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Message-ID: <87vc7lsj0w.fsf@openvz.org>
Date: Wed, 17 Apr 2013 11:39:27 +0400
From: Dmitry Monakhov <dmonakhov@...nvz.org>
To: Jan Kara <jack@...e.cz>
Cc: linux-ext4@...r.kernel.org, jack@...e.cz
Subject: Re: [PATCH 2/5] jbd2: optimize jbd2_journal_force_commit
On Mon, 15 Apr 2013 14:29:13 +0200, Jan Kara <jack@...e.cz> wrote:
> On Sun 14-04-13 23:01:34, Dmitry Monakhov wrote:
> > Current implementation of jbd2_journal_force_commit() is suboptimal because
> > result in empty and useless commits. But callers just want to force and wait
> > any unfinished commits. We already has jbd2_journal_force_commit_nested()
> > which does exactly what we want, except we are guaranteed that we do not hold
> > journal transaction open.
> Umm, I have a questions regarding this patch:
> Grep shows there are just two places in the code which use
> ext4_force_commit() (and thus jbd2_journal_force_commit()). These are
> ext4_write_inode() and ext4_sync_file() (in data=journal mode). The first
> callsite can use _nested() variant immediately as we even assert there's
> no handle started. The second call site can use the _nested variant as well
> because if we had the transaction started when entering ext4_sync_file() we
> would have serious problems (lock inversion, deadlocks in !data=journal
> modes) anyway. So IMO there's no need for !nested variant at all (at least
> in ext4, ocfs2 uses it as well, IMHO it can be converted as well but that's
> a different topic). Thoughts?
I'm not sure that I completely understand what you meant, but it seems
incorrect to use jbd2_journal_force_commit_nested() in
ext4_write_inode() and ext4_sync_file(). Because nested variant has
probabilistic behavior, It may skip real transaction commit if we hold
a transaction running. ext4_write_inode() and ext4_sync_file()
are the functions where we demand deterministic behavior. If we silently
miss real transaction commit because current->journal_info != NULL (due
to some bugs) this breaks data integrity assumptions and it is better to
make it loud and trigger a BUGON. This also true for ocfs2_sync_file().
At the same time ocfs2_file_aio_write() looks like a candidate for xxx_nested().
>
> Honza
>
> > Signed-off-by: Dmitry Monakhov <dmonakhov@...nvz.org>
> > ---
> > fs/jbd2/journal.c | 58 ++++++++++++++++++++++++++++++++++++------------
> > fs/jbd2/transaction.c | 23 -------------------
> > include/linux/jbd2.h | 2 +-
> > 3 files changed, 44 insertions(+), 39 deletions(-)
> >
> > diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c
> > index 886ec2f..cfe0aca 100644
> > --- a/fs/jbd2/journal.c
> > +++ b/fs/jbd2/journal.c
> > @@ -566,21 +566,19 @@ int jbd2_log_start_commit(journal_t *journal, tid_t tid)
> > }
> >
> > /*
> > - * Force and wait upon a commit if the calling process is not within
> > - * transaction. This is used for forcing out undo-protected data which contains
> > - * bitmaps, when the fs is running out of space.
> > - *
> > - * We can only force the running transaction if we don't have an active handle;
> > - * otherwise, we will deadlock.
> > - *
> > - * Returns true if a transaction was started.
> > + * Force and wait any uncommitted transactions. We can only force the running
> > + * transaction if we don't have an active handle, otherwise, we will deadlock.
> > */
> > -int jbd2_journal_force_commit_nested(journal_t *journal)
> > +int __jbd2_journal_force_commit(journal_t *journal, int nested, int *progress)
> > {
> > transaction_t *transaction = NULL;
> > tid_t tid;
> > - int need_to_start = 0;
> > + int need_to_start = 0, ret = 0;
> >
> > + J_ASSERT(!current->journal_info || nested);
> > +
> > + if (progress)
> > + *progress = 0;
> > read_lock(&journal->j_state_lock);
> > if (journal->j_running_transaction && !current->journal_info) {
> > transaction = journal->j_running_transaction;
> > @@ -590,16 +588,46 @@ int jbd2_journal_force_commit_nested(journal_t *journal)
> > transaction = journal->j_committing_transaction;
> >
> > if (!transaction) {
> > + /* Nothing to commit */
> > read_unlock(&journal->j_state_lock);
> > - return 0; /* Nothing to retry */
> > + return 0;
> > }
> > -
> > tid = transaction->t_tid;
> > read_unlock(&journal->j_state_lock);
> > if (need_to_start)
> > - jbd2_log_start_commit(journal, tid);
> > - jbd2_log_wait_commit(journal, tid);
> > - return 1;
> > + ret = jbd2_log_start_commit(journal, tid);
> > + if (!ret)
> > + ret = jbd2_log_wait_commit(journal, tid);
> > + if (!ret && progress)
> > + *progress = 1;
> > +
> > + return ret;
> > +}
> > +
> > +/**
> > + * Force and wait upon a commit if the calling process is not within
> > + * transaction. This is used for forcing out undo-protected data which contains
> > + * bitmaps, when the fs is running out of space.
