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Date: Thu, 17 Dec 2009 04:33:35 +0300
From: Dmitry Monakhov <dmonakhov@...nvz.org>
To: Jan Kara <jack@...e.cz>
Cc: linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org
Subject: Re: [PATCH] ext4: journalled quota seed-up
Dmitry Monakhov <dmonakhov@...nvz.org> writes:
Sorry forgot to add linux-ext4@...r.kernel.org list to CC
> Currently each quota modification result in write_dquot()
> and later dquot_commit(). This means what each quota modification
> function must wait for dqio_mutex. Which is *huge* performance
> penalty on big SMP systems. ASAIU The core idea of this implementation
> is to guarantee that each quota modification will be written to journal
> atomically. But in fact this is not always true, because dquot may be
> changed after dquot modification, but before it was committed to disk.
>
> | Task 1 | Task 2 |
> | alloc_space(nr) | |
> | ->spin_lock(dq_data_lock) | |
> | ->curspace += nr | |
> | ->spin_unlock(dq_data_lock) | |
> | ->mark_dirty() | free_space(nr) |
> | -->write_dquot() | ->spin_lock(dq_data_lock) |
> | --->dquot_commit() | ->curspace -= nr |
> | ---->commit_dqblk() | ->spin_unlock(dq_data_lock) |
> | ----->spin_lock(dq_data_lock) | |
> | ----->mem2disk_dqblk(ddq, dquot) | <<< Copy updated value |
> | ----->spin_unlock(dq_data_lock) | |
> | ----->quota_write() | |
> Quota corruption not happens only because quota modification caller
> started journal already. And ext3/4 allow only one running quota
> at a time. Let's exploit this fact and avoid writing quota each time.
> Act similar to dirty_for_io in general write_back path in page-cache.
> If we have found what other task already started on copy and write the
> dquot then we skip actual quota_write stage. And let that task do the job.
> Side effect: Task which skip real quota_write() will not get an error
> (if any). But this is not big deal because:
> 1) Any error has global consequences (RO_remount, err_print, etc).
> 2) Real IO is differed till the journall_commit.
>
> Signed-off-by: Dmitry Monakhov <dmonakhov@...nvz.org>
> ---
> fs/ext4/super.c | 37 ++++++++++++++++++++++++++++++-------
> fs/quota/dquot.c | 16 +++++++++++++---
> include/linux/quotaops.h | 1 +
> 3 files changed, 44 insertions(+), 10 deletions(-)
>
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index 3929091..164217e 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -3777,14 +3777,37 @@ static int ext4_release_dquot(struct dquot *dquot)
>
> static int ext4_mark_dquot_dirty(struct dquot *dquot)
> {
> - /* Are we journaling quotas? */
> - if (EXT4_SB(dquot->dq_sb)->s_qf_names[USRQUOTA] ||
> - EXT4_SB(dquot->dq_sb)->s_qf_names[GRPQUOTA]) {
> - dquot_mark_dquot_dirty(dquot);
> - return ext4_write_dquot(dquot);
> - } else {
> + int ret, err;
> + handle_t *handle;
> + struct inode *inode;
> +
> + /* Are we not journaling quotas? */
> + if (!EXT4_SB(dquot->dq_sb)->s_qf_names[USRQUOTA] &&
> + !EXT4_SB(dquot->dq_sb)->s_qf_names[GRPQUOTA])
> return dquot_mark_dquot_dirty(dquot);
> - }
> +
> + /* journaling quotas case */
> + inode = dquot_to_inode(dquot);
> + handle = ext4_journal_start(inode,
> + EXT4_QUOTA_TRANS_BLOCKS(dquot->dq_sb));
> + if (IS_ERR(handle))
> + return PTR_ERR(handle);
> + if (!__dquot_mark_dquot_dirty(dquot))
> + ret = dquot_commit(dquot);
> + else
> + /*
> + * Dquot was already dirty. This means that other task already
> + * started a transaction but not clear dirty bit yet (see
> + * dquot_commit). Since the only one running transaction is
> + * possible at a time. Then that task belongs to the same
> + * transaction. We don'n have to actually write dquot changes
> + * because that task will write it for for us.
> + */
> + ret = 0;
> + err = ext4_journal_stop(handle);
> + if (!ret)
> + ret = err;
> + return ret;
> }
>
> static int ext4_write_info(struct super_block *sb, int type)
> diff --git a/fs/quota/dquot.c b/fs/quota/dquot.c
> index 6979224..e03eea0 100644
> --- a/fs/quota/dquot.c
> +++ b/fs/quota/dquot.c
> @@ -311,14 +311,24 @@ static inline int mark_dquot_dirty(struct dquot *dquot)
> {
> return dquot->dq_sb->dq_op->mark_dirty(dquot);
> }
> -
> -int dquot_mark_dquot_dirty(struct dquot *dquot)
> +/* mark dquot dirty in atomic meaner, and return old dirty state */
> +int __dquot_mark_dquot_dirty(struct dquot *dquot)
> {
> + int ret = 1;
> spin_lock(&dq_list_lock);
> - if (!test_and_set_bit(DQ_MOD_B, &dquot->dq_flags))
> + if (!test_and_set_bit(DQ_MOD_B, &dquot->dq_flags)) {
> list_add(&dquot->dq_dirty, &sb_dqopt(dquot->dq_sb)->
> info[dquot->dq_type].dqi_dirty_list);
> + ret = 0;
> + }
> spin_unlock(&dq_list_lock);
> + return ret;
> +}
> +EXPORT_SYMBOL(__dquot_mark_dquot_dirty);
> +
> +int dquot_mark_dquot_dirty(struct dquot *dquot)
> +{
> + __dquot_mark_dquot_dirty(dquot);
> return 0;
> }
> EXPORT_SYMBOL(dquot_mark_dquot_dirty);
> diff --git a/include/linux/quotaops.h b/include/linux/quotaops.h
> index ae7ef25..c950f7d 100644
> --- a/include/linux/quotaops.h
> +++ b/include/linux/quotaops.h
> @@ -56,6 +56,7 @@ int dquot_commit(struct dquot *dquot);
> int dquot_acquire(struct dquot *dquot);
> int dquot_release(struct dquot *dquot);
> int dquot_commit_info(struct super_block *sb, int type);
> +int __dquot_mark_dquot_dirty(struct dquot *dquot);
> int dquot_mark_dquot_dirty(struct dquot *dquot);
>
> int vfs_quota_on(struct super_block *sb, int type, int format_id,
--
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