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Date:   Fri, 30 Oct 2020 16:28:31 +0100
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>,
        kernel test robot <lkp@...el.com>
Subject: Re: [PATCH v10 5/9] ext4: main fast-commit commit path

On Tue 27-10-20 10:38:19, harshad shirwadkar wrote:
> On Tue, Oct 27, 2020 at 7:29 AM Jan Kara <jack@...e.cz> wrote:
> > > > > +
> > > > > +     mutex_lock(&ei->i_fc_lock);
> > > > > +     if (running_txn_tid == ei->i_sync_tid) {
> > > > > +             update = true;
> > > > > +     } else {
> > > > > +             ext4_fc_reset_inode(inode);
> > > > > +             ei->i_sync_tid = running_txn_tid;
> > > > > +     }
> > > > > +     ret = __fc_track_fn(inode, args, update);
> > > > > +     mutex_unlock(&ei->i_fc_lock);
> > > > > +
> > > > > +     if (!enqueue)
> > > > > +             return ret;
> > > > > +
> > > > > +     spin_lock(&sbi->s_fc_lock);
> > > > > +     if (list_empty(&EXT4_I(inode)->i_fc_list))
> > > > > +             list_add_tail(&EXT4_I(inode)->i_fc_list,
> > > > > +                             (sbi->s_mount_state & EXT4_FC_COMMITTING) ?
> > > > > +                             &sbi->s_fc_q[FC_Q_STAGING] :
> > > > > +                             &sbi->s_fc_q[FC_Q_MAIN]);
> > > > > +     spin_unlock(&sbi->s_fc_lock);
> > > >
> > > > OK, so how do you prevent inode from being freed while it is still on
> > > > i_fc_list? I don't see anything preventing that and it could cause nasty
> > > > use-after-free issues. Note that for similar reasons JBD2 uses external
> > > > separately allocated inode for jbd2_inode so that it can have separate
> > > > lifetime (related to transaction commits) from struct ext4_inode_info.
> > > So, if you see the function ext4_fc_del() above, it's called from
> > > ext4_clear_inode(). What ext4_fc_del() does is that, if the inode is
> > > not being committed, it just removes it from the list. If that inode
> > > was deleted, we have a separate dentry queue which will record the
> > > deletion of the inode, so we don't really need the struct
> > > ext4_inode_info for recording that on-disk. However, if the inode is
> > > being committed (this is figured out by checking the per inode
> > > COMMITTING state), ext4_fc_del() waits until the completion.
> >
> > But I don't think this quite works. Consider the following scenario:
> >
> > inode I gets modified in transaction T
> >   you add I to FC list
> >
> > memory pressure reclaims I from memory
> >   you remove I from FC list
> >
> > open(I) -> inode gets loaded to memory again. Not tracked in FC list.
> > fsync(I) -> nothing to do, FC list is empty
> > <crash>
> >
> > And 'I' now doesn't contain data in should because T didn't commit yet and
> > FC was empty.
> Hmmm, I see. This needs to get fixed. However, I'm a little confused
> here. On memory pressure, the call chain would be like:
> VFS->ext4_evict_inode() -> ext4_free_inode() -> ext4_clear_inode(). In
> ext4_clear_inode(), we free up the jbd2_inode as well. If that's the
> case, how does jbd2_inode survive the memory pressure where its
> corresponding VFS inode is freed up?

Right (and I forgot about this detail of jbd2_inode lifetime). But with
jbd2_inode the thing is that it needs to stay around only as long as there
are dirty pages attached to the inode - once pages are written out (and
this always happens before inode can be evicted from memory), we are sure
the following transaction commit has nothing to do with the inode so we can
safely free it.

With your FC list, we need to track what has changed in the inode even
after all data pages have been written out.

> Assuming I'm missing something, one option would be to track
> jbd2_inode in the FC list instead of ext4_inode_info? Would that take
> care of the problem? Another option would be to trigger a fast_commit
> from ext4_evict_inode if the inode being freed is on fc list. But I'm
> worried that would increase the latency of unlink operation.

So tracking in jbd2_inode will not help - I was confused about that.
Forcing FC on inode eviction is IMO a no-go. That would regress some loads
and also make behavior under memory pressure worse (XFS was actually doing
something similar and they had serious trouble with that under heavy memory
pressure because they needed to write tens of thousands of inodes to the
log during reclaim).

I think that if we are evicting an inode that is in fastcommit and that isn't
unlinked, we just mark the fs as ineligible - to note we are loosing info
needed for fastcommit. This shouldn't happen frequently and if it does, it
means the machine is under heavy memory pressure and it likely isn't
beneficial to keep the info around or try to reload inode from disk on
fastcommit.

> > > > > +
> > > > > +     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...

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

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