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

From: Chi Zhiling
Date: Sun Dec 29 2024 - 21:43:17 EST



On 2024/12/29 06:17, Dave Chinner wrote:
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@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.

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...

Yes, I'm not particularly concerned about whether buffered I/O is atomic. I'm more concerned about the multithreading performance of buffered I/O.

Last week, it was mentioned that removing i_rwsem would have some impacts on truncate, fallocate, and PNFS operations.

(I'm not familiar with pNFS, so please correct me if I'm wrong.)

My understanding is that the current i_rwsem is used to protect both
the file's data and its size. Operations like truncate, fallocate,
and PNFS use i_rwsem because they modify both the file's data and its size. So, I'm thinking whether it's possible to use i_rwsem to protect only the file's size, without protecting the file's data.

So operations that modify the file's size need to be executed sequentially. For example, buffered writes to the EOF, fallocate operations without the "keep size" requirement, and truncate operations, etc, all need to hold an exclusive lock.

Other operations require a shared lock because they only need to access
the file's size without modifying it.


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.

Sorry, I overlooked some details from old discussion last time.
It seems that you are not satisfied with the effectiveness of
range locks.


Best regards,
Chi Zhiling