc(sizeof(*writers), GFP_NOFS);
if (!writers)
return ERR_PTR(-ENOMEM);

ret = percpu_counter_init(&writers->counter, 0, GFP_NOFS);
if (ret < 0) {
kfree(writers);
return ERR_PTR(ret);
}

init_waitqueue_head(&writers->wait);
return writers;
}

static void
btrfs_free_subvolume_writers(struct btrfs_subvolume_writers *writers)
{
percpu_counter_destroy(&writers->counter);
kfree(writers);
}

static void __setup_root(struct btrfs_root *root, struct btrfs_fs_info *fs_info,
u64 objectid)
{
Expand Down Expand Up @@ -1178,7 +1152,6 @@ static void __setup_root(struct btrfs_root *root, struct btrfs_fs_info *fs_info,
atomic_set(&root->log_writers, 0);
atomic_set(&root->log_batch, 0);
refcount_set(&root->refs, 1);
atomic_set(&root->will_be_snapshotted, 0);
atomic_set(&root->snapshot_force_cow, 0);
atomic_set(&root->nr_swapfiles, 0);
root->log_transid = 0;
Expand Down Expand Up @@ -1450,7 +1423,7 @@ struct btrfs_root *btrfs_read_tree_root(struct btrfs_root *tree_root,
static int btrfs_init_fs_root(struct btrfs_root *root)
{
int ret;
struct btrfs_subvolume_writers *writers;
unsigned int nofs_flag;

root->free_ino_ctl = kzalloc(sizeof(*root->free_ino_ctl), GFP_NOFS);
root->free_ino_pinned = kzalloc(sizeof(*root->free_ino_pinned),
Expand All @@ -1460,12 +1433,15 @@ static int btrfs_init_fs_root(struct btrfs_root *root)
goto fail;
}

writers = btrfs_alloc_subvolume_writers();
if (IS_ERR(writers)) {
ret = PTR_ERR(writers);
/*
* We might be called under a transaction (e.g. indirect backref
* resolution) which could deadlock if it triggers memory reclaim
*/
nofs_flag = memalloc_nofs_save();
ret = btrfs_drew_lock_init(&root->snapshot_lock);
memalloc_nofs_restore(nofs_flag);
if (ret)
goto fail;
}
root->subv_writers = writers;

if (root->root_key.objectid != BTRFS_TREE_LOG_OBJECTID) {
set_bit(BTRFS_ROOT_REF_COWS, &root->state);
Expand Down Expand Up @@ -3961,8 +3937,7 @@ void btrfs_free_fs_root(struct btrfs_root *root)
WARN_ON(!RB_EMPTY_ROOT(&root->inode_tree));
if (root->anon_dev)
free_anon_bdev(root->anon_dev);
if (root->subv_writers)
btrfs_free_subvolume_writers(root->subv_writers);
btrfs_drew_lock_destroy(&root->snapshot_lock);
free_extent_buffer(root->node);
free_extent_buffer(root->commit_root);
kfree(root->free_ino_ctl);
Expand Down
44 changes: 0 additions & 44 deletions fs/btrfs/extent-tree.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -5740,47 +5740,3 @@ int btrfs_trim_fs(struct btrfs_fs_info *fs_info, struct fstrim_range *range)
return bg_ret;
return dev_ret;
}

/*
* btrfs_{start,end}_write_no_snapshotting() are similar to
* mnt_{want,drop}_write(), they are used to prevent some tasks from writing
* data into the page cache through nocow before the subvolume is snapshoted,
* but flush the data into disk after the snapshot creation, or to prevent
* operations while snapshotting is ongoing and that cause the snapshot to be
* inconsistent (writes followed by expanding truncates for example).
*/
void btrfs_end_write_no_snapshotting(struct btrfs_root *root)
{
percpu_counter_dec(&root->subv_writers->counter);
cond_wake_up(&root->subv_writers->wait);
}

int btrfs_start_write_no_snapshotting(struct btrfs_root *root)
{
if (atomic_read(&root->will_be_snapshotted))
return 0;

percpu_counter_inc(&root->subv_writers->counter);
/*
* Make sure counter is updated before we check for snapshot creation.
*/
smp_mb();
if (atomic_read(&root->will_be_snapshotted)) {
btrfs_end_write_no_snapshotting(root);
return 0;
}
return 1;
}

void btrfs_wait_for_snapshot_creation(struct btrfs_root *root)
{
while (true) {
int ret;

