[Libguestfs] e2fsprogs concurrency questions

[Theodore Ts'o]

On Fri, Sep 17, 2021 at 04:06:55PM -0500, Eric Blake wrote:
> TL;DR summary: is there documented best practices for parallel access
> to the same inode of an ext2 filesystem from multiple threads?
> 
> First, a meta-question: is there a publicly archived mailing list for
> questions on e2fsprogs?  The README merely mentions Ted's email
> address, and http://e2fsprogs.sourceforge.net/ is silent on contact
> information, although with some googling, I found at least
> https://patchwork.ozlabs.org/project/linux-ext4/patch/20201205045856.895342-6-tytso@mit.edu/
> which suggests linux-ext4 at vger.kernel.org as a worthwhile list to
> mention on the web page.

Yes, discussions and patches relating to e2fsprogs take place on
linux-ext4 at vger.kernel.org.  (Just as xfsprogs patches and discussions
are sent to linux-xfs at vger.kernel.org.)

> Now, on to my real reason for writing.  The nbdkit project is using
> the ext2fs library to provide an ext2/3/4 filter on top of any data
> being served over NBD (Network Block Device protocol) in userspace:
> https://libguestfs.org/nbdkit-ext2-filter.1.html
> 
> Searching for the word 'thread' or 'concurrent' in libext2fs.info came
> up with no hits, so I'm going off of minimal documentation, and mostly
> what I can ascertain from existing examples (of which I'm not seeing
> very many).

Historically, libext2fs and e2fsprogs had no pthreads or concurrency
access at all.  This is because e2fsprogs predates Linux having
pthreads support at all.

This is _starting_ to change, but more on that in a little.

> Right now, the nbdkit filter forces some rather strict serialization
> in order to be conservatively safe: for every client that wants to
> connect, the nbdkit filter calls ext2fs_open(), then eventually
> ext2fs_file_open2(), then exposes the contents of that one extracted
> file over NBD one operation at a time, then closes everything back
> down before accepting a second client.  But we'd LOVE to add some
> parallelization; the NBD protocol allows multiple clients, as well as
> out-of-order processing of requests from a single client.
> 
> Right away, I already know that calling ext2fs_open() more than once
> on the same file system is a recipe for disaster (it is equivalent to
> mounting the same block device at once through more than one bare
> metal OS, and won't work).  So I've got a proposal for how to rework
> the nbdkit code to open the file system exactly once and share that
> handle among multiple NBD clients:
> https://listman.redhat.com/archives/libguestfs/2021-May/msg00028.html

So you are apparently calling ext2fs_open() before forking, and then
you want to use the ext2fs handle from separate processes.  Is that
correct?

That's not going to work if you are going to try to modify the file
system from different processes simultaneously.  That's because the
libext2fs using a writeback cache.  After the fork, each process has
its own copy of the wrteiback cache.

If you are using threads, older versions of libext2fs don't do any
locking before modifying data structures internal to the ext2_fs file
handle.  So if two threads simultaneously try to use the "ext2_fs fs"
handle, they might try to access the block allocation bitmap (for
example) at the same time, without locking, and so bad things will
happen.

You can do your own locking to make sure only one thread is trying to
use the fs handle at a time, at which point you should be fine.  So
you can have multiple clients accessing the file system without having
to open the file system, open a file, and then close the file and
close the file system before accepting the next client.  But only one
client can be using the ext2_fs handle at a time, and if you want to
share any libext2fs data structure across multiple threads,
appropriate read/write locking would be needed.

> Is it okay to have two concurrent handles open to the same inode, or
> do I need to implement a hash map on my end so that two NBD clients
> requesting access to the same file within the ext2 filesystem share a
> single inode?  If concurrent handles are supported, what mechanism can
> I use to ensure that a flush performed on one handle will be visible
> for reading from the other handle, as ext2fs_file_flush does not seem
> to be strong enough?

You could have two threads sharing the same file handle, with locking
so that only one thread is using a file handle at a time.  Also, since
we don't have an analogue for pread(2) and pwrite(2), each thread
would have to assume that the fseek position may have changed by some
other thread, so after it grabbed the file system lock, and then the
per-file lock, it would need to call ext2fs_file_llseek() to make sure
file's position is at a known location before calling
ext2fs_file_read() or ext2fs_file_write().

