Re: [PATCH] Add ext3 data=guarded mode

[Jan Kara]

On Mon 20-04-09 10:18:25, Chris Mason wrote:
> On Mon, 2009-04-20 at 15:44 +0200, Jan Kara wrote:
> > Hi Chris,
> > 
> > On Thu 16-04-09 15:42:01, Chris Mason wrote:
> > > 
> > > ext3 data=ordered mode makes sure that data blocks are on disk before
> > > the metadata that references them, which avoids files full of garbage
> > > or previously deleted data after a crash.  It does this by adding every dirty
> > > buffer onto a list of things that must be written before a commit.
> > > 
> > > This makes every fsync write out all the dirty data on the entire FS, which
> > > has high latencies and is generally much more expensive than it needs to be.
> > > 
> > > Another way to avoid exposing stale data after a crash is to wait until
> > > after the data buffers are written before updating the on-disk record
> > > of the file's size.  If we crash before the data IO is done, i_size
> > > doesn't yet include the new blocks and no stale data is exposed.
> > > 
> > > This patch adds the delayed i_size update to ext3, along with a new
> > > mount option (data=guarded) to enable it.  The basic mechanism works like
> > > this:
> > > 
> > > * Change block_write_full_page to take an end_io handler as a parameter.
> > > This allows us to make an end_io handler that queues buffer heads for
> > > a workqueue where the real work of updating the on disk i_size is done.
> > > 
> > > * Add an rbtree to the in-memory ext3 inode for tracking data=guarded
> > > buffer heads that are waiting to be sent to disk.
> > > 
> > > * Add an ext3 guarded write_end call to add buffer heads for newly
> > > allocated blocks into the rbtree.  If we have a newly allocated block that is
> > > filling a hole inside i_size, this is done as an old style data=ordered write
> > > instead.
> > > 
> > > * Add an ext3 guarded writepage call that uses a special buffer head
> > > end_io handler for buffers that are marked as guarded.  Again, if we find
> > > newly allocated blocks filling holes, they are sent through data=ordered
> > > instead of data=guarded.
> > > 
> > > * When a guarded IO finishes, kick a per-FS workqueue to do the
> > > on disk i_size updates.  The workqueue function must be very careful.  We
> > > only update the on disk i_size if all of the IO between the old on
> > > disk i_size and the new on disk i_size is complete.  This is why an
> > > rbtree is used to track the pending buffers, that way we can verify all
> > > of the IO is actually done.  The on disk i_size is incrementally updated to
> > > the largest safe value every time an IO completes.
> > > 
> > > * When we start tracking guarded buffers on a given inode, we put the
> > > inode into ext3's orphan list.  This way if we do crash, the file will
> > > be truncated back down to the on disk i_size and we'll free any blocks that
> > > were not completely written.  The inode is removed from the orphan list
> > > only after all the guarded buffers are done.
> > > 
> > > Signed-off-by: Chris Mason <chris.mason@xxxxxxxxxx>
> >   I've read the patch. I don't think I've got all the subtleties but before
> > diving into it more I'd like to ask why do we do the things in so
> > complicated way? 
> 
> Thanks for reviewing things!
> 
> > Maybe I'm missing some issues so let's see:
> >   1) If I got it right, hole filling goes through standard ordered mode so
> > we can ignore such writes. So why do we have special writepage? I should
> > look just like writepage for ordered mode and we could just tweak
> > ext3_ordered_writepage() (probably renamed) to do:
> > 	if (ext3_should_order_data(inode))
> > 		err = walk_page_buffers(handle, page_bufs, 0, PAGE_CACHE_SIZE,
> >                                         NULL, journal_dirty_data_fn);
> > 	else
> > 		err = walk_page_buffers(handle, page_bufs, 0, PAGE_CACHE_SIZE,
> >                                         NULL, journal_dirty_guarded_data_fn);
> 
> That would work.  My first writepage was more complex, it shrunk as the
> patch evolved.  Another question is if we want to use exactly the same
> writepage for guarded and ordered.  I've always though data=ordered
> should only order new blocks...
  Hmm, true. After my last change we already don't file data buffers in
ordered_writepage() if the page is fully mapped to disk so doing this in
all the cases is fine. Actually, I'll soon write ext3_page_mkwrite() to do
the block allocation on page fault time so after that we can get rid of most
of the code in ext3_ordered_writepage().

