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Date:   Thu, 13 Apr 2023 22:11:28 -0700
From:   Christoph Hellwig <hch@infradead.org>
To:     Miklos Szeredi <miklos@szeredi.hu>
Cc:     Bernd Schubert <bschubert@ddn.com>, axboe@kernel.dk,
        io-uring@vger.kernel.org, linux-ext4@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, linux-xfs@vger.kernel.org,
        dsingh@ddn.com
Subject: Re: [PATCH 1/2] fs: add FMODE_DIO_PARALLEL_WRITE flag
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On Thu, Apr 13, 2023 at 09:40:29AM +0200, Miklos Szeredi wrote:
> fuse_direct_write_iter():
> 
> bool exclusive_lock =
>     !(ff->open_flags & FOPEN_PARALLEL_DIRECT_WRITES) ||
>     iocb->ki_flags & IOCB_APPEND ||
>     fuse_direct_write_extending_i_size(iocb, from);
> 
> If the write is size extending, then it will take the lock exclusive.
> OTOH, I guess that it would be unusual for lots of  size extending
> writes to be done in parallel.
> 
> What would be the effect of giving the  FMODE_DIO_PARALLEL_WRITE hint
> and then still serializing the writes?

I have no idea how this flags work, but XFS also takes i_rwsem
exclusively for appends, when the positions and size aren't aligned to
the block size, and a few other cases.