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Date:   Tue, 3 Nov 2020 10:31:04 -0800
From:   harshad shirwadkar <harshadshirwadkar@...il.com>
To:     Jan Kara <jack@...e.cz>
Cc:     Andreas Dilger <adilger@...ger.ca>,
        "Theodore Y. Ts'o" <tytso@....edu>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>,
        kernel test robot <lkp@...el.com>
Subject: Re: [PATCH v10 5/9] ext4: main fast-commit commit path

On Tue, Nov 3, 2020 at 2:04 AM Jan Kara <jack@...e.cz> wrote:
>
> On Fri 30-10-20 09:45:10, harshad shirwadkar wrote:
> > > > > > > > +
> > > > > > > > +     return 0;
> > > > > > > > +}
> > > > > > > > +
> > > > > > > > +/*
> > > > > > > > + * Writes updated data ranges for the inode in question. Updates CRC.
> > > > > > > > + * Returns 0 on success, error otherwise.
> > > > > > > > + */
> > > > > > > > +static int ext4_fc_write_inode_data(struct inode *inode, u32 *crc)
> > > > > > > > +{
> > > > > > > > +     ext4_lblk_t old_blk_size, cur_lblk_off, new_blk_size;
> > > > > > > > +     struct ext4_inode_info *ei = EXT4_I(inode);
> > > > > > > > +     struct ext4_map_blocks map;
> > > > > > > > +     struct ext4_fc_add_range fc_ext;
> > > > > > > > +     struct ext4_fc_del_range lrange;
> > > > > > > > +     struct ext4_extent *ex;
> > > > > > > > +     int ret;
> > > > > > > > +
> > > > > > > > +     mutex_lock(&ei->i_fc_lock);
> > > > > > > > +     if (ei->i_fc_lblk_len == 0) {
> > > > > > > > +             mutex_unlock(&ei->i_fc_lock);
> > > > > > > > +             return 0;
> > > > > > > > +     }
> > > > > > > > +     old_blk_size = ei->i_fc_lblk_start;
> > > > > > > > +     new_blk_size = ei->i_fc_lblk_start + ei->i_fc_lblk_len - 1;
> > > > > > > > +     ei->i_fc_lblk_len = 0;
> > > > > > > > +     mutex_unlock(&ei->i_fc_lock);
> > > > > > > > +
> > > > > > > > +     cur_lblk_off = old_blk_size;
> > > > > > > > +     jbd_debug(1, "%s: will try writing %d to %d for inode %ld\n",
> > > > > > > > +               __func__, cur_lblk_off, new_blk_size, inode->i_ino);
> > > > > > > > +
> > > > > > > > +     while (cur_lblk_off <= new_blk_size) {
> > > > > > > > +             map.m_lblk = cur_lblk_off;
> > > > > > > > +             map.m_len = new_blk_size - cur_lblk_off + 1;
> > > > > > > > +             ret = ext4_map_blocks(NULL, inode, &map, 0);
> > > > > > > > +             if (ret < 0)
> > > > > > > > +                     return -ECANCELED;
> > > > > > >
> > > > > > > So isn't this actually racy with a risk of stale data exposure? Consider a
> > > > > > > situation like:
> > > > > > >
> > > > > > > Task 1:                         Task 2:
> > > > > > > pwrite(file, buf, 8192, 0)
> > > > > > > punch(file, 0, 4096)
> > > > > > > fsync(file)
> > > > > > >   writeout range 4096-8192
> > > > > > >   fastcommit for inode range 0-8192
> > > > > > >                                 pwrite(file, buf, 4096, 0)
> > > > > > >     ext4_map_blocks(file)
> > > > > > >       - reports that block at offset 0 is mapped so that is recorded in
> > > > > > >         fastcommit record. But data for that is not written so after a
> > > > > > >         crash we'd expose stale data in that block.
> > > > > > >
> > > > > > > Am I missing something?
> > > > > > So the way this gets handled is before entering this function, the
> > > > > > inode enters COMMITTING state (in ext4_fc_submit_inode_data_all
> > > > > > function). Once in COMMITTING state, all the inodes on this inode get
> > > > > > paused. Also, the commit path waits until all the ongoing updates on
> > > > > > that inode are completed. Once they are completed, only then its data
> > > > > > buffers are flushed and this ext4_map_blocks is called. So Task-2 here
> > > > > > would have either completely finished or would wait until the end of
> > > > > > this inode's commit. I realize that I should probably add more
> > > > > > comments to make this more clearer in the code. But is handling it
> > > > > > this way sufficient or am I missing any more cases?
> > > > >
> > > > > I see. In principle this should work. But I don't like that we have yet
> > > > > another mechanism that needs to properly wrap inode changes to make
> > > > > fastcommits work. And if we get it wrong somewhere, the breakage will be
> > > > > almost impossible to notice until someone looses data after a power
> > > > > failure. So it seems a bit fragile to me.
