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Message-ID: <20130731151114.GG22930@quack.suse.cz>
Date: Wed, 31 Jul 2013 17:11:14 +0200
From: Jan Kara <jack@...e.cz>
To: Dave Chinner <david@...morbit.com>
Cc: linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
akpm@...ux-foundation.org, davej@...hat.com,
viro@...iv.linux.org.uk, jack@...e.cz, glommer@...allels.com
Subject: Re: [PATCH 06/11] bdi: add a new writeback list for sync
On Wed 31-07-13 14:15:45, Dave Chinner wrote:
> From: Dave Chinner <dchinner@...hat.com>
>
> wait_sb_inodes() current does a walk of all inodes in the filesystem
> to find dirty one to wait on during sync. This is highly
> inefficient and wastes a lot of CPU when there are lots of clean
> cached inodes that we don't need to wait on.
>
> To avoid this "all inode" walk, we need to track inodes that are
> currently under writeback that we need to wait for. We do this by
> adding inodes to a writeback list on the bdi when the mapping is
> first tagged as having pages under writeback. wait_sb_inodes() can
> then walk this list of "inodes under IO" and wait specifically just
> for the inodes that the current sync(2) needs to wait for.
>
> To avoid needing all the realted locking to be safe against
> interrupts, Jan Kara suggested that we be lazy about removal from
> the writeback list. That is, we don't remove inodes from the
> writeback list on IO completion, but do it directly during a
> wait_sb_inodes() walk.
>
> This means that the a rare sync(2) call will have some work to do
> skipping clean inodes However, in the current problem case of
> concurrent sync workloads, concurrent wait_sb_inodes() calls only
> walk the very recently dispatched inodes and hence should have very
> little work to do.
>
> This also means that we have to remove the inodes from the writeback
> list during eviction. Do this in inode_wait_for_writeback() once
> all writeback on the inode is complete.
The patch looks OK, just two minor comments below.
Honza
>
> Signed-off-by: Dave Chinner <dchinner@...hat.com>
> ---
> fs/fs-writeback.c | 110 +++++++++++++++++++++++++++++++++++++-------
> fs/inode.c | 1 +
> include/linux/backing-dev.h | 3 ++
> include/linux/fs.h | 1 +
> mm/backing-dev.c | 1 +
> mm/page-writeback.c | 14 ++++++
> 6 files changed, 114 insertions(+), 16 deletions(-)
>
> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index b7ac1c2..638f122 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -176,6 +176,34 @@ void inode_wb_list_del(struct inode *inode)
> }
>
> /*
> + * mark an inode as under writeback on the given bdi
> + */
> +void bdi_mark_inode_writeback(struct backing_dev_info *bdi, struct inode *inode)
> +{
> + WARN_ON_ONCE(bdi != inode_to_bdi(inode));
> + if (list_empty(&inode->i_wb_list)) {
> + spin_lock(&bdi->wb.list_lock);
> + if (list_empty(&inode->i_wb_list))
> + list_add_tail(&inode->i_wb_list, &bdi->wb.b_writeback);
> + spin_unlock(&bdi->wb.list_lock);
> + }
> +}
> +
> +/*
> + * clear an inode as under writeback on the given bdi
> + */
> +static void bdi_clear_inode_writeback(struct backing_dev_info *bdi,
> + struct inode *inode)
> +{
> + WARN_ON_ONCE(bdi != inode_to_bdi(inode));
> + if (!list_empty(&inode->i_wb_list)) {
> + spin_lock(&bdi->wb.list_lock);
> + list_del_init(&inode->i_wb_list);
> + spin_unlock(&bdi->wb.list_lock);
> + }
> +}
Umm, are these list_empty() checks without lock really safe? Looking into
the code in more detail, it seems that mapping->tree_lock saves us from
races between insert & removal but it definitely deserves a comment (or maybe
even an assertion) that the function requires it.
bdi_clear_inode_writeback() is safe only because it is called only when the
inode is practically dead. Again, I think it deserves a comment...
> +
> +/*
> * Redirty an inode: set its when-it-was dirtied timestamp and move it to the
> * furthest end of its superblock's dirty-inode list.
> *
> @@ -330,13 +358,18 @@ static void __inode_wait_for_writeback(struct inode *inode)
> }
>
> /*
> - * Wait for writeback on an inode to complete. Caller must have inode pinned.
> + * Wait for writeback on an inode to complete during eviction. Caller must have
> + * inode pinned.
