From: Theodore Ts'o <tytso@mit.edu>
Subject: Re: [PATCH 4/5] jbd2: Speedup jbd2_journal_get_[write|undo]_access()
Date: Mon, 8 Jun 2015 12:47:26 -0400
Message-ID: <20150608164726.GM19168@thunk.org>
References: <1427983100-29889-1-git-send-email-jack@suse.cz>
 <1427983100-29889-5-git-send-email-jack@suse.cz>
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Cc: linux-ext4@vger.kernel.org
To: Jan Kara <jack@suse.cz>
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On Thu, Apr 02, 2015 at 03:58:19PM +0200, Jan Kara wrote:
> jbd2_journal_get_write_access() and jbd2_journal_get_create_access() are
> frequently called for buffers that are already part of the running
> transaction - most frequently it is the case for bitmaps, inode table
> blocks, and superblock. Since in such cases we have nothing to do, it is
> unfortunate we still grab reference to journal head, lock the bh, lock
> bh_state only to find out there's nothing to do.
> 
> Improving this is a bit subtle though since until we find out journal
> head is attached to the running transaction, it can disappear from under
> us because checkpointing / commit decided it's no longer needed. We deal
> with this by protecting journal_head slab with RCU. We still have to be
> careful about journal head being freed & reallocated within slab and
> about exposing journal head in consistent state (in particular
> b_modified and b_frozen_data must be in correct state before we allow
> user to touch the buffer).
> 
> FIXME: Performance data.
> 
> Signed-off-by: Jan Kara <jack@suse.cz>

Applied, so we can start getting some testing on this patch.  Did you
ever get performance data?

Thanks,

						- Ted