Re: [PATCH v2 0/4] ext4 fast commit API cleanup

From: riteshh <riteshh@linux.ibm.com>
To: harshad shirwadkar <harshadshirwadkar@gmail.com>
Cc: Ext4 Developers List <linux-ext4@vger.kernel.org>,
	"Theodore Y. Ts'o" <tytso@mit.edu>,
	Harshad Shirwadkar <harshads@google.com>
Subject: Re: [PATCH v2 0/4] ext4 fast commit API cleanup
Date: Tue, 11 Jan 2022 22:27:10 +0530	[thread overview]
Message-ID: <20220111165710.swbv2dvr75sraylh@riteshh-domain> (raw)
In-Reply-To: <CAD+ocbxw8oYg6RVXViwwwh1zuu4mrOQdJ6p0rT_FuXF=sMGaJA@mail.gmail.com>

On 22/01/11 08:19AM, harshad shirwadkar wrote:
> Hey Ritesh,
>
> Yes, your understanding is correct, this patch series does have a side
> effect that the entire file system gets locked before starting a fast
> commit. However, this regression is meant to be temporary (mainly to
> prevent merging of unnecessary correctness patches). I am working on
> another series which fixes this by only locking the inode that is
> being committed. That patch should be out shortly.
>
> The reason I hurried this patch series in without the inode locking
> patches first was that we had some consistency and file system hanging
> issues which needed to be fixed in the right way before the code
> becomes more cluttered with temporary correctness fixes which would
> eventually get dropped out. The hope is that with these patches, such
> fixes wouldn't need to be merged in.
>
> But yeah, I am fully aware of the performance degradation that this
> series introduces and you will soon see another patch series that
> fixes that issue.

Thanks Harshad, for taking time and helping me understand the reasoning behind.

-ritesh

>
> Thanks,
> Harshad
>
> On Tue, Jan 11, 2022 at 4:53 AM riteshh <riteshh@linux.ibm.com> wrote:
> >
> > On 21/12/23 12:21PM, Harshad Shirwadkar wrote:
> > > This patch series fixes up fast commit APIs. There are NO on-disk
> > > format changes introduced in this series. The main contribution of the
> > > series is that it drops fast commit specific transaction APIs and
> > > makes fast commits work with journal transaction APIs of JBD2
> > > journalling system. With these changes, a fast commit eligible
> > > transaction is simply enclosed in calls to "jbd2_journal_start()" and
> > > "jbd2_journal_stop()". If the update that is being performed is fast
> > > commit ineligible, one must simply call ext4_fc_mark_ineligible()
> > > after starting a transaction using "jbd2_journal_start()". The last
> > > patch in the series simplifies fast commit stats recording by moving
> > > it to a different function.
> > >
> > > I verified that the patch series introduces no regressions in "quick"
> > > and "log" groups when "fast_commit" feature is enabled.
> > >
> > > Changes from V1:
> > > ---------------
> > >
> > > - In the V1 of the patch series, there's performance regression. With
> > >   this patch series, we lock the entire file system from starting any
> > >   new handles during (which ensures consistency at the cost of
> > >   performance). What we ideally want to do is to lock individual
> > >   inodes from starting new updates during a commit. To do so, the V2
> > >   of this patch series retains the infrastructure of inode level
> > >   transactions (ext4_fc_start/stop_update()). In future (not in this
> > >   series), we would build on this infrastructure to lock individual
> > >   inodes and drop the file system level locking during the commit path.
> >
> > Hello Harshad,
> >
> > Sorry about being so late in the game :(
> >
> > So from what I understood from your above commit msg is that even the current
> > v2 patch series suffers from the same performance regression which is:-
> > we lock the filesystem from any starting transaction updates
> > (by taking j_barrier or say by calling jbd2_journal_lock_updates()) while
> > fast_commit's commit operation is in progress (which happens during a file fsync()).
> > This means when fast_commit's commit operation is in progress, then we can't even
> > start a new transaction for recording any metadata updates to any inodes of my FS.
> >
> > Is above understanding correct w.r.t this v2 patch series?
> > If yes, then why do we need to lock the full filesystem from starting any
> > journal txns? Why can't we let any process starts a new transaction while
> > the previous fast_commit's commit operation is in progress?
> >
> > JBD2 does allow us to do that right? i.e. while a jbd2 commit is in progress,
> > a new running transaction can be allocated and all the new metadata updates will
> > now be tracked in the new running txn, right?
> >
> > -ritesh
> >
> >
> > >
> > > Signed-off-by: Harshad Shirwadkar <harshadshirwadkar@gmail.com>
> > >
> > > Harshad Shirwadkar (4):
> > >   ext4: use ext4_journal_start/stop for fast commit transactions
> > >   ext4: drop ineligible txn start stop APIs
> > >   ext4: simplify updating of fast commit stats
> > >   ext4: update fast commit TODOs
> > >
> > >  fs/ext4/acl.c         |   2 -
> > >  fs/ext4/ext4.h        |   7 +-
> > >  fs/ext4/extents.c     |   9 +-
> > >  fs/ext4/fast_commit.c | 188 ++++++++++++++++--------------------------
> > >  fs/ext4/fast_commit.h |  27 +++---
> > >  fs/ext4/file.c        |   4 -
> > >  fs/ext4/inode.c       |   7 +-
> > >  fs/ext4/ioctl.c       |  13 +--
> > >  fs/ext4/super.c       |   1 -
> > >  fs/jbd2/journal.c     |   2 +
> > >  10 files changed, 96 insertions(+), 164 deletions(-)
> > >
> > > --
> > > 2.34.1.307.g9b7440fafd-goog
> > >