> */
> void inode_wait_for_writeback(struct inode *inode)
> {
> + BUG_ON(!(inode->i_state & I_FREEING));
> +
> spin_lock(&inode->i_lock);
> __inode_wait_for_writeback(inode);
> spin_unlock(&inode->i_lock);
> +
> + bdi_clear_inode_writeback(inode_to_bdi(inode), inode);
> }
>
> /*
> @@ -1218,7 +1251,9 @@ EXPORT_SYMBOL(__mark_inode_dirty);
> */
> static void wait_sb_inodes(struct super_block *sb)
> {
> - struct inode *inode, *old_inode = NULL;
> + struct backing_dev_info *bdi = sb->s_bdi;
> + struct inode *old_inode = NULL;
> + LIST_HEAD(sync_list);
>
> /*
> * We need to be protected against the filesystem going from
> @@ -1226,28 +1261,59 @@ static void wait_sb_inodes(struct super_block *sb)
> */
> WARN_ON(!rwsem_is_locked(&sb->s_umount));
>
> - mutex_lock(&sb->s_sync_lock);
> - spin_lock(&sb->s_inode_list_lock);
> -
> /*
> - * Data integrity sync. Must wait for all pages under writeback,
> - * because there may have been pages dirtied before our sync
> - * call, but which had writeout started before we write it out.
> - * In which case, the inode may not be on the dirty list, but
> - * we still have to wait for that writeout.
> + * Data integrity sync. Must wait for all pages under writeback, because
> + * there may have been pages dirtied before our sync call, but which had
> + * writeout started before we write it out. In which case, the inode
> + * may not be on the dirty list, but we still have to wait for that
> + * writeout.
> + *
> + * To avoid syncing inodes put under IO after we have started here,
> + * splice the io list to a temporary list head and walk that. Newly
> + * dirtied inodes will go onto the primary list so we won't wait for
> + * them. This is safe to do as we are serialised by the s_sync_lock,
> + * so we'll complete processing the complete list before the next
> + * sync operation repeats the splice-and-walk process.
> + *
> + * Inodes that have pages under writeback after we've finished the wait
> + * may or may not be on the primary list. They had pages under put IO
> + * after we started our wait, so we need to make sure the next sync or
> + * IO completion treats them correctly, Move them back to the primary
> + * list and restart the walk.
> + *
> + * Inodes that are clean after we have waited for them don't belong on
> + * any list, so simply remove them from the sync list and move onwards.
> */
> - list_for_each_entry(inode, &sb->s_inodes, i_sb_list) {
> + mutex_lock(&sb->s_sync_lock);
> + spin_lock(&bdi->wb.list_lock);
> + list_splice_init(&bdi->wb.b_writeback, &sync_list);
> +
> + while (!list_empty(&sync_list)) {
> + struct inode *inode = list_first_entry(&sync_list, struct inode,
> + i_wb_list);
> struct address_space *mapping = inode->i_mapping;
>
> + /*
> + * We are lazy on IO completion and don't remove inodes from the
> + * list when they are clean. Detect that immediately and skip
> + * inodes we don't ahve to wait on.
> + */
> + if (!mapping_tagged(mapping, PAGECACHE_TAG_WRITEBACK)) {
> + list_del_init(&inode->i_wb_list);
> + cond_resched_lock(&bdi->wb.list_lock);
> + continue;
> + }
> +
> spin_lock(&inode->i_lock);
> - if ((inode->i_state & (I_FREEING|I_WILL_FREE|I_NEW)) ||
> - (mapping->nrpages == 0)) {
> + if (inode->i_state & (I_FREEING|I_WILL_FREE|I_NEW)) {
> + list_move(&inode->i_wb_list, &bdi->wb.b_writeback);
> spin_unlock(&inode->i_lock);
> + cond_resched_lock(&bdi->wb.list_lock);
> continue;
> }
> __iget(inode);
> spin_unlock(&inode->i_lock);
> - spin_unlock(&sb->s_inode_list_lock);
> + spin_unlock(&bdi->wb.list_lock);
>
> /*
> * We hold a reference to 'inode' so it couldn't have been
> @@ -1264,9 +1330,21 @@ static void wait_sb_inodes(struct super_block *sb)
>
> cond_resched();
>
> - spin_lock(&sb->s_inode_list_lock);
> + /*
> + * the inode has been written back now, so check whether we
> + * still have pages under IO and move it back to the primary
> + * list if necessary
> + */
> + spin_lock_irq(&mapping->tree_lock);
> + spin_lock(&bdi->wb.list_lock);
> + if (mapping_tagged(mapping, PAGECACHE_TAG_WRITEBACK)) {
> + WARN_ON(list_empty(&inode->i_wb_list));
> + list_move(&inode->i_wb_list, &bdi->wb.b_writeback);
> + } else
> + list_del_init(&inode->i_wb_list);
> + spin_unlock_irq(&mapping->tree_lock);
Whitespace is damaged in the above hunk...
