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

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Message-ID: <2ab5f884-b157-477e-b495-16ad5925b1ec@163.com>
Date: Sat, 28 Dec 2024 15:37:41 +0800
From: Chi Zhiling <chizhiling@....com>
To: Dave Chinner <david@...morbit.com>
Cc: djwong@...nel.org, cem@...nel.org, linux-xfs@...r.kernel.org,
 linux-kernel@...r.kernel.org, Chi Zhiling <chizhiling@...inos.cn>
Subject: Re: [PATCH] xfs: Remove i_rwsem lock in buffered read



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@...inos.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.

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@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.

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