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

[Dave Chinner]

On Thu, Dec 26, 2024 at 02:16:02PM +0800, Chi Zhiling wrote:
> From: Chi Zhiling <chizhiling@xxxxxxxxxx>
> 
> 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.

A couple of old discussions from the list:

https://lore.kernel.org/linux-xfs/CAOQ4uxi0pGczXBX7GRAFs88Uw0n1ERJZno3JSeZR71S1dXg+2w@xxxxxxxxxxxxxx/
https://lore.kernel.org/linux-xfs/20190404165737.30889-1-amir73il@xxxxxxxxx/

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@xxxxxxxxxxxxx