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

From: Chi Zhiling
Date: Sat Dec 28 2024 - 02:38:16 EST




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@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.

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.

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.

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



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.

Sorry, I didn't find those discussions earlier.


Fundamentally, though, removing locking from the read side is not
the answer to this buffered write IO exclusion problem....

-Dave.

Best regards,
Chi Zhiling