> > + *
> > + * @journal: journal to force
> > + * Returns true if progress was made.
> > + */
> > +int jbd2_journal_force_commit_nested(journal_t *journal)
> > +{
> > + int progress;
> > +
> > + __jbd2_journal_force_commit(journal, 1, &progress);
> > + return progress;
> > +}
> > +
> > +/**
> > + * int journal_force_commit() - force any uncommitted transactions
> > + * @journal: journal to force
> > + *
> > + */
> > +int jbd2_journal_force_commit(journal_t *journal)
> > +{
> > + return __jbd2_journal_force_commit(journal, 0, NULL);
> > }
> >
> > /*
> > diff --git a/fs/jbd2/transaction.c b/fs/jbd2/transaction.c
> > index 325bc01..bb2a5c0 100644
> > --- a/fs/jbd2/transaction.c
> > +++ b/fs/jbd2/transaction.c
> > @@ -1515,29 +1515,6 @@ int jbd2_journal_stop(handle_t *handle)
> > return err;
> > }
> >
> > -/**
> > - * int jbd2_journal_force_commit() - force any uncommitted transactions
> > - * @journal: journal to force
> > - *
> > - * For synchronous operations: force any uncommitted transactions
> > - * to disk. May seem kludgy, but it reuses all the handle batching
> > - * code in a very simple manner.
> > - */
> > -int jbd2_journal_force_commit(journal_t *journal)
> > -{
> > - handle_t *handle;
> > - int ret;
> > -
> > - handle = jbd2_journal_start(journal, 1);
> > - if (IS_ERR(handle)) {
> > - ret = PTR_ERR(handle);
> > - } else {
> > - handle->h_sync = 1;
> > - ret = jbd2_journal_stop(handle);
> > - }
> > - return ret;
> > -}
> > -
> > /*
> > *
> > * List management code snippets: various functions for manipulating the
> > diff --git a/include/linux/jbd2.h b/include/linux/jbd2.h
> > index f9fe889..7b38517 100644
> > --- a/include/linux/jbd2.h
> > +++ b/include/linux/jbd2.h
> > @@ -1125,6 +1125,7 @@ extern void jbd2_journal_ack_err (journal_t *);
> > extern int jbd2_journal_clear_err (journal_t *);
> > extern int jbd2_journal_bmap(journal_t *, unsigned long, unsigned long long *);
> > extern int jbd2_journal_force_commit(journal_t *);
> > +extern int jbd2_journal_force_commit_nested(journal_t *);
> > extern int jbd2_journal_file_inode(handle_t *handle, struct jbd2_inode *inode);
> > extern int jbd2_journal_begin_ordered_truncate(journal_t *journal,
> > struct jbd2_inode *inode, loff_t new_size);
> > @@ -1199,7 +1200,6 @@ int __jbd2_log_space_left(journal_t *); /* Called with journal locked */
> > int jbd2_log_start_commit(journal_t *journal, tid_t tid);
> > int __jbd2_log_start_commit(journal_t *journal, tid_t tid);
> > int jbd2_journal_start_commit(journal_t *journal, tid_t *tid);
> > -int jbd2_journal_force_commit_nested(journal_t *journal);
> > int jbd2_log_wait_commit(journal_t *journal, tid_t tid);
> > int jbd2_complete_transaction(journal_t *journal, tid_t tid);
> > int jbd2_log_do_checkpoint(journal_t *journal);
> > --
> > 1.7.1
> >
> --
> Jan Kara <jack@...e.cz>
> SUSE Labs, CR
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