ret = btrfs_start_write_no_snapshotting(root);
if (ret)
break;
wait_var_event(&root->will_be_snapshotted,
!atomic_read(&root->will_be_snapshotted));
}
}
11 changes: 5 additions & 6 deletions fs/btrfs/file.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -1553,8 +1553,7 @@ static noinline int check_can_nocow(struct btrfs_inode *inode, loff_t pos,
u64 num_bytes;
int ret;

ret = btrfs_start_write_no_snapshotting(root);
if (!ret)
if (!btrfs_drew_try_write_lock(&root->snapshot_lock))
return -EAGAIN;

lockstart = round_down(pos, fs_info->sectorsize);
Expand All @@ -1569,7 +1568,7 @@ static noinline int check_can_nocow(struct btrfs_inode *inode, loff_t pos,
NULL, NULL, NULL);
if (ret <= 0) {
ret = 0;
btrfs_end_write_no_snapshotting(root);
btrfs_drew_write_unlock(&root->snapshot_lock);
} else {
*write_bytes = min_t(size_t, *write_bytes ,
num_bytes - pos + lockstart);
Expand Down Expand Up @@ -1675,7 +1674,7 @@ static noinline ssize_t btrfs_buffered_write(struct kiocb *iocb,
data_reserved, pos,
write_bytes);
else
btrfs_end_write_no_snapshotting(root);
btrfs_drew_write_unlock(&root->snapshot_lock);
break;
}

Expand Down Expand Up @@ -1779,7 +1778,7 @@ static noinline ssize_t btrfs_buffered_write(struct kiocb *iocb,

release_bytes = 0;
if (only_release_metadata)
btrfs_end_write_no_snapshotting(root);
btrfs_drew_write_unlock(&root->snapshot_lock);

if (only_release_metadata && copied > 0) {
lockstart = round_down(pos,
Expand Down Expand Up @@ -1808,7 +1807,7 @@ static noinline ssize_t btrfs_buffered_write(struct kiocb *iocb,

if (release_bytes) {
if (only_release_metadata) {
btrfs_end_write_no_snapshotting(root);
btrfs_drew_write_unlock(&root->snapshot_lock);
btrfs_delalloc_release_metadata(BTRFS_I(inode),
release_bytes, true);
} else {
Expand Down
8 changes: 4 additions & 4 deletions fs/btrfs/inode.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -4726,24 +4726,24 @@ static int btrfs_setsize(struct inode *inode, struct iattr *attr)
* truncation, it must capture all writes that happened before
* this truncation.
*/
btrfs_wait_for_snapshot_creation(root);
btrfs_drew_write_lock(&root->snapshot_lock);
ret = btrfs_cont_expand(inode, oldsize, newsize);
if (ret) {
btrfs_end_write_no_snapshotting(root);
btrfs_drew_write_unlock(&root->snapshot_lock);
return ret;
}

trans = btrfs_start_transaction(root, 1);
if (IS_ERR(trans)) {
btrfs_end_write_no_snapshotting(root);
btrfs_drew_write_unlock(&root->snapshot_lock);
return PTR_ERR(trans);
}

i_size_write(inode, newsize);
btrfs_inode_safe_disk_i_size_write(inode, 0);
pagecache_isize_extended(inode, oldsize, newsize);
ret = btrfs_update_inode(trans, root, inode);
btrfs_end_write_no_snapshotting(root);
btrfs_drew_write_unlock(&root->snapshot_lock);
btrfs_end_transaction(trans);
} else {

Expand Down
10 changes: 3 additions & 7 deletions fs/btrfs/ioctl.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -791,11 +791,7 @@ static int create_snapshot(struct btrfs_root *root, struct inode *dir,
* possible. This is to avoid later writeback (running dealloc) to
* fallback to COW mode and unexpectedly fail with ENOSPC.
*/
atomic_inc(&root->will_be_snapshotted);
smp_mb__after_atomic();
/* wait for no snapshot writes */
wait_event(root->subv_writers->wait,
percpu_counter_sum(&root->subv_writers->counter) == 0);
btrfs_drew_read_lock(&root->snapshot_lock);

ret = btrfs_start_delalloc_snapshot(root);
if (ret)
Expand Down Expand Up @@ -876,8 +872,8 @@ static int create_snapshot(struct btrfs_root *root, struct inode *dir,
dec_and_free:
if (snapshot_force_cow)
atomic_dec(&root->snapshot_force_cow);
if (atomic_dec_and_test(&root->will_be_snapshotted))
wake_up_var(&root->will_be_snapshotted);
btrfs_drew_read_unlock(&root->snapshot_lock);

free_pending:
kfree(pending_snapshot->root_item);
btrfs_free_path(pending_snapshot->path);
Expand Down

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