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

[Dave Chinner <david@fromorbit.com>]

On Sat, Dec 28, 2024 at 03:37:41PM +0800, Chi Zhiling wrote:
> 
> 
> On 2024/12/27 05:50, Dave Chinner 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.
> 
> You mean using shared locking for buffered reads and writes, right?
> 
> I think it's a great idea. In theory, write operations can be performed
> simultaneously if they write to different ranges.

Even if they overlap, the folio locks will prevent concurrent writes
to the same range.

Now that we have atomic write support as native functionality (i.e.
RWF_ATOMIC), we really should not have to care that much about
normal buffered IO being atomic. i.e. if the application wants
atomic writes, it can now specify that it wants atomic writes and so
we can relax the constraints we have on existing IO...

> So we should track all the ranges we are reading or writing,
> and check whether the new read or write operations can be performed
> concurrently with the current operations.

That is all discussed in detail in the discussions I linked.

> Do we have any plans to use shared locking for buffered writes?

All we are waiting for is someone to put the resources into making
the changes and testing it properly...

-Dave.
-- 
Dave Chinner
david@fromorbit.com