Re: [PATCH] xfs: Remove i_rwsem lock in buffered read

[Jinliang Zheng <alexjlzheng@gmail.com>]

On Fri, 27 Dec 2024 08:50:46 +1100, Dave Chinner <david@fromorbit.com> wrote:
> On Thu, Dec 26, 2024 at 02:16:02PM +0800, Chi Zhiling wrote:
> > From: Chi Zhiling <chizhiling@kylinos.cn>
> > 
> > Using an rwsem to protect file data ensures that we can always obtain a
> > completed modification. But due to the lock, we need to wait for the
> > write process to release the rwsem before we can read it, even if we are
> > reading a different region of the file. This could take a lot of time
> > when many processes need to write and read this file.
> > 
> > On the other hand, The ext4 filesystem and others do not hold the lock
> > during buffered reading, which make the ext4 have better performance in
> > that case. Therefore, I think it will be fine if we remove the lock in
> > xfs, as most applications can handle this situation.
> 
> Nope.
> 
> This means that XFS loses high level serialisation of incoming IO
> against operations like truncate, fallocate, pnfs operations, etc.
> 
> We've been through this multiple times before; the solution lies in
> doing the work to make buffered writes use shared locking, not
> removing shared locking from buffered reads.

Hi, Dave

I have a question that I haven't figured out: If shared locking are used
in buffer writes, how can the read/write atomicity mentioned by [0] provided
by xfs be guaranteed?

  "  "I/O is intended to be atomic to ordinary files and pipes and FIFOs.
  "  Atomic means that all the bytes from a single operation that started
  "  out together end up together, without interleaving from other I/O
  "  operations."
  "  
  "  i.e. that independent read()s should see a write() as a single
  "  atomic change. hence if you do a read() concurrently with a write(),
  "  the read should either run to completion before the write, or the
  "  write run to completion before the read().
  "  
  "  XFS is the only linux filesystem that provides this behaviour.

[0] https://lore.kernel.org/linux-xfs/20190325001044.GA23020@dastard/

thanks,
Jinliang Zheng :)

> 
> A couple of old discussions from the list:
> 
> https://lore.kernel.org/linux-xfs/CAOQ4uxi0pGczXBX7GRAFs88Uw0n1ERJZno3JSeZR71S1dXg+2w@mail.gmail.com/
> https://lore.kernel.org/linux-xfs/20190404165737.30889-1-amir73il@gmail.com/
> 
> There are likely others - you can search for them yourself to get
> more background information.
> 
> Fundamentally, though, removing locking from the read side is not
> the answer to this buffered write IO exclusion problem....
> 
> -Dave.
> -- 
> Dave Chinner
> david@fromorbit.com