From: Andreas Dilger Subject: Re: [PATCH] Adding support to freeze and unfreeze a journal Date: Tue, 10 May 2011 15:07:41 -0600 Message-ID: <1744A781-0223-4A21-9322-979A6D2AD461@dilger.ca> References: <20110509150852.GQ4122@quack.suse.cz> <1305040070-2343-1-git-send-email-surbhi.palande@canonical.com> Mime-Version: 1.0 (Apple Message framework v1082) Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 8BIT Cc: jack@suse.cz, marco.stornelli@gmail.com, toshi.okajima@jp.fujitsu.com, tytso@mit.edu, m.mizuma@jp.fujitsu.com, sandeen@redhat.com, linux-ext4@vger.kernel.org, linux-fsdevel@vger.kernel.org To: Surbhi Palande Return-path: Received: from idcmail-mo2no.shaw.ca ([64.59.134.9]:18145 "EHLO idcmail-mo2no.shaw.ca" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751401Ab1EJVHt convert rfc822-to-8bit (ORCPT ); Tue, 10 May 2011 17:07:49 -0400 In-Reply-To: <1305040070-2343-1-git-send-email-surbhi.palande@canonical.com> Sender: linux-ext4-owner@vger.kernel.org List-ID: Mostly minor cleanups to the commit message and comments. On May 10, 2011, at 09:07, Surbhi Palande wrote: > The journal should be frozen when a F.S freezes. s/F.S/filesystem/g > What this means is that till s/till/until/ > the F.S is thawed again, no new transactions should be accepted by the > journal. When the F.S thaws, inturn it should thaw the journal and this should > allow the journal to resume accepting new transactions. > While the F.S has frozen the journal, the clients of journal on calling > jbd2_journal_start() will sleep on a wait queue. Thawing the journal will wake > up the sleeping clients and journalling can progress normally. > > An example of the race condition that can happen without this patch is as > follows: > > Say the F.S is thawed when we begin. Let tx be the time at unit x > > P1: Process doing an aio write > t1) ext4_file_write() > t2) generic_file_aio_write() > t3) __generic_file_aio_write() > // F.S is not frozen, so we do not block in the next check. > t4) vfs_check_frozen() > t5) generic_write_checks() > ----------------- Prempted------------------ > > P2: Process that does fs freeze > > t6) freeze_super() > t7) sync_filesystem() > t8) sync_blockdev() > t9) sb->s_op->freeze_fs() (= ext4_freeze) > t10) jbd2_journal_lock_updates() > t11) jbd2_journal_flush() > // Need to unlock the journal before returning to user space. > t12) jbd2_journal_unlock_updates() > // Journal is unlocked and so we can start accepting new transactions now. > > // freezing process completes execution. Page cache is now clean and should > // remain clean till the F.S is frozen. > -------------------------------------------- > > P1: writing process gets the control back > t13) generic_file_buffered_write() > t14) generic_perform_write() > t15) a_ops->write_begin() (= ext4_write_begin) > t16) ext4_journal_start() > // New handle is started. We do not block here! Write continues > // dirtying the page cache while the F.S is frozen! > > Signed-off-by: Surbhi Palande > Acked-by: Jan Kara > --- > fs/ext4/super.c | 20 ++++++-------------- > fs/jbd2/journal.c | 1 + > fs/jbd2/transaction.c | 42 ++++++++++++++++++++++++++++++++++++++++++ > include/linux/jbd2.h | 10 ++++++++++ > 4 files changed, 59 insertions(+), 14 deletions(-) > > diff --git a/fs/ext4/super.c b/fs/ext4/super.c > index 8553dfb..796aa4c 100644 > --- a/fs/ext4/super.c > +++ b/fs/ext4/super.c > @@ -4179,23 +4179,15 @@ static int ext4_freeze(struct super_block *sb) > > journal = EXT4_SB(sb)->s_journal; > > - /* Now we set up the journal barrier. */ > - jbd2_journal_lock_updates(journal); > - > + error = jbd2_journal_freeze(journal); > /* > - * Don't clear the needs_recovery flag if we failed to flush > + * Don't clear the needs_recovery flag if we failed to freeze > * the journal. > */ > - error = jbd2_journal_flush(journal); > - if (error < 0) > - goto out; > - > - /* Journal blocked and flushed, clear needs_recovery flag. */ > - EXT4_CLEAR_INCOMPAT_FEATURE(sb, EXT4_FEATURE_INCOMPAT_RECOVER); > - error = ext4_commit_super(sb, 1); > -out: > - /* we rely on s_frozen to stop further updates */ > - jbd2_journal_unlock_updates(EXT4_SB(sb)->s_journal); > + if (error >= 0) { > + EXT4_CLEAR_INCOMPAT_FEATURE(sb, EXT4_FEATURE_INCOMPAT_RECOVER); > + error = ext4_commit_super(sb, 1); > + } > return error; > } > > diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c > index e0ec3db..5e46333 100644 > --- a/fs/jbd2/journal.c > +++ b/fs/jbd2/journal.c > @@ -842,6 +842,7 @@ static journal_t * journal_init_common (void) > init_waitqueue_head(&journal->j_wait_checkpoint); > init_waitqueue_head(&journal->j_wait_commit); > init_waitqueue_head(&journal->j_wait_updates); > + init_waitqueue_head(&journal->j_wait_frozen); > mutex_init(&journal->j_barrier); > mutex_init(&journal->j_checkpoint_mutex); > spin_lock_init(&journal->j_revoke_lock); > diff --git a/fs/jbd2/transaction.c b/fs/jbd2/transaction.c > index 05fa77a..b040293 100644 > --- a/fs/jbd2/transaction.c > +++ b/fs/jbd2/transaction.c > @@ -171,6 +171,17 @@ repeat: > journal->j_barrier_count == 0); > goto repeat; > } > + /* dont let a new handle start when a journal is frozen. s/dont/Don't/ or s/dont/Do not/ > + * jbd2_journal_freeze calls jbd2_journal_unlock_updates() only after > + * the jflags indicate that the journal is frozen. So if the s/jflags/j_flags/ > + * j_barrier_count is 0, then check if this was made 0 by the freezing > + * process > + */ > + if (journal->j_flags & JBD2_FROZEN) { > + read_unlock(&journal->j_state_lock); > + jbd2_check_frozen(journal); > + goto repeat; > + } > > if (!journal->j_running_transaction) { > read_unlock(&journal->j_state_lock); > @@ -489,6 +500,37 @@ int jbd2_journal_restart(handle_t *handle, int nblocks) > } > EXPORT_SYMBOL(jbd2_journal_restart); > > +int jbd2_journal_freeze(journal_t *journal) > +{ > + int error = 0; > + /* Now we set up the journal barrier. */ > + jbd2_journal_lock_updates(journal); > + > + /* > + * Don't clear the needs_recovery flag if we failed to flush > + * the journal. > + */ > + error = jbd2_journal_flush(journal); > + if (error >= 0) { > + write_lock(&journal->j_state_lock); > + journal->j_flags |= JBD2_FROZEN; > + write_unlock(&journal->j_state_lock); > + } > + jbd2_journal_unlock_updates(journal); > + return error; > +} > +EXPORT_SYMBOL(jbd2_journal_freeze); > + > +void jbd2_journal_thaw(journal_t * journal) > +{ > + write_lock(&journal->j_state_lock); > + journal->j_flags &= ~JBD2_FROZEN; > + write_unlock(&journal->j_state_lock); > + wake_up(&journal->j_wait_frozen); > +} > +EXPORT_SYMBOL(jbd2_journal_thaw); > + > + > /** > * void jbd2_journal_lock_updates () - establish a transaction barrier. > * @journal: Journal to establish a barrier on. > diff --git a/include/linux/jbd2.h b/include/linux/jbd2.h > index a32dcae..c7885b2 100644 > --- a/include/linux/jbd2.h > +++ b/include/linux/jbd2.h > @@ -718,6 +718,7 @@ jbd2_time_diff(unsigned long start, unsigned long end) > * @j_wait_checkpoint: Wait queue to trigger checkpointing > * @j_wait_commit: Wait queue to trigger commit > * @j_wait_updates: Wait queue to wait for updates to complete > + * @j_wait_frozen: Wait queue to wait for journal to thaw > * @j_checkpoint_mutex: Mutex for locking against concurrent checkpoints > * @j_head: Journal head - identifies the first unused block in the journal > * @j_tail: Journal tail - identifies the oldest still-used block in the > @@ -835,6 +836,9 @@ struct journal_s > /* Wait queue to wait for updates to complete */ > wait_queue_head_t j_wait_updates; > > + /* Wait queue to wait for journal to thaw*/ > + wait_queue_head_t j_wait_frozen; > + > /* Semaphore for locking against concurrent checkpoints */ > struct mutex j_checkpoint_mutex; > > @@ -1013,7 +1017,11 @@ struct journal_s > #define JBD2_ABORT_ON_SYNCDATA_ERR 0x040 /* Abort the journal on file > * data write error in ordered > * mode */ > +#define JBD2_FROZEN 0x080 /* Journal thread is frozen as the filesystem is frozen */ Need to wrap this to 80 columns: +#define JBD2_FROZEN 0x080 /* Journal thread frozen along with filesystem */ > +#define jbd2_check_frozen(journal) \ > + wait_event(journal->j_wait_frozen, (journal->j_flags & JBD2_FROZEN)) Having this macro, which is now only used in one place, isn't really clarifying the code because the name "check_frozen" doesn't really imply "wait until it journal is unfrozen". It would be better to just put the wait_event() inline at the one callsite and remove this macro entirely. > /* > * Function declarations for the journaling transaction and buffer > * management > @@ -1121,6 +1129,8 @@ extern void jbd2_journal_invalidatepage(journal_t *, > struct page *, unsigned long); > extern int jbd2_journal_try_to_free_buffers(journal_t *, struct page *, gfp_t); > extern int jbd2_journal_stop(handle_t *); > +extern int jbd2_journal_freeze(journal_t *); > +extern void jbd2_journal_thaw(journal_t *); > extern int jbd2_journal_flush (journal_t *); > extern void jbd2_journal_lock_updates (journal_t *); > extern void jbd2_journal_unlock_updates (journal_t *); Once these minor changes have been made you can add: Reviewed-by: Andreas Dilger Cheers, Andreas