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

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Message-ID: <dca3db30-0e8f-4387-9d4d-974def306502@oracle.com>
Date: Wed, 8 Jan 2025 12:15:52 +0000
From: John Garry <john.g.garry@...cle.com>
To: Amir Goldstein <amir73il@...il.com>, Chi Zhiling <chizhiling@....com>
Cc: Dave Chinner <david@...morbit.com>, 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


> diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> index c488ae26b23d0..2542f15496488 100644
> --- a/fs/xfs/xfs_file.c
> +++ b/fs/xfs/xfs_file.c
> @@ -777,9 +777,10 @@ xfs_file_buffered_write(
>          ssize_t                 ret;
>          bool                    cleared_space = false;
>          unsigned int            iolock;
> +       bool                    atomic_write = iocb->ki_flags & IOCB_ATOMIC;
> 
>   write_retry:
> -       iolock = XFS_IOLOCK_EXCL;
> +       iolock = atomic_write ? XFS_IOLOCK_SHARED : XFS_IOLOCK_EXCL;
>          ret = xfs_ilock_iocb(iocb, iolock);
> --
> 
> xfs_file_write_checks() afterwards already takes care of promoting
> XFS_IOLOCK_SHARED to XFS_IOLOCK_EXCL for extending writes.
> 
> It is possible that XFS_IOLOCK_EXCL could be immediately demoted
> back to XFS_IOLOCK_SHARED for atomic_writes as done in
> xfs_file_dio_write_aligned().
> 
> TBH, I am not sure which blockdevs support 4K atomic writes that could
> be used to test this.
> 
> John, can you share your test setup instructions for atomic writes?

Please note that IOCB_ATOMIC is not supported for buffered IO, so we 
can't do this - we only support direct IO today.

And supporting buffered IO has its challenges; how to handle overlapping 
atomic writes of differing sizes sitting in the page cache is the main 
issue which comes to mind.

Thanks,
John