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Date:   Wed, 21 Oct 2020 10:31:48 -0700
From:   harshad shirwadkar <harshadshirwadkar@...il.com>
To:     Jan Kara <jack@...e.cz>
Cc:     Ext4 Developers List <linux-ext4@...r.kernel.org>,
        "Theodore Y. Ts'o" <tytso@....edu>
Subject: Re: [PATCH v10 2/9] ext4: add fast_commit feature and handling for
 extended mount options

On Wed, Oct 21, 2020 at 9:18 AM Jan Kara <jack@...e.cz> wrote:
>
> On Thu 15-10-20 13:37:54, Harshad Shirwadkar wrote:
> > We are running out of mount option bits. Add handling for using
> > s_mount_opt2. Add ext4 and jbd2 fast commit feature flag and also add
> > ability to turn off the fast commit feature in Ext4.
> >
> > Signed-off-by: Harshad Shirwadkar <harshadshirwadkar@...il.com>
> > ---
> >  fs/ext4/ext4.h       |  4 ++++
> >  fs/ext4/super.c      | 27 ++++++++++++++++++++++-----
> >  include/linux/jbd2.h |  5 ++++-
> >  3 files changed, 30 insertions(+), 6 deletions(-)
> >
> > diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> > index 1879531a119f..02d7dc378505 100644
> > --- a/fs/ext4/ext4.h
> > +++ b/fs/ext4/ext4.h
> > @@ -1213,6 +1213,8 @@ struct ext4_inode_info {
> >  #define EXT4_MOUNT2_EXPLICIT_JOURNAL_CHECKSUM        0x00000008 /* User explicitly
> >                                               specified journal checksum */
> >
> > +#define EXT4_MOUNT2_JOURNAL_FAST_COMMIT      0x00000010 /* Journal fast commit */
> > +
> >  #define clear_opt(sb, opt)           EXT4_SB(sb)->s_mount_opt &= \
> >                                               ~EXT4_MOUNT_##opt
> >  #define set_opt(sb, opt)             EXT4_SB(sb)->s_mount_opt |= \
> > @@ -1813,6 +1815,7 @@ static inline bool ext4_verity_in_progress(struct inode *inode)
> >  #define EXT4_FEATURE_COMPAT_RESIZE_INODE     0x0010
> >  #define EXT4_FEATURE_COMPAT_DIR_INDEX                0x0020
> >  #define EXT4_FEATURE_COMPAT_SPARSE_SUPER2    0x0200
> > +#define EXT4_FEATURE_COMPAT_FAST_COMMIT              0x0400
> >  #define EXT4_FEATURE_COMPAT_STABLE_INODES    0x0800
>
> Is fast commit really a compat feature? IMO if there are fast commits
> stored in the journal, the filesystem is actually incompatible with the
> old kernels because data we guranteed to be permanenly stored may be
> invisible for the old kernel (since it won't replay fastcommit
> transactions).
>
> ...
>
> Oh, now I see that the journal FAST_COMMIT is actually incompat. So what's
> the point of compat ext4 feature with incompat JBD2 feature?
So having fast commits enabled on an ext4 file system doesn't
immediately make it incompatible with the older kernels. FS becomes
incompatible only if there are fast commits blocks that are stored in
the journal. So, one of the tricks that this patchset does is on a
clean unmount, since it's guaranteed that there are no fast commit
blocks in journal, we clear out the JBD2 incompat flag and preserve
the compat flag in ext4. So, we can think of ext4 compat flag as "FS
will try fast commits when possible" while jbd2 incompat flag as
"There are fast commits blocks present in the journal". Does that make
sense?
>
> > diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> > index 901c1c938276..70256a240442 100644
> > --- a/fs/ext4/super.c
> > +++ b/fs/ext4/super.c
> > @@ -1709,7 +1709,7 @@ enum {
> >       Opt_dioread_nolock, Opt_dioread_lock,
> >       Opt_discard, Opt_nodiscard, Opt_init_itable, Opt_noinit_itable,
> >       Opt_max_dir_size_kb, Opt_nojournal_checksum, Opt_nombcache,
> > -     Opt_prefetch_block_bitmaps,
> > +     Opt_prefetch_block_bitmaps, Opt_no_fc,
>
> It would be more consistent to use a name 'Opt_nofc' and IMHO 'fc' is
> really too short an ambiguous. I agree "nofastcommit" is somewhat long but
> still OK and much more descriptive...
Ack
>
> >  };
> >
> >  static const match_table_t tokens = {
> > @@ -1796,6 +1796,7 @@ static const match_table_t tokens = {
> >       {Opt_init_itable, "init_itable=%u"},
> >       {Opt_init_itable, "init_itable"},
> >       {Opt_noinit_itable, "noinit_itable"},
> > +     {Opt_no_fc, "no_fc"},
>
> And here "nofastcommit", or perhaps "nofast_commit".
Ack

Thanks,
Harshad
>
>                                                                 Honza
> --
> Jan Kara <jack@...e.com>
> SUSE Labs, CR

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