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Date:   Tue, 18 Apr 2023 19:29:50 -0600
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Subject: Re: [PATCH 1/2] fs: add FMODE_DIO_PARALLEL_WRITE flag
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To:     Christoph Hellwig <hch@infradead.org>,
        "Darrick J. Wong" <djwong@kernel.org>
Cc:     Miklos Szeredi <miklos@szeredi.hu>,
        Bernd Schubert <bschubert@ddn.com>, io-uring@vger.kernel.org,
        linux-ext4@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-xfs@vger.kernel.org, dsingh@ddn.com
References: <20230307172015.54911-2-axboe@kernel.dk>
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 <ZDuNqQgpHUw+gi9G@infradead.org>
From:   Jens Axboe <axboe@kernel.dk>
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On 4/15/23 11:54?PM, Christoph Hellwig wrote:
> On Fri, Apr 14, 2023 at 08:36:12AM -0700, Darrick J. Wong wrote:
>> IIUC uring wants to avoid the situation where someone sends 300 writes
>> to the same file, all of which end up in background workers, and all of
>> which then contend on exclusive i_rwsem.  Hence it has some hashing
>> scheme that executes io requests serially if they hash to the same value
>> (which iirc is the inode number?) to prevent resource waste.
>>
>> This flag turns off that hashing behavior on the assumption that each of
>> those 300 writes won't serialize on the other 299 writes, hence it's ok
>> to start up 300 workers.
>>
>> (apologies for precoffee garbled response)
> 
> It might be useful if someone (Jens?) could clearly document the
> assumptions for this flag.

I guess it can be summed up as the common case should not be using
exclusive (per file/inode) locking. If file extensions need exclusive
locking that's less of a concern, as I don't think it's unreasonable to
expect that to require stricter locking.

-- 
Jens Axboe