> }
> - spin_unlock(&sb->s_inode_list_lock);
> + spin_unlock(&bdi->wb.list_lock);
> iput(old_inode);
> mutex_unlock(&sb->s_sync_lock);
> }
> diff --git a/fs/inode.c b/fs/inode.c
> index f8e0f2f..de62d2b 100644
> --- a/fs/inode.c
> +++ b/fs/inode.c
> @@ -365,6 +365,7 @@ void inode_init_once(struct inode *inode)
> INIT_HLIST_NODE(&inode->i_hash);
> INIT_LIST_HEAD(&inode->i_devices);
> INIT_LIST_HEAD(&inode->i_io_list);
> + INIT_LIST_HEAD(&inode->i_wb_list);
> INIT_LIST_HEAD(&inode->i_lru);
> address_space_init_once(&inode->i_data);
> i_size_ordered_init(inode);
> diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h
> index c388155..45edd8a 100644
> --- a/include/linux/backing-dev.h
> +++ b/include/linux/backing-dev.h
> @@ -58,6 +58,7 @@ struct bdi_writeback {
> struct list_head b_dirty; /* dirty inodes */
> struct list_head b_io; /* parked for writeback */
> struct list_head b_more_io; /* parked for more writeback */
> + struct list_head b_writeback; /* inodes under writeback */
> spinlock_t list_lock; /* protects the b_* lists */
> };
>
> @@ -125,6 +126,8 @@ void bdi_writeback_workfn(struct work_struct *work);
> int bdi_has_dirty_io(struct backing_dev_info *bdi);
> void bdi_wakeup_thread_delayed(struct backing_dev_info *bdi);
> void bdi_lock_two(struct bdi_writeback *wb1, struct bdi_writeback *wb2);
> +void bdi_mark_inode_writeback(struct backing_dev_info *bdi,
> + struct inode *inode);
>
> extern spinlock_t bdi_lock;
> extern struct list_head bdi_list;
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index b99eb39..5bb84b1 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -579,6 +579,7 @@ struct inode {
> struct list_head i_io_list; /* backing dev IO list */
> struct list_head i_lru; /* inode LRU list */
> struct list_head i_sb_list;
> + struct list_head i_wb_list; /* backing dev writeback list */
> union {
> struct hlist_head i_dentry;
> struct rcu_head i_rcu;
> diff --git a/mm/backing-dev.c b/mm/backing-dev.c
> index 024d4ca..b0cc55a 100644
> --- a/mm/backing-dev.c
> +++ b/mm/backing-dev.c
> @@ -419,6 +419,7 @@ static void bdi_wb_init(struct bdi_writeback *wb, struct backing_dev_info *bdi)
> INIT_LIST_HEAD(&wb->b_dirty);
> INIT_LIST_HEAD(&wb->b_io);
> INIT_LIST_HEAD(&wb->b_more_io);
> + INIT_LIST_HEAD(&wb->b_writeback);
> spin_lock_init(&wb->list_lock);
> INIT_DELAYED_WORK(&wb->dwork, bdi_writeback_workfn);
> }
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 3f0c895..038c893 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -2262,11 +2262,25 @@ int test_set_page_writeback(struct page *page)
> spin_lock_irqsave(&mapping->tree_lock, flags);
> ret = TestSetPageWriteback(page);
> if (!ret) {
> + bool on_wblist;
> +
> + on_wblist = mapping_tagged(mapping,
> + PAGECACHE_TAG_WRITEBACK);
> +
> radix_tree_tag_set(&mapping->page_tree,
> page_index(page),
> PAGECACHE_TAG_WRITEBACK);
> if (bdi_cap_account_writeback(bdi))
> __inc_bdi_stat(bdi, BDI_WRITEBACK);
> +
> + /*
> + * we can come through here when swapping anonymous
> + * pages, so we don't necessarily have an inode to
> + * track for sync. Inodes are remove lazily, so there is
> + * no equivalent in test_clear_page_writeback().
> + */
> + if (!on_wblist && mapping->host)
> + bdi_mark_inode_writeback(bdi, mapping->host);
> }
> if (!PageDirty(page))
> radix_tree_tag_clear(&mapping->page_tree,
> --
> 1.8.3.2
>
--
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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