> >   2) Why is there any RB tree after all? Since guarded are just extending
> > writes, we can have a linked list of guarded buffers. We always append
> > at the end, we update i_size if the current buffer has no predecestor in the
> > list.
> 
> A guarded write is anything from write() that is past the disk i_size.
> lseek and friends mean it could happen in any order.
  Are you sure? Looking and ext3_guarded_write_end():
...
+       if (test_clear_buffer_datanew(bh)) {
+               /*
+                * if we're filling a hole inside i_size, we need to
+                * fall back to the old style data=ordered
+                */
+               if (offset < inode->i_size) {
+                       ret = ext3_journal_dirty_data(handle, bh);
+                       goto out;
+               }
...
  So it seems we always to ordered write unless we are appending / have
blocks allocated. You could have i_disksize in the check but is it really
worth it? IMO getting rid of the RB tree might be better ;)

> >   3) Currently truncate() does filemap_write_and_wait() - is it really
> > needed? Each guarded bh could carry with itself i_disksize it should update
> > to when IO is finished. Extending truncate will just update this i_disksize
> > at the last member of the list (or update i_disksize when the list is
> > empty). 
> >
> > Shortening truncate will walk the list of guarded bh's, removing from
> > the list those beyond new i_size, then it will behave like the extending
> > truncate (it works even if current i_disksize is larger than new i_size).
> > Note, that before we get to ext3_truncate() mm walks all the pages beyond
> > i_size and waits for page writeback so by the time ext3_truncate() is
> > called, all the IO is finished and dirty pages are canceled.
> 
> The problem here was the disk i_size being updated by ext3_setattr
> before the vmtruncate calls calls ext3_truncate().  So the guarded IO
> might wander in and change the i_disksize update done by setattr.
> 
> It all made me a bit dizzy and I just tossed the write_and_wait in
> instead.
> 
> At the end of the day, we're waiting for guarded writes only, and we
> probably would have ended up waiting on those exact same pages in
> vmtruncate anyway.  So, I do agree we could avoid the write with more
> code, but is this really a performance critical section?
  Well, not really critical but also not negligible - mainly because with
your approach we end up *submitting* new writes we could just be canceled
otherwise. Without fdatawrite(), data of short-lived files need not ever
reach the disk similarly as in writeback mode (OK, this is connected with
the fact that you actually don't have fdatawrite() before ext3_truncate()
in ext3_delete_inode() and that's what initially puzzled me).

> >   IO finished callback will update i_disksize to carried value if the
> > buffer is the first in the list, otherwise it will copy it's value to the
> > previous member of the list.
> >   4) Do we have to call end_page_writeback() from the work queue? That
> > could make IO completion times significantly longer on a good disk array,
> > couldn't it? 
> 
> My understanding is that XFS is doing something similar with the
> workqueue already, without big performance problems.
  OK.

> > There is a way how to solve this I believe although it might
> > be too hacky / complicated. We have to update i_disksize before calling
> > end_page_writeback() because of truncate races and generally for
> > filemap_fdatawrite() to work. So what we could do is:
> >   guarded_end_io():
> >     update i_disksize
> >     call something like __mark_inode_dirty(inode, I_DIRTY_DATASYNC) but
> >       avoid calling ext3_dirty_inode() or somehow make that we immediately
> >       return from it.
> >     call end_buffer_async_write()
> >     queue addition of inode to the transaction / removal from orphan list
> 
> It could, but we end up with a list of inodes that must be logged before
> they can be freed.  This was a problem in the past (before the
> dirty_inode operation was added) because logging an inode is relatively
> expensive, and we have no mechanism to throttle them.
> 
> In the past it lead to deadlocks because kswapd would try and log all
> the dirty inodes, and someone else had the transaction pinned while
> waiting on kswapd to find free ram.  We might be able to do better
> today, but I didn't want to cram that into this patch series as well.
  Ah, OK. I didn't know this. Anyway, if we find getting rid of the work
queue is useful, we can do it later. It would be rather local change.

								Honza
-- 
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR
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