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Date: Thu, 22 Oct 2020 15:09:08 +0200 From: Jan Kara <jack@...e.cz> To: harshad shirwadkar <harshadshirwadkar@...il.com> Cc: Jan Kara <jack@...e.cz>, 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 21-10-20 10:31:48, harshad shirwadkar wrote: > 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? Yes, understood. That's clever. Thanks for explanation! But please add the above justification to the description of EXT4_FEATURE_COMPAT_FAST_COMMIT feature or somewhere around that. Honza -- Jan Kara <jack@...e.com> SUSE Labs, CR
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