> > > > Ack
> > > > >
> > > > > Ideally I think we would reuse the current transaction machinery for this
> > > > > somehow (so that changes added through one transaction handle would behave
> > > > > atomically wrt to fastcommits) but the details are not clear to me yet. I
> > > > > need to think more about this...
> > > > Yeah, I thought about that too. All we need to do is to atomically
> > > > increment an "number of ongoing updates" counter on an inode, which
> > > > could be done by existing ext4_journal_start()/stop() functions.
> > > > However, the problem is that current ext4_journal_start()/stop() don't
> > > > take inode as an argumen. I considered changing all the
> > > > ext4_journal_start/stop calls but that would have inflated the size of
> > > > this patch series which is already pretty big. But we can do that as a
> > > > follow up cleanup. Does that sound reasonable?
> > >
> > > So ext4_journal_start() actually does take inode as an argument and we use
> > > it quite some places (we also have ext4_journal_start_sb() which takes just
> > > the superblock). What I'm not sure about is whether that's the inode you
> > > want to protect for fastcommit purposes (would need some code auditing) or
> > > whether there are not more inodes that need the protection for some
> > > operations. ext4_journal_stop() could be handled by recording the inode in
> > > the handle on ext4_journal_start() so ext4_journal_stop() then knows for
> > > which inode to decrement the counter.
> > >
> > > Another possibility would be to increment the counter in
> > > ext4_get_inode_loc() - that is a clear indication we are going to change
> > > something in the inode. This also automatically handles the situation when
> > > multiple inodes are modified by the operation or that proper inodes are
> > > being protected. With decrementing the counter it is somewhat more
> > > difficult. I think we can only do that at ext4_journal_stop() time so we
> > > need to record in the handle for which inodes we acquired the update
> > > references and drop them from ext4_journal_stop(). This would look as a
> > > rather robust solution to me...
> > ..the only problem here is that the same handle can be returned by
> > multiple calls to ext4_journal_start(). That means a handle returned
> > by ext4_journal_start() could be associated with multiple inodes. One
>
> That is not quite true. ext4_journal_start() returns always a new handle
> (unless that process has already a handle started, but nested handles are
> not interesting for our case). Just multiple handles may refer to the same
> transaction which is what confused you I guess. So each handle has 1:1
> correspondence with a logical operation that needs to be performed
> atomically and you can store your inode in handle_t (==
> jbd2_journal_handle). Maybe to make the layering clear, you could add a
> helper jbd2_associate_handle_with_inode() or something like that for the
> storing and similar helper for fetching the inode.
Ah I see, I was definitely confused about that. Thanks for the
explanation. I understand it now.
>
> Now I'm not certain that each logical operation has only single inode that
> gets modified in it - e.g. rename may modify multiple inodes. Now I suspect
> that you are marking the fs as inelligible in all the cases that modify
> more inodes but it's difficult to be sure with your current scheme. That's
> another way that should be automated by the scheme (which is easy enough -
> you can mark fs as ineligible if handle already has different inode
> associated with it in ext2_get_inode_loc()).
Makes sense, I'll recheck what happens in case of multiple inodes
modification in 1 logical operation. But this solution sounds good to
me.
>
> I don't think you need to play any games with fs private structure at this
> point as you describe below...
I agree, thanks,
Harshad
>
>                                                                 Honza
>
>
> > way to deal with this would be to define ext4 specific handle
> > structure. So, each call to ext4_journal_start would return a struct
> > that looks like following:
> >
> > struct ext4_handle {
> >     handle_t *jbd2_handle;
> >     struct inode *inode;
> > }
> >
> > So now on ext4_journal_stop(), we know for which inode we need to drop
> > counters. The objects of this struct would either need to have their
> > own kmem_cache or would need to be defined on stack (I think the
> > latter is preferred). Should we do this? If we do this, this is going
> > to be a pretty big change (will have to inspect all the existing
> > callers of ext4_journal_start() and ext4_journal_stop()).
> >
> > Another option would be to change the definition of handle_t such that
> > on every call to jbd2_journal_start(), we get a new wrapper object
> > that takes a reference on handle_t. Such an object would have a
> > private pointer that FS can use the way it wants. This will be a
> > relatively smaller change but it would impact OCFS too. But if we go
> > this route, we can't avoid using a new kmem_cache, since now these new
> > handle wrappers would need to be allocated inside of JBD2.
> >
> > I kind of like the second option better because it keeps the change
> > comparatively smaller. Wdyt? Also, Ted / Andreas, wdyt?
> >
> > Thanks,
> > Harshad
> >
> > Thank
> > >
> > >                                                                 Honza
> > > --
> > > Jan Kara <jack@...e.com>
> > > SUSE Labs, CR
> --
> Jan Kara <jack@...e.com>
> SUSE Labs, CR

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