(The reason why ext2fs_file_flush() is not strong enough is because
that will force writeback, but it doesn't invaludate any cached
information about the file's inode or extent tree structure.  So if
the file inode or extent tree is getting modified by some other thread
out from under it, you're going to have a bad time.)

> Finally, I see with
> https://patchwork.ozlabs.org/project/linux-ext4/patch/20201205045856.895342-6-tytso@mit.edu/
> that you recently added EXT2_FLAG_THREADS, as well as
> CHANNEL_FLAGS_THREADS.  I think it should be fairly straightforward to
> tweak my nbdkit custom IO manager to advertise CHANNEL_FLAGS_THREADS
> (as the NBD protocol really DOES support parallel outstanding IO
> requests), and then add EXT2_FLAG_THREADS into the flags I pss to
> ext2fs_file_open2(), to try and get ext2fs to take advantage of
> parallel access to the underlying storage (regardless of whether the
> clients are parallel coming into ext2fs).  Are there any concurrency
> issues I should be aware of on that front when updating my code?

So this is the _beginning_ of adding threaded support into libext2fs.
At the moment, we now have locking for the unix_io.c data structures.
This allows multiple threads to safely do read-only operations in
parallel.  But this is *all* that it allows.

This was implemented as part of preparatory work to do parallel
e2fsck.  The strategy is that we will have several different threads
reading from disjoint parts of the file system.  So for example, one
thread might be reading from block groups 0 -- 100.  Another thread
might be reading from block groups 101 -- 200.  And so on.  Each
thread will have its own copy of struct e2fsck_struct, and when they
are done they will merge their data to the global e2fsck_struct.  If
there are any inconsistencies that need to be fixed, such that the
file system needs to be modified, this will require waiting until all
of the threads are done, or other specialized locking inside e2fsck.
Of course, in the "happy path", where the file system does not need
any repairs, we won't need to do any special locking or waiting, since
the workload will be read-only.

So we do not have any concurrency support for allocating inodes, or
allocating blocks, or assigning blocks to an inode's extent tree, etc.
Nor do we currently have any plans to add more concureency support to
libext2fs.

To do this would require a huge amount of effort, and it would also
require making a lot of changes to the underlying data structures.
For example, even if we added locking to all of the various data
structures hanging off of the ext2_fs handle, if two threads tried to
open the same inode using ext2fs_file_open(), the two file handles are
completely independent, and there is no way for one thread to do any
kind of cache invalidation of another thread's file handle after it
has modified the inode.  The same is true if one thread is using a
directory iterator while another process tries to modify that
directory.

> Obviously, when the kernel accesses an ext2/3/4 file system, it DOES
> support full concurrency (separate user space processes can open
> independent handles to the same file....

Yes, and that's because the kernel was designed with that in mind from
the beginning.  The ext2fs library was originally designed to support
programs like e2fsck, mke2fs, and debugfs.  None of these tools
required concurrency, and as I've mentioned, at the time when
libext2fs was first implemented, Linux didn't even *have* threads
support.  So concurrency wasn't even possible, even if it had been
needed at that time.

> process must be observed from another).  But nbdkit is all about
> accessing the data of an ext2 filesystem from userspace, without any
> kernel bio involvement, and is thus reliant on whatever concurrency
> guarantees the ext2progs library has (or lacks).

The e2fsprogs library pretty much doesn't have any concurrency
guarantees, sorry.  I suspect you could create a layer on top of
libext2fs which actually implemented a global inode cache ala the
kernel, so that when two threads call something like ext2fs_iget()
function, it works like the kernel's iget() function and they get the
same inode structure, which is reference counted.  Things like the
directory iterator would have to be changed into something more like
the functions exported by the kernel VFS layer, which would make this
layer useless for e2fsck, but it would be more useful for a threaded
client that wanted concurrent read/write access to the filesystem from
mulitple threads.

Cheers,

						- Ted